Climate change

Class 12 Biology, unit 13, book pages 244–260

Practise 52 MCQs Take the unit quiz Review 48 cards

Climate change is the long-term shift in the climate of the Earth. This unit separates weather from climate, explains the natural and human causes of climate change, and shows how it affects plants, animals and the ocean through higher temperature and falling pH. It ends with the species that have gone extinct or become endangered, which is why the topic matters for biodiversity.

Learning outcomes (3)

Describe how climate change impacts flora and fauna.
Describe how climate change can impact ocean biology in terms of its temperature and acidity as well as the resulting harmful effects.
Name species that have gone extinct due to climate change.

Weather and climateOften examined

Weather and climate are two closely linked terms that many people treat as synonyms, but they are quite different ideas. This chapter uses them to study climate change, its causes, and its impacts on the biotic (living) and abiotic (non-living) components of the Earth's ecosystem.

Weather: Short-term changes in environmental conditions: temperature (hot and cold), water vapour in the atmosphere (humid or dry air), precipitation (rain, snow, hail or dew), daylight (cloudy or sunny) and water availability (flooding or drought).
Climate: The overall pattern of weather in an area that prevails over a period of a few decades to even centuries.

Weather is the day-to-day ups and downs; climate is the long-term pattern (the average) of those ups and downs.

Weather compared with climate
Point	Weather	Climate
Time span	Short term (hours, days, a few weeks)	Long term: a few decades to even centuries
What it describes	The condition of the atmosphere at a given time and place (hot, cold, humid, dry, rainy, sunny)	The overall pattern of weather that prevails in an area
How it changes	Changes quickly and often, even within a day	Changes slowly; shifts take many years
Example	A cloudy, rainy afternoon; a flood or drought this season	Deserts are hot and dry; forests are humid and green; each ocean, forest, desert and plain has its own climate

Worked example: telling weather from climate

Decide whether each statement describes weather or climate, and give the reason. (a) Heavy rain fell on the city yesterday evening. (b) Deserts are hot and dry for most years. (c) Floods damaged crops this week. (d) Over several centuries the northern plains have stayed cold in winter.

Statement	Answer	Reason
(a) Heavy rain yesterday evening	Weather	It is one short-term event of precipitation.
(b) Deserts are hot and dry for most years	Climate	It is the usual pattern of an area over many years.
(c) Floods damaged crops this week	Weather	Flooding is a short-term change in water availability.
(d) Cold winters for centuries	Climate	The pattern is described over a period of centuries.

Clue to use: ask "how long?". A single day or week points to weather; decades or more points to climate.

13.1Climate change, its causes and impacts on biodiversityOften examined

Climate change: The long-term, gradual and apparently irreversible changes in the climatic conditions of an area that may affect the whole world with the passage of time.
Anthropogenic: Human-caused. The term climate change today often refers to human-caused (anthropogenic) climate change happening rapidly over the last century, after the Industrial Revolution.

Major events of climate change include:

average rise in global air (atmospheric) temperature;
ocean acidification;
changes in the seasonal duration;
changes in the monsoon raining pattern;
extreme hot and cold;
extreme flooding and drought, and so on.

Climate changes naturally over thousands and millions of years because of factors such as volcanic eruptions and variations in the Earth's orbit. The climate change of today is different: it is happening rapidly and is mainly caused by human activity.

Natural and anthropogenic climate change
Point	Natural climate change	Anthropogenic climate change
Cause	Volcanic eruptions, variations in Earth's orbit and other natural factors	Human activities since the Industrial Revolution
Speed	Over thousands and millions of years	Rapid, over the last century

The factors that change the climate, grouped as natural and anthropogenic (human-caused). Each is explained in the sections that follow.

13.1.1Causes of climate change: natural factorsOften examined

The Earth's climate is driven by the balance of energy entering and leaving the atmosphere. This balance is disturbed by both natural and anthropogenic factors. Natural processes influence the Earth's climate over long periods of time, though their effects are often gradual and cyclical. The book marks this topic as extra reading material.

i. Volcanic activity

Large volcanic eruptions release huge amounts of dust, ash and gases such as sulfur dioxide into the atmosphere. These particles can block sunlight, causing temporary cooling of the Earth's surface. However, volcanic eruptions also release $CO X_{2}$ , contributing to the greenhouse effect over time.

Figure 13.1: Volcanic eruption in 2018 near Antigua, Guatemala, Central America. Dust, ash and sulfur dioxide cool the surface for a short time, while the

CO X_{2}

released adds to the greenhouse effect over time.

ii. Solar variability

The Sun's energy output is not constant and can vary over time because of solar cycles. Changes in solar radiation can influence Earth's climate. During periods of higher solar activity, more solar energy reaches the Earth, causing temporary warming.

iii. Milankovitch cycles

Milankovitch cycles: Long-term changes in the Earth's orbit and axial tilt that affect the amount of sunlight the planet receives. The shape of the orbit (eccentricity), the tilt of the axis (obliquity) and the direction of the axis (precession) change slowly and periodically over time, so the amount of solar energy reaching the Earth varies.

Figure 13.2: Milankovitch cycles. Precession (26,000 years), obliquity (41,000 years) and eccentricity (100,000 years).

The three Milankovitch cycles
Cycle	What changes	Duration
Eccentricity	The shape of the Earth's orbit around the Sun, alternating between more circular and more elliptical orbits	About 100,000 years
Axial tilt (obliquity)	The angle of the Earth's tilt on its axis, which varies between $22. 1^{\circ}$ and $24. 5^{\circ}$	About 41,000 years
Precession	The wobble, or change in direction, of the Earth's axis of rotation, like a spinning top	About 26,000 years

Worked example: matching Milankovitch cycles

Name the Milankovitch cycle described by each statement, then arrange the three cycles from the shortest to the longest. (a) The Earth's axis wobbles like a spinning top. (b) The orbit alternates between more circular and more elliptical. (c) The tilt of the axis changes between

22. 1^{\circ}

and

24. 5^{\circ}

Statement	Cycle	Duration
(a) Axis wobbles like a spinning top	Precession	26,000 years
(b) Orbit alternates between circular and elliptical	Eccentricity	100,000 years
(c) Tilt changes between $22. 1^{\circ}$ and $24. 5^{\circ}$	Axial tilt (obliquity)	41,000 years

Match each description to the cycle using the table above.
Write the durations: precession 26,000, obliquity 41,000, eccentricity 100,000 years.
Order them from the smallest number to the largest.

Order from shortest to longest: precession (26,000 years), then axial tilt (41,000 years), then eccentricity (100,000 years). All three change the amount of solar energy that reaches the Earth.

iv. Ocean currents

The oceans store and transport heat around the world through currents. Changes in ocean circulation patterns, such as El Niño and La Niña events, can cause short-term climate changes, affecting weather patterns such as rainfall and temperature distribution.

v. Natural greenhouse gas emissions

Naturally occurring processes such as the decay of organic matter, the respiration of plants and animals, and wetlands release greenhouse gases such as carbon dioxide ( $CO X_{2}$ ) and methane ( $CH X_{4}$ ) into the atmosphere. These gases trap heat, contributing to the natural greenhouse effect.

The greenhouse effect. Sunlight passes through the atmosphere and warms the surface, which gives off infrared (accent). Greenhouse gases absorb some of it and send part back down, so with more greenhouse gas more heat is kept and the surface is warmer.

Natural factors causing climate change
Natural factor	Effect on climate
Volcanic activity	Dust, ash and sulfur dioxide block sunlight: temporary cooling. $CO X_{2}$ adds to the greenhouse effect over time.
Solar variability	Higher solar activity: more solar energy reaches the Earth, temporary warming.
Milankovitch cycles	Slow periodic changes in orbit shape, axial tilt and axis direction vary the solar energy received.
Ocean currents	Changes such as El Niño and La Niña cause short-term climate changes in rainfall and temperature.
Natural greenhouse gases	Decay, respiration and wetlands release $CO X_{2}$ and $CH X_{4}$ that trap heat.

13.1.1Anthropogenic (human-caused) factors causing climate changeOften examined

Human activities since the Industrial Revolution have been the primary drivers of rapid climate change. These activities significantly increase the concentration of greenhouse gases in the atmosphere, accelerating global warming. The major anthropogenic factors are listed below.

The greenhouse effect. Greenhouse gases trap heat that would otherwise escape back into space; more greenhouse gases trap more heat.

i. Deforestation and urbanization

Environmental buffers: Trees, so called because they absorb $CO X_{2}$ through photosynthesis and help to regulate the Earth's climate.
Deforestation: The cutting of trees by man for various purposes.
Urbanization: The expansion of cities, that is, urban development.

When forests are cut down for agriculture, logging or urban development, this carbon sink is lost (Figure 13.3). In addition, burning trees during deforestation releases the stored $CO X_{2}$ into the atmosphere.

Figure 13.3: Deforestation and urbanization contribute to climate change. A forest absorbs

CO X_{2}

; when it is cut and burnt, the sink is lost and stored

CO X_{2}

is released.

ii. Burning of fossil fuels

The combustion of coal, oil and natural gas for energy production, transportation and industry is the largest source of $CO X_{2}$ emissions. This leads to an enhanced greenhouse effect, where more heat is trapped in the atmosphere, causing the planet to warm.

iii. Industrial processes and agriculture

Industrial activities such as cement production and certain chemical processes release $CO X_{2}$ and other gases (Figure 13.4). Agriculture is a significant source of methane ( $CH X_{4}$ ), especially from livestock (cows and sheep) and rice paddies, and of nitrous oxide ( $N X_{2} O$ ) from fertilizers.

iv. Land use changes

Beyond deforestation, other land use changes such as urbanization and soil degradation also contribute to climate change by reducing natural vegetation that absorbs carbon and by increasing heat absorption by cities (urban heat islands).

v. Waste management

Landfills produce methane ( $CH X_{4}$ ) as organic waste breaks down. Improperly managed waste, including food waste and discarded products, increases the release of greenhouse gases.

vi. Transportation

The global reliance on vehicles powered by gasoline and diesel engines emits significant amounts of $CO X_{2}$ . Air travel and shipping also contribute to greenhouse gas emissions.

vii. Aerosols and chemicals

Human-made aerosols, such as those from industrial pollution, can have both cooling and warming effects on the climate. Some aerosols reflect sunlight (cooling effect), but others, such as black carbon (soot), absorb heat and contribute to warming. Chlorofluorocarbons (CFCs) and other chemicals not only damage the ozone layer but are also potent greenhouse gases.

Anthropogenic factors at a glance
Anthropogenic factor	Human activity	Gas or effect
Deforestation and urbanization	Cutting forests for agriculture, logging and city growth; burning trees	Carbon sink lost; stored $CO X_{2}$ released
Burning of fossil fuels	Coal, oil and natural gas for energy, transport and industry	Largest source of $CO X_{2}$ ; enhanced greenhouse effect
Industrial processes and agriculture	Cement production, chemical processes, livestock, rice paddies, fertilizers	$CO X_{2}$ , $CH X_{4}$ , $N X_{2} O$
Land use changes	Urbanization, soil degradation	Less vegetation to absorb carbon; urban heat islands
Waste management	Landfills, food waste	$CH X_{4}$
Transportation	Petrol and diesel vehicles, air travel, shipping	$CO X_{2}$
Aerosols and chemicals	Industrial pollution, soot, CFCs	Aerosols cool or warm; black carbon and CFCs warm

13.1.2Impact of climate change on biodiversity: flora (plants)Often examined

Biodiversity: The flora (overall plants) and fauna (overall animals) of an area taken together. Climate change affects all environmental compartments, but biodiversity shows a significant impact.

Plants depend on specific conditions such as temperature, rainfall and soil quality to grow. Climate change alters these conditions, making it difficult for many plants to survive.

i. Changes in growing seasons

The growing season is the time of year when plants can grow because of favourable temperature conditions. With climate change, growing seasons are becoming longer in some regions and shorter in others. This inconsistency affects when plants flower, produce fruit or go dormant.

ii. Increased stress from extreme weather

Climate change causes more frequent and intense storms, floods and droughts. Droughts can dry out the soil, preventing plants from getting enough water, while floods can waterlog their roots.

iii. Migration of plants

Plants cannot move like animals, but seeds can spread to new areas through wind, water or animals. As climates change, plants that once thrived in a particular area may begin growing in new regions better suited to the altered climate, while others may disappear from their original habitats.

Impact of climate change on flora
Impact on plants	Cause	Result
Changes in growing seasons	Temperature conditions change	Seasons longer in some regions, shorter in others; flowering, fruiting and dormancy are affected
Stress from extreme weather	More frequent and intense storms, floods and droughts	Drought: not enough water. Flood: waterlogged roots
Migration of plants	Climate shifts	Seeds spread by wind, water or animals; plants grow in new regions and may vanish from old habitats

Activity: the impact of changes in growing conditions

Figure 13.5: Activity to demonstrate the impact of changes in growing seasons. One seedling gets normal sunlight and regular water; the other is kept in a colder, less favourable place.

Method

Plant two seeds in two small pots.
Place one pot in normal sunlight and water it regularly.
Place the other pot in a colder or less favourable environment, for example a shaded area or near a fan to simulate a colder climate.
Observe how differently the two plants grow.

The plant in normal conditions grows better. This shows how environmental conditions affect plant growth.

13.1.2Impact of climate change on fauna (animals)Often examined

Animals rely on stable ecosystems to find food, shelter and mates. Climate change disrupts these ecosystems, causing a wide range of problems for wildlife.

i. Loss of habitat

As temperatures rise, many animals lose their habitats. For example, polar bears rely on sea ice to hunt for seals. When the ice melts because of warming, they struggle to find food.

Figure 13.6: Impact of climate change on the polar bear's habitat. As sea ice melts, the bear loses its hunting platform.

ii. Shifting habitats

Just like plants, animals often need to move to new regions to find suitable climates. For example, birds that once migrated to warmer regions during winter may have to adjust their migration patterns as temperatures shift.

iii. Food chain disruption

Animals depend on specific plants and other animals for food. When climate change affects one species (a plant or a prey animal), it also impacts the animals that rely on it. For example, if a type of plant that an insect feeds on goes extinct because of the changing climate, the insect population will decline, which then affects the animals that eat those insects.

Figure 13.7: Network of food chains (food web). Arrows point from the food to the animal that eats it. Losing one species affects every animal linked to it.

iv. Breeding and hibernation cycles

Many animals rely on specific temperature and weather conditions for breeding, hibernation and migration. Climate change can throw off these cycles. For example, some animals hibernate in winter and wake up in spring when food is available. If spring comes too early or too late, they may wake up at the wrong time, with no food around.

v. Ecosystem balance and extinction

Flora and fauna are part of an interconnected web. When one species struggles or disappears, it affects the whole system. If plants or animals cannot adapt to the rapid changes in climate, they may go extinct, disrupting the balance of the ecosystem.

vi. Invasive species

Invasive species: Species that move to new areas to escape a changing climate and invade habitats where they do not naturally belong. They can outcompete native plants or animals for resources, which can harm the ecosystem.

Figure 13.8: Effect of invasive species on other species. Native species decline as the invader takes their resources.

vii. Species adaptation

Some species are better at adapting to new climates, but many cannot keep up with the speed of climate change. Species that evolve quickly might survive, while others face extinction.

Impact of climate change on fauna
Impact on fauna	Example from the book
Loss of habitat	Polar bears lose sea ice for hunting seals
Shifting habitats	Birds change their migration patterns
Food chain disruption	An insect declines when its food plant goes extinct, then animals that eat the insect are affected
Breeding and hibernation cycles	Animals wake from hibernation at the wrong time, with no food around
Ecosystem balance and extinction	Species that cannot adapt go extinct and the balance is disturbed
Invasive species	Species that move in outcompete native species for resources
Species adaptation	Fast-evolving species may survive; others face extinction

13.2Climate change impacts on ocean ecosystem

Climate change impacts the ocean ecosystem in three ways, that is, by causing:

a rise in ocean temperature;
a decrease in ocean pH (increased acidity);
effects on the growth of ocean biodiversity.

The three ways
Impact	Where it is explained
Rise in ocean temperature	13.2.1 Climate change impacts ocean temperature
Decrease in ocean pH (increased acidity)	13.2.2 Climate change impacts ocean acidification
Effects on ocean biodiversity	13.2.3 Harmful effects of high ocean temperature and ocean acidification on marine biology

The three ways in which climate change affects the ocean ecosystem, with the section that explains each.

13.2.1Climate change impacts ocean temperatureOften examined

Climate change impacts ocean temperatures through several human and natural factors.

Global warming and the greenhouse gases

Global warming: The process, driven primarily by the build-up of greenhouse gases (such as carbon dioxide, methane and nitrous oxide) in the atmosphere, in which the Earth warms.

Human activities, particularly the burning of fossil fuels (coal, oil, gas), deforestation and industrial processes, have increased the concentration of greenhouse gases in the atmosphere. These gases trap heat, preventing it from escaping back into space, which leads to global warming. The oceans absorb much of this excess heat, resulting in higher ocean temperatures.

Scientific enquiry: thermal expansion

Thermal expansion: Warmer water expands because its molecules move further apart when heated, which increases the ocean's volume. This contributes to rising sea levels.

The book asks: how can you prove thermal expansion in the laboratory?

A laboratory demonstration of thermal expansion. When the water in the flask is heated it expands and rises up the narrow tube.

Possible experiment

Fill a flask completely with coloured water and fit a stopper carrying a narrow glass tube, so that the water stands part-way up the tube.
Mark the water level in the tube.
Heat the flask gently (for example in a hot water bath).
Watch the level: it rises up the tube. The glass changes very little, so the rise shows that the water itself has expanded.
Cool the flask and the level falls again, confirming that the change was caused by temperature.

High heat capacity of the ocean

High heat capacity: The ability to absorb and store vast amounts of heat without a dramatic increase in temperature.

The oceans have a high heat capacity, so they can absorb and store vast amounts of heat without a dramatic rise in temperature. The book states that this property also causes the rise in ocean temperature.

Large surface area of the oceans

The oceans cover more than 70% of the Earth's surface, so they absorb about 90% of the atmospheric heat, resulting in higher ocean temperatures.

Why the ocean warms
Factor	How it raises ocean temperature
Global warming and greenhouse gases	Gases trap heat; the oceans absorb much of the excess heat
High heat capacity	The oceans store vast amounts of heat
Large surface area	Oceans cover more than 70% of the Earth and absorb about 90% of atmospheric heat
Ocean circulation and heat distribution	Altered currents give uneven heat distribution (next section)

13.2.1Ocean circulation and heat distributionOften examined

Ocean circulation: The movement of ocean water vertically or horizontally.

Ocean circulation and heat distribution play a critical role in regulating ocean temperatures across different regions of the world. These processes usually help to maintain a balance in ocean heat distribution, but climate change is altering these patterns, causing a rise in ocean temperatures in several ways: through the Gulf Stream, the Atlantic Meridional Overturning Circulation (AMOC), El Niño and La Niña events, and sea ice and ocean temperature.

Gulf Stream

The Gulf Stream is a powerful and warm ocean current that originates in the Gulf of Mexico and flows along the eastern coast of the United States before crossing the Atlantic Ocean toward Europe. It plays a key role in regulating the climate of the North Atlantic region and has a significant impact on weather patterns, especially in North America and Western Europe.

Figure 13.9: Geographical distribution of the Earth. The numbers such as

2 3^{\circ} 2 6^{'} 2 2^{''}

N are geographic coordinates that give the exact latitude. Buenos Aires is marked as an example.

Reading latitude: degrees, minutes and seconds

Reading a geographic coordinate
Symbol	Meaning (example $2 3^{\circ} 2 6^{'} 2 2^{''}$ N)
$2 3^{\circ}$	23 degrees of latitude north of the equator
$2 6^{'}$ (single apostrophe)	26 minutes of latitude; there are 60 minutes in one degree
$2 2^{''}$ (double apostrophe)	22 seconds of latitude; there are 60 seconds in one minute
N	The latitude is in the Northern Hemisphere

Meaning: the Tropic of Cancer is located at 23 degrees, 26 minutes and 22 seconds north of the equator.

Worked example: converting degrees, minutes and seconds

Express the latitude of the Tropic of Cancer,

2 3^{\circ} 2 6^{'} 2 2^{''}

N, in decimal degrees, and state on which side of the equator Buenos Aires (

3 4^{\circ} 3 4^{'} 1 3^{''}

S) lies.

Convert the minutes to degrees by dividing by 60.
$2 6^{'} = \frac{26}{60} = 0.433 3^{\circ}$
Convert the seconds to degrees by dividing by 3600 (60 seconds in a minute and 60 minutes in a degree).
$2 2^{''} = \frac{22}{3600} = 0.006 1^{\circ}$
Add the three parts.
$23 + 0.4333 + 0.0061 = 23.439 4^{\circ} N$
Read the letter. S means the Southern Hemisphere, so Buenos Aires lies south of the equator. Its latitude is larger than that of the Tropic of Capricorn ( $2 3^{\circ} 2 6^{'} 2 2^{''}$ S), so it is south of that circle as well.

23.4 4^{\circ} N

Normally this heat is distributed more evenly across the globe. Climate change is causing these currents to weaken or shift, leading to an uneven distribution of heat. For example, some regions may experience a build-up of heat, causing ocean temperatures to rise more rapidly.

Atlantic Meridional Overturning Circulation (AMOC)

Atlantic Meridional Overturning Circulation (AMOC): A type of ocean current that carries warm surface water from the tropics to the North Atlantic, where it cools and sinks before flowing back toward the equator as cold, deep water.

The AMOC. Warm surface water flows north, cools and sinks in the North Atlantic, and returns south as cold, deep water.

How climate change slows the AMOC

Global temperatures rise because of climate change.
Polar ice melts and adds large amounts of fresh water to the North Atlantic.
The fresh water reduces the water's salinity, so it is less dense and is prevented from sinking as usual.
The AMOC slows down, so less heat is carried to the poles.
Tropical and subtropical ocean temperatures become warmer, which adds to rising ocean temperatures and disrupts weather patterns globally.

How climate change slows the AMOC. The slowdown (accent) is the turning point of the chain.

13.2.1El Niño and La Niña eventsOften examined

El Niño and La Niña are climate phenomena that result from changes in ocean circulation in the Pacific Ocean, affecting global weather and ocean temperatures.

El Niño: An event in which warm surface waters shift eastward toward the coast of South America, leading to warmer ocean temperatures in the central and eastern Pacific. It also affects weather patterns around the world and contributes to rising temperatures.
La Niña: The opposite of El Niño, where cooler waters dominate.
El Niño-Southern Oscillation (ENSO): The climate pattern of which El Niño and La Niña are the opposite phases.

The two phases of ENSO across the Pacific. In La Niña (cold phase) cooler waters dominate the central and eastern Pacific; in El Niño (warm phase) warm surface water shifts east towards South America.

El Niño compared with La Niña
Point	El Niño (warm phase)	La Niña (cold phase)
Sea surface temperature	Warmer in the central and eastern Pacific	Cooler
Movement of warm water	Warm surface waters shift eastward toward the coast of South America	Cooler waters dominate
Effect on rainfall and temperature	Wetter conditions in some regions; warmer conditions and drought in others	More rainfall in some areas (like Southeast Asia); drier, cooler conditions in others (like South America)
Duration	Several months	Several months

The book adds that strong El Niño events often cause a significant increase in global ocean temperatures. Both phenomena have widespread effects on weather, agriculture, ecosystems and economies globally.

13.2.1Sea ice and ocean temperatureOften examined

As ocean temperatures rise, sea ice and glaciers at the poles (particularly in the Arctic and Antarctic) are melting at an accelerated rate. Sea ice acts as a reflective shield, bouncing sunlight back into space. When it melts, darker ocean water is exposed, which absorbs more heat, further accelerating ocean warming.

The sea ice feedback loop. Melting exposes dark water that absorbs more heat, which melts more ice.

Worked example: why melting sea ice speeds up ocean warming

Explain, in steps, why the melting of sea ice makes the ocean warm faster.

Sea ice is bright and reflective, so it bounces sunlight back into space and keeps the polar ocean cooler.
Warm temperatures melt some of the ice, so some of the ocean surface is no longer covered.
The exposed ocean water is darker, so it reflects less and absorbs more of the sunlight as heat.
The ocean warms, and the extra warmth melts more ice. The cycle repeats and speeds up.

Because each change makes the next change bigger, this is a feedback loop that amplifies warming.

Critical thinking 1: is the feedback positive or negative?

The book describes a feedback loop: as ice melts and more heat is absorbed, the ocean's ability to store carbon dioxide diminishes. This increases atmospheric $CO X_{2}$ levels, trapping more heat and causing further warming of both the atmosphere and the oceans.

Critical thinking 2: why does committed warming happen?

The book states that even if greenhouse gas emissions stopped today, the heat already absorbed by the oceans would continue to warm the planet for centuries. This is known as committed warming.

13.2.2Climate change impacts ocean acidification (low pH)Often examined

Ocean acidification: The long-term lowering of the pH of the ocean (it becomes more acidic) because of the absorption of increasing amounts of carbon dioxide ( $CO X_{2}$ ) from the atmosphere.

The ocean as a carbon sink

The ocean acts as a carbon sink, meaning it absorbs large quantities of $CO X_{2}$ from the atmosphere. Since the Industrial Revolution the concentration of $CO X_{2}$ in the atmosphere has increased dramatically because of human activities such as burning fossil fuels (coal, oil and gas), deforestation and industrial processes. To balance the increased atmospheric $CO X_{2}$ , the ocean has absorbed about 30% of this excess $CO X_{2}$ .

$CO X_{2}$ dissolving in ocean water

When $CO X_{2}$ dissolves in seawater it reacts with water to form carbonic acid ( $H X_{2} CO X_{3}$ ).
$CO X_{2} + H X_{2} O H X_{2} CO X_{3}$
Carbonic acid is unstable and dissociates into bicarbonate ions ( $HCO X_{3} X^{-}$ ) and hydrogen ions ( $H X^{+}$ ).
$H X_{2} CO X_{3} HCO X_{3} X^{-} + H X^{+}$
The increase in hydrogen ions is what lowers the pH of the ocean, making it more acidic.

Ocean acidification in four steps, from atmospheric

CO X_{2}

to a lower pH.

Ocean acidification and climate change are interlinked

The link

Climate change makes the ocean temperature rise.
Warmer water holds less gas, so the ocean absorbs less $CO X_{2}$ .
More $CO X_{2}$ remains in the atmosphere in the future.
That accelerates global warming, which warms the ocean further.

As the ocean becomes more acidic it affects the delicate balance of ocean chemistry, which has serious consequences for marine life and ecosystems.

13.2.2Regional variations in ocean acidification

Ocean acidification does not happen evenly across the globe. Some regions, like the polar oceans, are more vulnerable. Cold water absorbs more $CO X_{2}$ than warm water, which means that areas like the Arctic and Southern Oceans are acidifying faster. This puts polar marine life, such as cold-water corals (species that thrive in these environments), at greater risk.

Regional differences in ocean acidification
Point	Polar oceans (Arctic and Southern Oceans)	Warm regions
Water temperature	Cold	Warm
$CO X_{2}$ absorbed	More	Less
Rate of acidification	Faster	Slower
Marine life at risk	Polar marine life such as cold-water corals	Less at risk from this cause

13.2.3Harmful effects of high ocean temperature on marine biologyOften examined

High ocean temperatures and ocean acidification, both consequences of climate change, have profound and harmful effects on marine biology. These changes threaten the survival of numerous marine species and ecosystems, disrupting food webs, biodiversity and the services oceans provide to humanity. High ocean temperature affects marine life in several ways.

Coral bleaching

Zooxanthellae: The symbiotic algae that live within the tissues of corals and provide them with nutrients.
Coral bleaching: The phenomenon in which corals expel their symbiotic algae, causing the coral to turn completely white or pale.

Coral reefs are among the most diverse ecosystems in the ocean, but they are extremely sensitive to temperature changes.

Coral bleaching (see also Figure 13.10, bleaching recorded in coral reefs of the National Marine Sanctuary of American Samoa between 2014 and 2015). A rise in ocean temperature makes the coral expel its algae.

What happens in coral bleaching

Ocean temperatures rise and the corals experience stress.
The stressed corals expel the zooxanthellae living in their tissues.
Without the algae the coral loses its colour and its nutrient supply, and its white calcium carbonate skeleton shows: the coral looks bleached.
Bleached corals are not dead, but they are more vulnerable to diseases and unable to grow or reproduce efficiently.
Prolonged exposure to high temperatures leads to coral death.

The loss of coral reefs destroys habitats for many marine species, threatening biodiversity and the ecosystem services which reefs provide, such as coastal protection and fisheries.

Disruption of marine ecosystems

Temperature-sensitive species, such as certain types of fish and plankton, are particularly affected by rising temperatures. Many species must migrate to cooler waters, shifting the balance of ecosystems and affecting food chains. Species that cannot migrate quickly enough, or that depend on specific habitats, may face local or global extinction. For example, fish populations that live in cooler waters may decline, disrupting the marine food web from small plankton to large predators.

Impact on fish reproduction and growth

Many fish species are cold-blooded, meaning their body temperatures are influenced by the surrounding water temperature. Warmer oceans can alter the metabolism, growth and reproduction of these species. Warmer waters can cause fish to grow smaller, affecting their reproductive success and survival rates. For example, certain fish species may produce fewer offspring or experience developmental problems in warmer environments.

Marine heatwaves

Marine heatwaves: Periods of extreme heat in the ocean, like heatwaves on land. They have intensified with climate change, causing mass die-offs of marine species, including fish, shellfish and marine mammals.

Melting polar ice and habitat loss

In polar regions, rising ocean temperatures are causing sea ice to melt. Many species, such as polar bears, seals and walruses, depend on sea ice for hunting and breeding. As the ice disappears, these species face habitat loss and are forced to migrate or adapt to unfamiliar environments, leading to population declines. The loss of sea ice also impacts the food chains in these regions, particularly affecting krill, an important food source for whales, seals and penguins.

Figure 13.11: Antarctic krill (genus Euphausia), a crustacean arthropod. Loss of sea ice reduces krill, which whales, seals and penguins eat.

Ocean deoxygenation

Hypoxic zones: Areas of low oxygen concentration in warmer ocean waters.
Anoxic zones (dead zones): Areas where oxygen is so depleted that marine life cannot survive.

Warmer ocean waters hold less oxygen. Fish, crabs and other marine species either die or move to other regions, further stressing marine ecosystems. Deoxygenation disrupts marine biodiversity and can lead to declines in fisheries.

Worked example: hypoxic zone and anoxic zone

Differentiate between a hypoxic zone and an anoxic zone in the ocean, and say what causes both.

Point	Hypoxic zone	Anoxic zone
Oxygen level	Low oxygen concentration	Oxygen so depleted that almost none is left
Effect on marine life	Fish and crabs die or move to other regions	Marine life cannot survive
Other name	Low-oxygen zone	Dead zone

Cause of both: warmer ocean water holds less oxygen (ocean deoxygenation). It disrupts marine biodiversity and can lead to declines in fisheries.

Harmful effects of high ocean temperature
Effect of high ocean temperature	Result for marine life
Coral bleaching	Corals expel zooxanthellae, turn white, are vulnerable to disease; prolonged heat kills them; reef habitats and fisheries are lost
Disruption of marine ecosystems	Temperature-sensitive fish and plankton migrate to cooler waters; food chains shift; extinction possible
Fish reproduction and growth	Cold-blooded fish change in metabolism, grow smaller, produce fewer offspring
Marine heatwaves	Mass die-offs of fish, shellfish and marine mammals
Melting polar ice and habitat loss	Polar bears, seals, walruses lose habitat; krill decline
Ocean deoxygenation	Hypoxic and anoxic zones; deaths, migration, falling fisheries

13.2.3Harmful effects of ocean acidification on marine biologyOften examined

Ocean acidification affects marine life in several ways.

Impact on calcifying organisms

Calcifying organisms: Marine organisms, such as corals, shellfish (oysters, clams), sea urchins and plankton, that rely on calcium carbonate ( $CaCO X_{3}$ ) to build their skeletons and shells.

As ocean acidification lowers the pH of seawater, it reduces the availability of carbonate ions ( $CO X_{3} X^{2 -}$ ), which these organisms need to form calcium carbonate. This makes it harder for them to build and maintain their skeletons and shells.

Figure 13.12: Calcifying organisms (corals, zooplankton, sea urchin, oyster). At normal pH carbonate ions are plentiful and the shell is strong; at lower pH there are fewer carbonate ions and the shell is thin and weak.

Oyster larvae struggle to form shells in acidic water,
leading to high mortality rates in oyster populations.
Coral reefs, which provide critical habitats for many marine species, are at risk of erosion and collapse as ocean acidification weakens their structures. The loss of coral reefs would have cascading effects throughout marine ecosystems.

Disruption of food chains

Plankton, particularly species like pteropods (tiny marine snails), are at the base of many marine food webs and are especially vulnerable to ocean acidification. Pteropods and other planktonic organisms struggle to form their shells in acidic water. As plankton populations decline, the species that depend on them for food, such as fish, whales and seabirds, face reduced food availability, disrupting entire food webs.

How ocean acidification disrupts food chains. Plankton (accent) are at the base of many marine food webs.

Behavioural changes in marine life

Ocean acidification can also affect the behaviour of marine animals. Studies show that elevated $CO X_{2}$ levels can impair the ability of fish to smell, navigate and avoid predators. Fish exposed to more acidic water may have trouble finding food, recognizing danger or navigating their environments, making them more vulnerable to predators and less successful in finding mates or reproducing.

Worked example: predicting the effect of rising atmospheric

CO X_{2}

If atmospheric

CO X_{2}

keeps rising, predict the change in each quantity below and give the reason.

Quantity	Change	Reason
$CO X_{2}$ dissolved in the ocean	Increases	The ocean is a carbon sink and absorbs about 30% of excess $CO X_{2}$
Hydrogen ions, $H X^{+}$	Increases	$CO X_{2} + H X_{2} O H X_{2} CO X_{3}$ then $H X_{2} CO X_{3} HCO X_{3} X^{-} + H X^{+}$
pH of seawater	Falls	More $H X^{+}$ means the water is more acidic
Carbonate ions, $CO X_{3} X^{2 -}$	Decrease in availability	Acidification reduces the carbonate ions available for calcium carbonate
Shells of oysters and pteropods	Thinner and weaker	Not enough carbonate to build and maintain them
Food available to fish, whales and seabirds	Falls	Pteropods and other plankton decline at the base of the food web

The reasoning chain is: more $CO X_{2}$ , more $H X^{+}$ , lower pH, fewer carbonate ions, weaker shells, fewer plankton, hungrier predators.

13.3Species extinction due to climate changeOften examined

Extinction: The complete disappearance of a species. In this chapter, extinction is caused when climate change pushes species beyond their ability to adapt.

Climate change contributes to species extinction by drastically altering the habitats and ecosystems that species depend on for survival. Rapid changes in temperature, precipitation and the frequency of extreme weather events caused by climate change can push species beyond their ability to adapt.

Climate change alters temperature, precipitation and the frequency of extreme weather.
The habitat and ecosystem that a species depends on change quickly.
The species cannot adapt, migrate or evolve fast enough.
Its numbers fall until it becomes endangered and may finally go extinct.

The chain from climate change to extinction. The final box (accent) is the outcome for a species that cannot keep pace with the change.

13.3.1Species that have gone extinct due to climate changeOften examined

Many species have gone extinct due to climate change. Some of them are listed below (Figure 13.13).

Figure 13.13: Species that have gone extinct due to climate change
Species	Where it lived	What happened
Bramble Cay melomys (Melomys rubicola)	The tiny island of Bramble Cay off the coast of Australia	A small rodent, considered the first mammal to go extinct due to climate change. Rising sea levels and storm surges wiped out its habitat.
Golden toad (Incilius periglenes)	Costa Rica's Monteverde Cloud Forest	Believed to have gone extinct in the late 1980s due to climate change-induced drought and shifts in temperature. The changing climate made the forest less hospitable, and disease further decimated the population.
Pinta Island tortoise (Chelonoidis abingdonii)	Galápagos Islands	The last individual, known as Lonesome George, died in 2012. Habitat loss and human activity contributed, but climate change played a role by disrupting the ecosystem, particularly the food and water availability for the tortoises.
Atelopus frogs (several species; harlequin frogs)	Highland areas	Many species have gone extinct in recent decades, largely due to the spread of the chytrid fungus, a pathogenic fungus that affects amphibians worldwide. Climate change promotes it: rising temperatures and changes in humidity have helped the fungus spread, leading to the collapse of amphibian populations.

13.3.2Species that have gone endangered due to climate change

While several species have gone extinct, many species have become endangered due to the impacts of climate change. This part of the book is marked as extra reading material. Notable examples (Figure 13.14) are given below.

Figure 13.14: Species that have gone endangered due to climate change
Species	How climate change threatens it
Polar bear (Ursus maritimus)	Relies on sea ice to hunt for seals. As Arctic ice melts, the hunting grounds shrink.
Adélie penguin (Pygoscelis adeliae)	Depends on Antarctic sea ice for breeding and feeding. Warming and changing ice patterns reduce its habitat.
Koala (Phascolarctos cinereus)	Increased heatwaves and bushfires, made worse by climate change, along with habitat loss, push koalas closer to endangerment.
Snow leopard (Panthera uncia)	Rising temperatures affect the mountain ecosystems it inhabits, shrinking its prey base and increasing human-wildlife conflict.
Coral species (for example staghorn coral)	Coral reefs are devastated by ocean warming and acidification, leading to large-scale coral bleaching events.
Monarch butterfly (Danaus plexippus)	Climate change alters its migration patterns and reduces the availability of milkweed, its primary food source.
Atlantic cod (Gadus morhua)	Rising ocean temperatures disrupt breeding grounds and food availability, causing population declines.

These species are just a few of the many that are struggling to adapt to the rapidly changing climate.

Extinct and endangered species
Point	Extinct species	Endangered species
Status	No living individuals remain	Still alive, but threatened with extinction
Examples	Bramble Cay melomys, golden toad, Pinta Island tortoise, Atelopus frogs	Polar bear, Adélie penguin, koala, snow leopard, staghorn coral, monarch butterfly, Atlantic cod

Key terms

Weather: Short-term changes in environmental conditions such as temperature, atmospheric water vapour, precipitation, daylight and water availability.
Climate: The overall pattern of weather in an area that prevails over a period of a few decades to even centuries.
Climate change: Long-term, gradual and apparently irreversible changes in the climatic conditions of an area that may affect the whole world.
Anthropogenic: Human-caused; refers to climate change happening rapidly over the last century after the Industrial Revolution.
Milankovitch cycles: Long-term changes in the Earth's orbit and axial tilt that affect the amount of sunlight the planet receives.
Eccentricity: The change in the shape of the Earth's orbit around the Sun between more circular and more elliptical, over about 100,000 years.
Axial tilt (obliquity): The change in the angle of the Earth's tilt on its axis between 22.1^\circ and 24.5^\circ, over about 41,000 years.
Precession: The wobble or change in direction of the Earth's axis of rotation, like a spinning top, over about 26,000 years.
El Niño-Southern Oscillation (ENSO): The climate pattern whose opposite phases are El Niño and La Niña.
Environmental buffers: Trees, which absorb \ce{CO2} through photosynthesis and help regulate the Earth's climate.
Deforestation: The cutting of trees by man for various purposes.
Urbanization: The expansion of cities (urban development).
Greenhouse gases: Gases such as carbon dioxide, methane and nitrous oxide that trap heat in the atmosphere.
Urban heat islands: Cities that absorb more heat because natural vegetation has been reduced.
Aerosols: Human-made particles, for example from industrial pollution, that can cool (reflect sunlight) or warm (black carbon) the climate.
Biodiversity: The flora and fauna of an area together.
Flora: The overall plants of an area.
Fauna: The overall animals of an area.
Growing season: The time of year when plants can grow because of favourable temperature conditions.
Invasive species: Species that move into areas where they do not naturally belong and outcompete native species for resources.
Global warming: The warming of the Earth driven mainly by the build-up of greenhouse gases in the atmosphere.
Thermal expansion: The expansion of warmer water as its molecules move further apart, which contributes to rising sea levels.
Heat capacity: The ability to absorb and store large amounts of heat without a dramatic rise in temperature; the oceans have a high heat capacity.
Ocean circulation: The movement of ocean water vertically or horizontally.
Gulf Stream: A powerful warm ocean current that starts in the Gulf of Mexico and crosses the Atlantic toward Europe.
Atlantic Meridional Overturning Circulation (AMOC): A current carrying warm surface water from the tropics to the North Atlantic, where it cools and sinks and returns as cold, deep water.
El Niño: Warm surface waters shift eastward toward South America, giving warmer ocean temperatures in the central and eastern Pacific.
La Niña: The opposite of El Niño: cooler waters dominate the Pacific.
Feedback loop: A process in which a change causes further change of the same kind, for example melting ice leading to more warming.
Committed warming: The continued warming of the planet for centuries from heat already absorbed by the oceans, even if emissions stopped.
Carbon sink: Something that absorbs large quantities of \ce{CO2} from the atmosphere, such as the ocean or forests.
Ocean acidification: The long-term lowering of ocean pH because of absorption of atmospheric \ce{CO2}.
Carbonic acid: \ce{H2CO3}, formed when \ce{CO2} dissolves in seawater; it dissociates into bicarbonate and hydrogen ions.
Bicarbonate ion: \ce{HCO3-}, formed when carbonic acid dissociates.
Zooxanthellae: Symbiotic algae living in coral tissues that provide the coral with nutrients.
Coral bleaching: Corals expelling their symbiotic algae so that they turn white or pale.
Marine heatwaves: Periods of extreme ocean heat that cause mass die-offs of marine species.
Hypoxic zones: Ocean areas of low oxygen concentration.
Anoxic zones: Dead zones where oxygen is so depleted that marine life cannot survive.
Ocean deoxygenation: The fall in dissolved oxygen in warmer ocean water.
Cold-blooded: Having a body temperature influenced by the surrounding temperature, as in many fish.
Krill: An important food source for whales, seals and penguins; genus Euphausia, a crustacean arthropod.
Calcifying organisms: Organisms such as corals, shellfish, sea urchins and plankton that use calcium carbonate to build skeletons and shells.
Pteropods: Tiny marine snails, plankton at the base of many marine food webs.
IPCC: Intergovernmental Panel on Climate Change, established in 1988 by UNEP and WMO to assess scientific research on climate change.
Extinction: The complete disappearance of a species.
Endangered species: A species threatened by the impacts of climate change that is close to going extinct.
Chytrid fungus: A pathogenic fungus that affects amphibians worldwide and whose spread is promoted by climate change.

Multiple choice questionsp. 258–259

What is the primary difference between weather and climate?

AWeather is long-term environment and climate is short-term environment.
BWeather is short-term changes, while climate is the long-term pattern of weather.Answer
CWeather affects larger areas, while climate affects smaller regions.
DClimate changes daily, while weather remains constant.

Why

Weather is the short-term change in conditions such as temperature, rainfall and daylight. Climate is the overall pattern of weather in an area that prevails over a few decades to even centuries. Option A reverses the two terms, and option D does the same: weather changes daily while climate stays fairly constant. Option C is wrong because the difference is the time scale, not the size of the area.

Which of the following is an example of a natural factor that causes climate change?

ADeforestation
BBurning of fossil fuels
CVolcanic activityAnswer
DUrbanization

Why

Volcanic eruptions are a natural factor: they release dust, ash, sulfur dioxide and \ce{CO2} into the atmosphere. Deforestation, burning of fossil fuels and urbanization are anthropogenic (human-caused) factors, so options A, B and D are wrong even though they do cause climate change.

How does ocean acidification relate to climate change?

AIt is caused by an increase in solar radiation.
BIt results from increased levels of carbon dioxide in the atmosphere.Answer
CIt occurs due to the natural release of methane.
DIt is caused by fluctuations in the Earth's orbit.

Why

The ocean absorbs extra \ce{CO2} from the atmosphere. The \ce{CO2} forms carbonic acid, which releases \ce{H+} ions and lowers the pH of the seawater. Option A (solar radiation) and option D (changes in the Earth's orbit) are natural climate drivers, but they do not lower ocean pH. Option C is wrong because methane is a greenhouse gas, not the cause of ocean acidification.

Which of the following best describes the term anthropogenic climate change?

AClimate change caused by natural factors such as volcanic eruptions.
BClimate change that occurs gradually over millions of years.
CClimate change results from human activities like deforestation and industrialization.Answer
DClimate change due to changes in the Earth's orbit.

Why

Anthropogenic means human-caused. Option C matches this definition, and deforestation and industrialization are the human activities named in the chapter. Option A describes natural climate change, option B describes slow natural change over long periods, and option D describes Milankovitch cycles, which are natural.

How do Milankovitch cycles affect Earth's climate?

ABy increasing volcanic activity.
BBy changing the Earth's orbit and tilt, affecting the sunlight received.Answer
CBy reducing the amount of greenhouse gases.
DBy causing ocean currents to circulate heat.

Why

Milankovitch cycles are slow, periodic changes in the shape of the Earth's orbit (eccentricity), the tilt of its axis (obliquity) and the direction of its axis (precession). Together they change the amount of solar energy that reaches the Earth. Option A and option C have nothing to do with orbital changes. Option D describes ocean circulation, which is a separate factor.

Which of the following impacts of climate change affects animals' migration patterns?

AIncreased volcanic eruptions
BOcean acidification
CShifting habitats due to temperature changesAnswer
DUrbanization

Why

As temperatures change, animals must move to new regions to find a suitable climate, so their migration patterns shift. Birds that once migrated to warmer regions are an example. Ocean acidification affects the shells and skeletons of marine organisms, not migration. Volcanic eruptions are a natural factor, and urbanization is a human cause of climate change, not an impact of it.

What happens to the ecosystem balance when a species goes extinct due to climate change?

AThe balance remains unaffected.
BIt strengthens the remaining species.
CIt disrupts the entire ecosystem balance.Answer
DIt causes new species to evolve immediately.

Why

Flora and fauna form an interconnected web. When a species disappears, the species that feed on it lose food, and the species it feeds on can grow unchecked, so the whole ecosystem is disturbed. Option A and option B are wrong because removing one link changes the rest of the web. Option D is wrong because evolution takes many generations, so new species do not appear immediately.

Why is deforestation a major contributor to climate change?

AIt releases sulfur dioxide, leading to global cooling.
BIt increases the production of solar radiation.
CIt reduces the Earth's capacity to absorb CO2.Answer
DIt has no impact on climate change.

Why

Trees absorb \ce{CO2} through photosynthesis and act as a carbon sink. When forests are cut down this sink is lost, and burning the trees releases the carbon they have stored. Option A is tempting because sulfur dioxide from volcanoes does cool the Earth, but deforestation does not release sulfur dioxide as its main effect. Trees do not produce solar radiation (option B), and option D is false.

Which of the following factors directly contributes to the rise in ocean temperatures due to climate change?

AIncreased ocean salinity
BMelting glaciers releasing fresh water
CEnhanced greenhouse effect trapping heat in the atmosphereAnswer
DReduced marine biodiversity

Why

Greenhouse gases trap heat in the atmosphere, and the oceans absorb much of this excess heat, so their temperature rises. Melting glaciers (option B) is tempting because it affects ocean circulation, but it is a result of warming and does not add heat directly. Fresh water lowers salinity rather than raising it (option A), and reduced biodiversity is an effect of warm oceans, not a cause (option D).

Why does the ocean have a high heat capacity, affecting the global climate?

AIt reflects sunlight back into space.
BIt can store large amounts of heat without a significant temperature increase.Answer
CIt has a high salt content.
DIt has a lower density than freshwater.

Why

Heat capacity is the ability to absorb heat with only a small rise in temperature. Because the ocean can store large amounts of heat this way, it takes up much of the excess heat from the atmosphere. Reflecting sunlight (option A) is a property of sea ice, not of ocean water. Salt content (option C) is not the reason for heat capacity, and option D is false because seawater is denser than freshwater.

What role does the Atlantic Meridional Overturning Circulation (AMOC) play in ocean temperature regulation?

AIt reduces ocean acidity.
BIt transfers warm surface water to the North Atlantic and returns cold water to the tropics.Answer
CIt blocks the flow of ocean currents towards Europe.
DIt increases the salinity of tropical waters.

Why

The AMOC carries warm surface water from the tropics to the North Atlantic. There it cools and sinks, and it flows back toward the equator as cold, deep water. Option A is unrelated to acidity. Option C is wrong because the AMOC is a current that carries heat, not a barrier. Option D is wrong because the AMOC moves heat; melting ice, not the AMOC, lowers salinity in the North Atlantic.

What is coral bleaching, and why does it occur?

AIt is the whitening of coral due to loss of algae, caused by higher ocean temperatures.Answer
BIt is the natural growth process of coral reefs in warmer waters.
CIt is the formation of new coral structures due to ocean acidification.
DIt is the process of corals adapting to cooler ocean temperatures.

Why

When ocean temperature rises, corals are stressed and expel the symbiotic algae (zooxanthellae) that live in their tissues. The coral loses its algae, turns white or pale, and its white calcium carbonate skeleton shows. Options B and D describe growth and adaptation, but bleaching is a harmful stress response. Option C is wrong because acidification weakens coral skeletons rather than forming new structures.

Which oceanic region is more susceptible to ocean acidification?

AEquatorial regions
BArctic and Southern OceansAnswer
CSubtropical regions
DMediterranean Sea

Why

Cold water absorbs more \ce{CO2} than warm water, so the polar oceans, the Arctic and the Southern Oceans, are acidifying faster. Equatorial and subtropical waters are warm and absorb less \ce{CO2}, so options A and C are wrong. The chapter does not discuss the Mediterranean Sea, so option D is not supported by the text.

Which phenomenon is characterized by a significant increase in ocean temperatures due to shifts in warm water toward the coast of South America?

ALa Niña
BGulf Stream strengthening
COcean deoxygenation
DEl NiñoAnswer

Why

In El Niño, warm surface water shifts eastward toward the coast of South America, so the central and eastern Pacific warms. La Niña (option A) is the opposite cool phase. The Gulf Stream (option B) flows in the North Atlantic and is not part of this pattern. Ocean deoxygenation (option C) is the loss of oxygen from warm water, not a shift of warm water.

Which of the following species is considered the first mammal to go extinct directly due to climate change?

APinta Island Tortoise
BGolden Toad
CBramble Cay MelomysAnswer
DAdélie Penguin

Why

The Bramble Cay melomys (Melomys rubicola) is a small rodent. It is considered the first mammal to go extinct due to climate change, after rising sea levels and storm surges destroyed its island habitat. The Pinta Island tortoise is a reptile and the golden toad is an amphibian, so options A and B are not mammals. The Adélie penguin is a bird, and it is endangered rather than extinct (option D).

Short answer questionsp. 259–260

13 marks

How does climate change impact ocean temperature?

Solution

Climate change raises ocean temperature in three main ways.

Greenhouse gases: the enhanced greenhouse effect traps heat, and the oceans absorb much of this excess heat, so their temperature rises.
Changed circulation: currents such as the Gulf Stream, the AMOC, El Niño and La Niña spread heat around the world. Climate change weakens or shifts these currents, so some regions gain heat faster.
Loss of sea ice: sea ice reflects sunlight, but when it melts the darker water exposed absorbs more heat, which further warms the ocean.

23 marks

What is the role of the ocean's high heat capacity in climate change?

Solution

The ocean's high heat capacity makes it a major store of heat in the climate system.

It can absorb and store vast amounts of heat from the atmosphere without a dramatic rise in its own temperature. With its large surface area, the ocean takes up most of the excess atmospheric heat.
The stored heat still raises the ocean temperature over time, because heat builds up faster than it can be released.
Heat already stored in the ocean keeps the planet warming for centuries, even if greenhouse gas emissions stop (committed warming), and warm water expands, which contributes to rising sea levels.

32026 Annual I, Set B, Q.2 (xii) OR2026 Annual I, Set D, Q.2 (xii) OR3 marks

Name three animal species that went extinct due to climate change.

Solution

Three species that went extinct because of climate change are:

Bramble Cay melomys (Melomys rubicola): a small rodent of Bramble Cay, an island off Australia. Rising sea levels and storm surges destroyed its habitat.
Golden toad (Incilius periglenes): once found in Costa Rica's Monteverde Cloud Forest. Climate-induced drought and shifts in temperature made the forest less hospitable, and it was last seen in the late 1980s.
Pinta Island tortoise (Chelonoidis abingdonii): the last individual died in 2012. Climate change disrupted the ecosystem of the Galápagos Islands, affecting its food and water supply.

43 marks

What phenomenon results in extreme ocean heat events, like heatwaves on land?

Solution

The phenomenon is the marine heatwave.

A marine heatwave is a period of extreme heat in the ocean, in the same way that heatwaves occur on land.
Climate change has intensified these events, because the ocean is already warmer as greenhouse gases build up in the atmosphere.
Marine heatwaves cause mass die-offs of fish, shellfish and marine mammals, and they damage the food webs that these animals belong to.

53 marks

How does ocean acidification occur?

Solution

Ocean acidification happens in three steps:

Carbon dioxide from the atmosphere dissolves in seawater and reacts with water to form carbonic acid: \ce{CO2 + H2O -> H2CO3}.
Carbonic acid is unstable and breaks down into bicarbonate ions and hydrogen ions: \ce{H2CO3 -> HCO3- + H+}.
The extra \ce{H+} ions lower the pH of seawater, so the ocean becomes more acidic. This long-term lowering of pH is ocean acidification.

63 marks

What is coral bleaching?

Solution

Coral bleaching is the loss of symbiotic algae from coral tissue.

Corals live with symbiotic algae (zooxanthellae) in their tissues, which provide the coral with nutrients.
When ocean temperature rises, the coral is stressed and expels the algae. The coral turns completely white or pale, because its white calcium carbonate skeleton shows through.
Bleached corals are not dead, but they are vulnerable to disease and cannot grow or reproduce efficiently. Prolonged exposure to high temperature leads to coral death.

73 marks

Which ocean current plays a critical role in regulating the climate of the North Atlantic region?

Solution

The Gulf Stream plays this role.

The Gulf Stream is a powerful, warm ocean current that starts in the Gulf of Mexico.
It flows along the east coast of the United States and across the Atlantic Ocean toward Europe, carrying heat northwards.
This heat strongly affects the weather patterns and climate of North America and Western Europe, which is why the current regulates the climate of the North Atlantic region.

83 marks

Why are polar bears threatened by climate change?

Solution

Polar bears depend on sea ice, and climate change is melting it.

Polar bears rely on sea ice to hunt seals.
As Arctic ice melts because of rising temperatures, their hunting grounds shrink and they struggle to find food.
Hunger, habitat loss and the need to migrate or adapt to unfamiliar environments lead to population decline, so the species is threatened.

93 marks

What impact does warmer ocean water have on oxygen levels?

Solution

Warmer ocean water causes ocean deoxygenation.

Warmer water holds less dissolved oxygen than cold water.
Areas of low oxygen (hypoxic zones) and areas with no oxygen (anoxic zones) develop, where marine life cannot survive.
Fish and crabs die or move to other regions, which disrupts marine biodiversity and can lead to declines in fisheries.

103 marks

How has climate change contributed to the spread of the chytrid fungus in amphibians?

Solution

Climate change has helped the chytrid fungus spread among amphibians.

The chytrid fungus is a pathogenic fungus that affects amphibians worldwide.
Rising temperatures and changes in humidity have helped the fungus spread into highland areas where it had not reached before.
The spread of the fungus leads to the collapse of amphibian populations, as happened to many species of Atelopus (harlequin) frogs, which have gone extinct in recent decades.

113 marks

What is the primary factor causing ocean warming?

Solution

The primary factor is the build-up of greenhouse gases from human activities.

Human activities such as burning fossil fuels, deforestation and industrial processes increase the concentration of greenhouse gases (\ce{CO2}, methane and nitrous oxide) in the atmosphere.
These gases trap heat that would otherwise escape back into space, which is the enhanced greenhouse effect and causes global warming.
The oceans absorb much of this excess heat, so their temperature rises.

123 marks

Which species went extinct in Costa Rica due to climate change-induced drought?

Solution

The species is the golden toad (Incilius periglenes).

It was once found in Costa Rica's Monteverde Cloud Forest.
Climate-induced drought and shifts in temperature made the forest less hospitable in the late 1980s, and the species became extinct.
Disease further decimated the remaining population, so climate change was the main cause of the extinction.

133 marks

What is the relationship between sea ice and ocean temperature?

Solution

Sea ice and ocean temperature affect each other in a feedback loop.

Sea ice reflects sunlight back into space, which helps keep the ocean cool.
As ocean temperature rises, sea ice and glaciers at the poles melt at an accelerated rate.
When the ice melts, the darker ocean water is exposed. It absorbs more heat, which accelerates ocean warming, so the warming feeds back on itself.

143 marks

How does ocean acidification affect calcifying organisms?

Solution

Ocean acidification makes it harder for calcifying organisms to build their skeletons and shells.

Corals, shellfish such as oysters, sea urchins and plankton build their skeletons and shells from calcium carbonate.
Lower pH reduces the availability of carbonate ions, so these organisms struggle to form and maintain their shells and skeletons.
Oyster larvae struggle to form shells, which causes high mortality, coral reefs are at risk of erosion and collapse, and plankton such as pteropods decline, which disrupts the food webs that depend on them.

153 marks

Differentiate between weather and climate.

Solution

Weather and climate differ mainly in their time scale.

Feature	Weather	Climate
Time period	Short term: conditions over hours, days or weeks	Long term: a pattern over a few decades to even centuries
What it describes	Temperature, water vapour, precipitation, daylight and water availability at a given time	The overall pattern of weather that prevails in an area
Variation	Changes from day to day	Changes slowly and is fairly stable over time

For example, a cloudy, rainy day is weather, while a desert with a hot, dry climate is the long-term pattern of weather in that region.

Extensive answer questionsp. 260

16 marks

Explain how global warming and greenhouse gases contribute to rising ocean temperatures.

Solution

Principle

Greenhouse gases trap heat in the atmosphere, which causes global warming. The oceans absorb much of the extra heat, so their temperature rises.

Main greenhouse gases

Gas	Main human sources
Carbon dioxide (\ce{CO2})	Burning fossil fuels, deforestation, cement production and industrial processes
Methane (\ce{CH4})	Livestock such as cows and sheep, rice paddies and landfill waste
Nitrous oxide (\ce{N2O})	Fertilizers used in agriculture

Steps

Human activities add greenhouse gases to the atmosphere, so their concentration increases.
The gases trap outgoing heat that would otherwise escape back into space. This is the enhanced greenhouse effect, and the atmosphere warms.
The oceans absorb much of this excess heat because of their large surface area and high heat capacity. About 90% of the atmospheric heat is taken up by the oceans.
The water heats up, so the ocean temperature rises. Warm water also expands (thermal expansion), which adds to rising sea levels.
Warmer water holds less dissolved \ce{CO2}, so more \ce{CO2} stays in the atmosphere, which traps even more heat.

Figure

Greenhouse gases warm the ocean in a sequence of steps.

26 marks

Describe the process of ocean acidification and its impact on marine life.

Solution

Process

Carbon dioxide from the atmosphere dissolves in seawater, where it reacts with water to form carbonic acid.
\ce{CO2 + H2O -> H2CO3}
Carbonic acid is unstable and dissociates into bicarbonate ions and hydrogen ions.
\ce{H2CO3 -> HCO3- + H+}
The extra hydrogen ions lower the pH, so the ocean becomes more acidic. This long-term lowering of pH is ocean acidification.
Lower pH reduces the availability of carbonate ions, which marine organisms need to build calcium carbonate skeletons and shells.

Ocean acidification, from dissolved carbon dioxide to weaker shells and skeletons.

Impact on marine life

Calcifying organisms such as corals, oysters, sea urchins and plankton find it hard to build and maintain their shells and skeletons. Oyster larvae struggle to form shells, which leads to high mortality in oyster populations.
Coral reefs are at risk of erosion and collapse as their structures weaken. Reefs provide critical habitats, so their loss has cascading effects on marine ecosystems.
Plankton such as pteropods are at the base of many food webs. As they decline, the fish, whales and seabirds that eat them face reduced food, which disrupts the whole food web.
Higher \ce{CO2} levels can impair the ability of fish to smell, navigate and avoid predators, so they become more vulnerable and less successful at finding food or mates.

36 marks

How do El Niño and La Niña events influence ocean temperatures and global climate patterns?

Solution

What the events are

El Niño and La Niña are opposite phases of a pattern of changes in ocean circulation in the Pacific Ocean, known together as the El Niño-Southern Oscillation (ENSO). Both can last for several months and affect weather, agriculture and ecosystems across the world.

Comparison

Feature	El Niño (warm phase)	La Niña (cold phase)
Ocean surface temperature	Warmer in the central and eastern Pacific	Cooler sea surface temperature
Movement of surface water	Warm surface water shifts eastward toward South America	Cooler waters dominate the Pacific
Effect on rainfall and weather	Wetter and warmer in some regions, drought in others	More rainfall in some areas such as Southeast Asia, drier and cooler in others such as South America
Effect on global ocean temperature	Strong events often cause a significant increase in global ocean temperatures	Cooler ocean conditions in the Pacific

Figure

The two phases of the Pacific pattern, with the direction of the warm water.

Influence on climate

El Niño brings warmer ocean temperatures in the central and eastern Pacific, which leads to wetter and warmer conditions in some regions and drought in others.
The later strong El Niño events often cause a significant increase in global ocean temperatures.
La Niña brings cooler ocean temperatures, so the regional rainfall and drought patterns it causes are the opposite of those produced by El Niño.

46 marks

What role does the Atlantic Meridional Overturning Circulation (AMOC) play in ocean heat distribution, and how is it affected by climate change?

Solution

What the AMOC is

The AMOC is a type of ocean current. It carries warm surface water from the tropics to the North Atlantic, and it returns cold deep water back toward the equator.

Role in heat distribution

Warm surface water flows from the tropics toward the North Atlantic, carrying heat northwards.
In the North Atlantic the water cools and sinks, releasing its heat to the region.
The cold, deep water flows back toward the equator, which completes the loop and keeps heat spread across the oceans.

Effect of climate change

As temperatures rise, polar ice melts and adds large amounts of fresh water to the North Atlantic.
The fresh water lowers the salinity of the sea water, so it becomes less dense.
The less dense water cannot sink as usual, so the AMOC slows down.
Less heat is transported toward the poles, so the tropical and subtropical oceans become warmer, and the warming disrupts weather patterns globally.

The AMOC loop, with the sinking in the North Atlantic highlighted.

56 marks

Discuss the impact of high ocean temperatures on coral reefs and marine biodiversity.

Solution

Coral bleaching

Corals live with symbiotic algae (zooxanthellae) in their tissues. The algae provide nutrients, and the coral provides a home.
High ocean temperature stresses the coral, which expels the algae.
Without the algae the coral turns white, because its white calcium carbonate skeleton shows through. The coral is not yet dead, but it is vulnerable to disease and cannot grow or reproduce efficiently.
Prolonged exposure to high temperature leads to coral death, and the reef habitat is lost.

Healthy coral (left) and bleached coral (right) in cross-section.

Effects on marine biodiversity

Effect of high temperature	Result for marine life
Loss of coral reefs	Destruction of habitats for many marine species, and loss of coastal protection and fisheries
Changes in fish growth and reproduction	Cold-blooded fish grow smaller and may produce fewer offspring
Marine heatwaves	Mass die-offs of fish, shellfish and marine mammals
Ocean deoxygenation	Hypoxic and anoxic zones, where fish and crabs die or move away
Temperature-sensitive species	Fish and plankton move to cooler waters, which shifts the food chain

66 marks

How does climate change contribute to species extinction, and provide an example?

Solution

How climate change causes extinction

Habitat loss: rising temperatures destroy the habitats species depend on, such as the sea ice used by polar bears or the island habitat of the Bramble Cay melomys.
Shifting habitats: animals must move to new regions with suitable climates, and species that cannot move fast enough may disappear.
Food chain disruption: if one species disappears, the species that depend on it lose their food.
Mismatched cycles: breeding, hibernation and migration may no longer match the food supply.
Disease and invasive species: climate change helps some pathogens, such as the chytrid fungus, and invasive species spread into new areas.
Failure to adapt: many species cannot adapt quickly enough to the speed of climate change.

Food chain disruption

How the loss of one plant species can reach the top of a food chain.

Example: the golden toad

The golden toad lived in Costa Rica's Monteverde Cloud Forest.
Climate-induced drought and shifts in temperature made the forest less hospitable.
Disease further decimated the population.
The species was last seen in the late 1980s and is now considered extinct.

76 marks

Describe how climate change has affected polar ecosystems and the species that inhabit them.

Solution

Changes in polar ecosystems

Polar regions are warming, and sea ice and glaciers are melting at an accelerated rate. Sea ice is a reflective shield, so its loss turns the ocean into a heat absorber, and the warming becomes self-reinforcing.

The sea ice feedback loop in the polar regions.

Species that inhabit polar regions

Species	How climate change affects it
Polar bears (Ursus maritimus)	They hunt seals from sea ice, so shrinking ice reduces their hunting grounds and food supply
Adélie penguins (Pygoscelis adeliae)	They depend on Antarctic sea ice for breeding and feeding, and that habitat is shrinking
Seals and walruses	They depend on sea ice for hunting and breeding, so habitat loss forces them to migrate or adapt
Antarctic krill (Euphausia)	The loss of sea ice disrupts the food chains that krill support, and krill are an important food for whales, seals and penguins

86 marks

Explain the concept of marine heatwaves and their impact on ocean ecosystems.

Solution

What a marine heatwave is

Just like heatwaves on land, the ocean experiences periods of extreme heat called marine heatwaves. These events have intensified with climate change.

Cause

Greenhouse gases trap heat, and the oceans absorb much of the excess heat, so the water is already warmer.
On top of this warm background, extreme periods of heat occur more often and more intensely.

Impact on ocean ecosystems

Steps from a warmer ocean to damage in marine ecosystems.

Fish, shellfish and marine mammals die in large numbers.
Plankton and other food species decline, so the predators that depend on them lose food.
Coral reefs bleach and may die, which removes habitat for many marine species.

96 marks

What are the challenges faced by species that become endangered due to climate change, such as the snow leopard?

Solution

The snow leopard

Rising temperatures are affecting the mountain ecosystems that snow leopards (Panthera uncia) inhabit.
Their prey base shrinks, so they have less food.
Increased human-wildlife conflict puts the animals and people at risk.

Challenges shared by endangered species

Loss of habitat as the places they live in shrink or change.
Shortage of food when the prey or the plants they depend on decline.
Disrupted breeding, hibernation and migration, because the seasons no longer match the food supply.
Extreme events such as heatwaves and bushfires, which damage habitat and kill animals.
Increased conflict with humans as animals move into new areas.

Other examples

Species	Challenge caused by climate change
Koalas (Phascolarctos cinereus)	Heatwaves and bushfires, together with habitat loss
Monarch butterflies (Danaus plexippus)	Changed migration patterns and a reduced supply of milkweed, their food
Atlantic cod (Gadus morhua)	Warmer oceans disrupt breeding grounds and food availability
Staghorn coral	Large-scale bleaching events caused by warming and acidification

106 marks

How does the melting of polar ice influence global climate patterns and ocean circulation?

Solution

Process

How melting polar ice changes ocean circulation and climate, step by step.

Effects on ocean circulation

Fresh water lowers the salinity and density of North Atlantic water, so it sinks less.
The AMOC slows, so less heat is carried toward the poles.
Tropical and subtropical oceans warm because heat builds up there instead.

Effects on global climate patterns

Changes in ocean heat distribution disrupt weather patterns across the world.
Melting sea ice exposes dark ocean water, which absorbs more heat and speeds up global warming.
Melted ice adds water to the oceans, which contributes to rising sea levels.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

MCQ 122026 Annual I, Set B, MCQ 121 mark

Which organisms are most affected by ocean acidification?

AMarine Mammals
BFish
CBirds
DShell-forming organismsAnswer

Why

Ocean acidification lowers the pH of seawater and reduces the carbonate ions that calcifying organisms need to make calcium carbonate. Shell-forming organisms (corals, oysters, clams, sea urchins and plankton such as pteropods) therefore find it hard to build and keep their shells and skeletons.

Fish are affected mainly in behaviour and through loss of food, and marine mammals and birds only suffer indirectly through the food web. They are not the group that is most affected.

Q.2 (x)2026 Annual I, Set B, Q.2 (x)1x3 = 3 marks

State any three harmful effects of climate change on ocean biology.

Solution

Any three of the following harmful effects, one mark each.

Effect (1 mark each)	What happens
Coral bleaching	Warm water stresses the corals, so they expel their zooxanthellae and turn white; prolonged heat kills them and reef habitats are lost.
Disruption of marine ecosystems	Temperature-sensitive fish and plankton migrate to cooler waters, food chains shift, and species that cannot move may become extinct.
Harm to fish growth and reproduction	Cold-blooded fish change in metabolism, grow smaller and produce fewer offspring.
Marine heatwaves	Extreme ocean heat causes mass die-offs of fish, shellfish and marine mammals.
Melting polar ice	Polar bears, seals and walruses lose habitat, and krill, the food of whales, seals and penguins, decline.
Ocean deoxygenation	Warmer water holds less oxygen, creating hypoxic and anoxic (dead) zones where marine life dies or moves away.
Ocean acidification	Lower pH makes shell and skeleton building difficult for calcifying organisms and disturbs food chains.

Q.2 (xii) OR2026 Annual I, Set B, Q.2 (xii) OR1.5+1.5 = 3 marks

Write names and briefly discuss any two species that have gone extinct.

Solution

Extinction is the complete disappearance of a species. Any two of the species below, with the name and a short discussion, earn the marks (1.5 each).

1. Bramble Cay melomys (Melomys rubicola)

A small rodent of the tiny island of Bramble Cay off the coast of Australia. It is considered the first mammal to go extinct because of climate change: rising sea levels and storm surges wiped out its habitat.

2. Golden toad (Incilius periglenes)

It lived in Costa Rica's Monteverde Cloud Forest and is believed to have gone extinct in the late 1980s. Climate change brought drought and shifts in temperature that made the forest less hospitable, and disease further reduced the population.

Other species from the book

Pinta Island tortoise (Chelonoidis abingdonii), Galapagos Islands: the last individual, Lonesome George, died in 2012; climate change disrupted its food and water.
Atelopus frogs (harlequin frogs): many species were lost as the chytrid fungus spread with rising temperature and changing humidity.

MCQ 132026 Annual I, Set D, MCQ 131 mark

An increase in ocean temperature mainly leads to:

ACoral bleachingAnswer
BReduced sunlight
CIncreased salinity
DDecreased evaporation

Why

A rise in ocean temperature stresses corals, so they expel the symbiotic algae (zooxanthellae) that live in their tissues and turn white. This is coral bleaching.

Warming does not reduce sunlight. Salinity and evaporation are not the effects the book links to a rise in ocean temperature; the book's effects are coral bleaching, shifting ecosystems, smaller fish, marine heatwaves, melting ice and low oxygen.

Q.2 (x) OR2026 Annual I, Set D, Q.2 (x) OR1x3 = 3 marks

Highlight any three harmful effects of ocean acidification.

Solution

Ocean acidification (a fall in the pH of seawater because dissolved \ce{CO2} forms carbonic acid and releases \ce{H+}) harms marine life in three main ways, one mark each.

Calcifying organisms are weakened. Lower pH reduces the carbonate ions (\ce{CO3^2-}) needed to build calcium carbonate, so corals, oysters, clams and sea urchins make thin, weak shells and skeletons. Oyster larvae die in large numbers and coral reefs risk erosion and collapse.
Food chains are disrupted. Plankton such as pteropods cannot form shells properly and decline. Fish, whales and seabirds that feed on them then have less food.
Behaviour of marine animals changes. High \ce{CO2} impairs the ability of fish to smell, navigate and avoid predators, so they find less food and are less successful in finding mates and reproducing.

Q.2 (xii) OR2026 Annual I, Set D, Q.2 (xii) OR1.5+1.5 = 3 marks

Write names and briefly discuss any two species that have gone extinct.

Solution

Extinction is the complete disappearance of a species. Any two species with a short discussion earn the marks (1.5 each).

1. Golden toad (Incilius periglenes)

It lived in Costa Rica's Monteverde Cloud Forest. It is believed to have gone extinct in the late 1980s because climate change brought drought and shifts in temperature that made the forest less hospitable, and disease further decimated the population.

2. Bramble Cay melomys (Melomys rubicola)

A small rodent of Bramble Cay, a tiny island off the coast of Australia, and the first mammal considered to go extinct due to climate change. Rising sea levels and storm surges wiped out its habitat.

Q.3 (a)2026 Annual I, Set D, Q.3 (a)3 marks

Discuss any three impacts of climate change on plants.

Solution

Plants depend on suitable temperature, rainfall and soil quality. Climate change alters these conditions. Any three impacts, one mark each.

Impact (1 mark each)	Explanation
Changes in growing seasons	Growing seasons become longer in some regions and shorter in others. This changes when plants flower, produce fruit or go dormant.
Stress from extreme weather	More frequent and intense storms, floods and droughts. Drought dries the soil so plants cannot get enough water; floods waterlog the roots.
Migration of plants	Plants cannot move, but seeds spread by wind, water or animals to new regions better suited to the changed climate, while the plants may disappear from their old habitats.

Q.3 (b)2026 Annual I, Set D, Q.3 (b)4 marks

How are animals affected by climate change? (Four points)

Solution

Animals rely on stable ecosystems for food, shelter and mates, and climate change disrupts these. Any four points, one mark each.

Point (1 mark each)	How animals are affected
Loss of habitat	Rising temperature melts sea ice; polar bears need it to hunt seals and struggle to find food.
Shifting habitats	Animals move to new regions to find a suitable climate; birds change their migration patterns.
Food chain disruption	If a plant or prey species declines, the animals that depend on it decline too (for example an insect that loses its food plant, then the animals that eat the insect).
Breeding and hibernation cycles	Cycles that depend on temperature are upset; animals may wake from hibernation too early or too late, with no food around.
Ecosystem balance and extinction	Species that cannot adapt go extinct and the balance of the interconnected web is disturbed.
Invasive species	Species that move in to escape the changing climate outcompete native species for resources.

Exam focus

Impacts of climate change on plants (growing seasons, extreme weather, migration) and on animals (four points). (long)
Set D Q.3 (a) (3 marks, plants) and Q.3 (b) (4 marks, animals) were the long questions of the unit.
Go to answer
Harmful effects of ocean acidification (calcifying organisms, food chains, behaviour). (short)
Set D Q.2 (x) OR, Set B MCQ 12 (shell-forming organisms); book long question 2 and short question 14.
Go to answer
Three harmful effects of climate change and high ocean temperature on ocean biology. (short)
Set B Q.2 (x), Set D MCQ 13 (coral bleaching); book long question 5.
Go to answer
Name and discuss two species that have gone extinct because of climate change. (short)
Repeated in both 2026 sets (Set B Q.2 (xii) OR, Set D Q.2 (xii) OR); book short question 3 and MCQ 15.
Go to answer
How does ocean acidification occur? (two equations and the fall in pH) (short)
Not asked directly in 2026, but it underlies Set B MCQ 12 and Set D Q.2 (x) OR; book short question 5 and long question 2.
Go to answer
Marine heatwaves, coral bleaching and deoxygenation. (long)
Set D MCQ 13 asked coral bleaching; book long questions 5 and 8 and short questions 4, 6 and 9.
Go to answer
Endangered species such as the snow leopard and polar bear, and why they are threatened. (long)
No paper question in 2026; book long question 9 and short question 8.
Go to answer

Weather and climateOften examined

13.1Climate change, its causes and impacts on biodiversityOften examined

13.1.1Causes of climate change: natural factorsOften examined

i. Volcanic activity

ii. Solar variability

iii. Milankovitch cycles

iv. Ocean currents

v. Natural greenhouse gas emissions

13.1.1Anthropogenic (human-caused) factors causing climate changeOften examined

i. Deforestation and urbanization

ii. Burning of fossil fuels

iii. Industrial processes and agriculture

iv. Land use changes

v. Waste management

vi. Transportation

vii. Aerosols and chemicals

13.1.2Impact of climate change on biodiversity: flora (plants)Often examined

i. Changes in growing seasons

ii. Increased stress from extreme weather

iii. Migration of plants

Activity: the impact of changes in growing conditions

13.1.2Impact of climate change on fauna (animals)Often examined

i. Loss of habitat

ii. Shifting habitats

iii. Food chain disruption

iv. Breeding and hibernation cycles

v. Ecosystem balance and extinction

vi. Invasive species

vii. Species adaptation

13.2Climate change impacts on ocean ecosystem

13.2.1Climate change impacts ocean temperatureOften examined

Global warming and the greenhouse gases

Scientific enquiry: thermal expansion

High heat capacity of the ocean

Large surface area of the oceans

13.2.1Ocean circulation and heat distributionOften examined

Gulf Stream

Reading latitude: degrees, minutes and seconds

Atlantic Meridional Overturning Circulation (AMOC)

13.2.1El Niño and La Niña eventsOften examined

13.2.1Sea ice and ocean temperatureOften examined

Critical thinking 1: is the feedback positive or negative?

Critical thinking 2: why does committed warming happen?

13.2.2Climate change impacts ocean acidification (low pH)Often examined

The ocean as a carbon sink

COX2​ dissolving in ocean water

Ocean acidification and climate change are interlinked

13.2.2Regional variations in ocean acidification

13.2.3Harmful effects of high ocean temperature on marine biologyOften examined

Coral bleaching

Disruption of marine ecosystems

Impact on fish reproduction and growth

Marine heatwaves

Melting polar ice and habitat loss

Ocean deoxygenation

13.2.3Harmful effects of ocean acidification on marine biologyOften examined

Impact on calcifying organisms

Disruption of food chains

Behavioural changes in marine life

13.3Species extinction due to climate changeOften examined

13.3.1Species that have gone extinct due to climate changeOften examined

13.3.2Species that have gone endangered due to climate change

Key terms

Multiple choice questionsp. 258–259

Short answer questionsp. 259–260

Extensive answer questionsp. 260

Principle

Main greenhouse gases

Steps

Figure

Process

Impact on marine life

What the events are

Comparison

Figure

Influence on climate

What the AMOC is

Role in heat distribution

Effect of climate change

Coral bleaching

$CO X_{2}$ dissolving in ocean water