D4.3 Climate change | IB DP Biology

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Climate change is the final topic in the syllabus and draws on almost everything before it: the carbon cycle, photosynthesis and respiration, ecosystem stability, adaptation and natural selection. The biology you are asked about is specific. It covers the anthropogenic causes, the positive feedbacks that accelerate warming, named examples of habitats and ranges changing, and what can be done to take carbon back out of the air. At HL it adds the disruption of the timing of biological events.

🎯What you need to be able to do

Explain the anthropogenic causes of climate change: increases in carbon dioxide and methane, and distinguish correlation from causation.
Explain positive feedback cycles in global warming.
Explain the change from carbon sink to carbon source in boreal forests as a tipping point.
Describe polar habitat change, using emperor penguins and walruses.
Explain how changes in ocean currents alter nutrient upwelling.
Describe poleward and upslope range shifts, using New Guinea birds and North American trees.
Explain threats to coral reefs from acidification and bleaching.
Evaluate afforestation, forest regeneration and restoration of peat-forming wetlands as ways of sequestering carbon.
AHL Explain phenology, disruption of phenological synchrony, increased insect life cycles, and evolution in response to climate change.

📚The biology

Anthropogenic causes

Carbon dioxide and methane are greenhouse gases: they allow incoming short-wave solar radiation through but absorb and re-emit longer-wave infrared radiation from the warmed Earth, trapping heat in the lower atmosphere. Human activities have greatly increased their atmospheric concentrations:

Carbon dioxide
Combustion of fossil fuels (coal, oil, natural gas) for energy, transport and industry; deforestation and burning of forests; cement production. Concentration has risen from about 280 ppm before the industrial revolution to over 420 ppm.

Methane
Livestock (from fermentation in the guts of cattle and sheep); rice paddies and wetlands, where anaerobic bacteria produce it; landfill; leaks from natural gas extraction. Methane traps far more heat per molecule than CO₂, though it lasts less long in the atmosphere.

Correlation and causation. Data from Antarctic ice cores — bubbles of ancient air trapped in the ice, and chemical indicators of past temperature — show a positive correlation between global temperature and atmospheric CO₂ concentration over hundreds of thousands of years: when one was high, so was the other. A correlation alone does not prove that CO₂ causes warming; the relationship could run the other way, or both could be caused by something else. Other evidence does confirm the causal link: the laboratory-measured ability of CO₂ to absorb infrared radiation, satellite measurements showing less infrared escaping at exactly the wavelengths CO₂ absorbs, and climate models that reproduce observed warming only when rising greenhouse gases are included.

Positive feedback cycles in global warming

Warming triggers processes that cause further warming — positive feedback — so change accelerates:

CO₂ from the deep ocean
Warmer water holds less dissolved CO₂, and warming changes ocean circulation, so the oceans absorb less and may release CO₂ stored in deep water.

Loss of reflective snow and ice
Snow and ice reflect most sunlight. As they melt, darker ocean and land beneath absorb more solar radiation, warming further and melting more ice.

Decomposition of peat and permafrost
Warmer temperatures accelerate decomposition of peat and of organic matter frozen in permafrost for thousands of years, releasing CO₂.

Methane from melting permafrost
Where thawed soils are waterlogged, anaerobic decomposition releases methane.

Droughts and forest fires
Hotter, drier conditions cause more frequent and intense droughts and fires, which kill trees and release their carbon as CO₂.

Boreal forests: a tipping point

The boreal forest (taiga) of Canada, Scandinavia and Russia stores enormous amounts of carbon in trees and soils, and has long been a net carbon sink. Climate change threatens to turn it into a net carbon source:

Warmer temperatures and decreased winter snowfall (so less meltwater in spring) lead to increased incidence of drought.
Drought stresses trees and reduces primary production; large areas show forest browning as trees die back.
Dry, dead vegetation fuels more frequent and intense forest fires.
Intense fires burn not only the trees but deep soil organic layers built up over centuries: legacy carbon combustion, releasing carbon that previous fires had left untouched.

Once photosynthesis falls below respiration and combustion over large areas, the forest releases more carbon than it takes up. This change, driven by positive feedback, may be very hard to reverse: a tipping point.

Polar habitat change

Emperor penguins (Antarctic): landfast ice
Emperor penguins breed on landfast ice, sea ice attached to the coast. They need it to stay stable from April until chicks fledge in December. Warming causes early breakout of landfast ice, before chicks have waterproof feathers; if the ice breaks up, chicks drown or freeze. Complete breeding failures have already occurred at some colonies, threatening the loss of breeding grounds.

Walruses (Arctic): sea ice
Walruses feed on shellfish on the shallow seabed and use floating sea ice as a platform to rest between dives, near their feeding areas. As summer sea ice retreats far from the shallow feeding grounds, walruses crowd onto beaches in huge numbers, must swim much further to feed, and young animals are killed in stampedes.

Ocean currents and upwelling

In some regions, currents bring cold, nutrient-rich water up from the deep ocean to the surface (upwelling). Nutrients such as nitrates and phosphates fuel phytoplankton growth, supporting some of the most productive fisheries on Earth. Climate change can alter currents and the timing and extent of upwelling. Warmer surface water is less dense and forms a stable layer that prevents mixing with the cold, nutrient-rich water below. Less nutrient reaches the surface, so primary production decreases, and less energy flows through marine food chains to fish, seabirds and marine mammals.

Poleward and upslope range shifts

As temperatures rise, species tend to shift their ranges towards cooler conditions — towards the poles or up mountains.

Tropical montane birds in New Guinea: surveys repeated decades apart on mountains in New Guinea found that many bird species had shifted their ranges upslope, living at higher elevations than before. Species already near the summits have nowhere left to go, and risk extinction as their habitat shrinks.
North American tree species: forest inventory data show that many tree species are experiencing range contraction at the southern (warmer) edge of their range, while young trees are appearing further north than older trees, indicating northward spread. Because trees disperse and grow slowly, many may not keep pace with the rate of warming.

Threats to coral reefs

Ocean acidification
The oceans absorb much of the extra CO₂, which forms carbonic acid and lowers the pH of seawater. This reduces the availability of carbonate ions, suppressing calcification: corals cannot build their calcium carbonate skeletons as fast, and existing reef structures can dissolve.

Coral bleaching
When water temperature rises even 1–2 °C above the usual summer maximum for weeks, corals expel their zooxanthellae (C4.1), losing colour and their main food source. If high temperatures persist, the corals starve and die.

Corals are the foundation of reef ecosystems, providing structure, shelter and food for a huge diversity of species. Loss of corals causes the collapse of the whole reef ecosystem, with loss of fisheries and coastal protection — a clear example of potential ecosystem collapse (B4.1).

Carbon sequestration

Carbon sequestration means removing CO₂ from the atmosphere and storing it for long periods. Three biological approaches:

Afforestation
Planting forests on land that has not recently been forest. Growing trees fix CO₂ into wood and roots for decades.

Forest regeneration
Allowing or helping forests to regrow on previously deforested land, often with native species, as in rewilding (D4.2).

Restoration of peat-forming wetlands
Blocking drainage ditches to rewet peatlands. In waterlogged, anaerobic soils, dead plant matter decomposes very slowly and accumulates as peat, storing carbon for thousands of years. Peat forms naturally in temperate and boreal zones, and very rapidly in some tropical ecosystems such as peat swamp forests of Southeast Asia.

There is active scientific debate over the best approach. Plantations of fast-growing, non-native trees (such as eucalyptus or pine) can fix carbon quickly, but they support little biodiversity, may be harvested and burned, can use large amounts of water, and are vulnerable to pests and fire. Rewilding with native species grows more slowly but produces diverse, resilient ecosystems that store carbon in soil as well as trees and support native wildlife.

Phenology AHL

Phenology is the study of the timing of biological events in relation to seasons and climate. Examples include flowering; budburst in spring and bud set in autumn in deciduous trees; bird migration and nesting; insect emergence. Organisms time these events using environmental cues, of which the most important are photoperiod (day length, which is predictable and unaffected by climate change) and temperature patterns (which are changing).

Disruption of phenological synchrony AHL

Within an ecosystem, species depend on events happening at the right time relative to each other — for example, young animals appearing when their food is most abundant. If one population uses temperature as its cue and another uses photoperiod, warming shifts the timing of one but not the other, and the events fall out of synchrony (a mismatch).

Arctic mouse-ear chickweed and reindeer. In Greenland, the spring growth of plants such as the Arctic mouse-ear chickweed (Cerastium arcticum) is triggered by temperature and has moved earlier as springs have warmed. The migration of reindeer (caribou, Rangifer tarandus) to their calving grounds is cued by photoperiod and has not shifted. Reindeer increasingly arrive after the peak of plant growth, when food quality has declined, and calf survival has fallen.
Great tits and caterpillars. In north European forests, the peak biomass of caterpillars, which hatch as oak leaves open, has moved earlier with warmer springs. Great tits (Parus major) time egg-laying partly by photoperiod, and have advanced their breeding less. Their chicks increasingly hatch after the caterpillar peak, when food for nestlings is scarcer.

More insect life cycles per year AHL

Insect development is strongly temperature-dependent. With warmer temperatures and longer summers, some insects complete more generations per year. The spruce bark beetle (Ips typographus) in Europe traditionally completed one generation a year in cooler regions; it now often completes two, and sometimes three. More generations mean much faster population growth, and outbreaks have killed vast areas of spruce forest, especially where trees are already weakened by drought — releasing carbon and changing forest ecosystems.

Evolution as a consequence of climate change AHL

Climate change is a selection pressure, and some populations are already evolving in response. Tawny owls (Strix aluco) occur in two colour variants controlled by genes: grey and brown. In Finland, grey owls are better camouflaged against snow and survive cold, snowy winters better; brown owls have lower survival in such winters. As winters have become milder with less snow cover, the survival disadvantage of brown owls has disappeared, the fitness of the brown variant has increased, and the frequency of brown owls in the population has risen over recent decades — a change in allele frequency driven by climate change (D4.1).

✏️Worked example

(a) The minimum extent of Arctic sea ice in September was about 7.5 million km² in 1980 and about 3.9 million km² in 2020. Calculate the percentage change, and explain how this change could itself contribute to further warming.
(b) AHL In a European forest, the peak of caterpillar biomass moved from day 150 of the year in 1990 to day 135 in 2020. Over the same period, the date of peak food demand by great tit nestlings moved from day 150 to day 145. Calculate the rate of change of each, in days per year, and the size of the mismatch in 2020. Explain the difference, and suggest a consequence for the great tit population.

(a)

\[ \frac{3.9 - 7.5}{7.5} \times 100 = -48\% \]

The minimum sea ice extent fell by 48%. Sea ice is highly reflective; the dark ocean water that replaces it absorbs much more solar radiation, warming the water and the air, which melts more ice — a positive feedback cycle that accelerates warming.

(b) Over 30 years:

\[ \text{caterpillars: } \frac{150 - 135}{30} = 0.5\ \text{days earlier per year} \qquad \text{great tits: } \frac{150 - 145}{30} = 0.17\ \text{days earlier per year} \]

In 1990 the two events were synchronized (both day 150). In 2020, peak demand (day 145) is 10 days after the caterpillar peak (day 135): a mismatch of 10 days.

Caterpillar emergence is cued by temperature (via oak budburst), which has risen, so it has moved much earlier. Great tits time breeding partly by photoperiod, which has not changed, so their timing has shifted less. Consequence: nestlings hatch after the peak of food, so parents find fewer caterpillars; fewer chicks survive or fledge in good condition, and the population may decline. Over time, natural selection may favour birds that breed earlier.

Check it. A loss of 3.6 out of 7.5 is just under half, so −48% is sensible. In (b), the mismatch should equal the difference between the two shifts: 15 − 5 = 10 days, which agrees with day 145 − day 135.

Stating that the great tits “have not adapted” or “failed to notice”. The birds are responding exactly to their cue; the problem is that their cue (day length) and the caterpillars’ cue (temperature) no longer change together. Always name the two different cues when explaining a phenological mismatch.

📝Practise

Work through these on paper, then reveal the answer. Questions 5 and 6 are AHL.

1. Ice core data show a positive correlation between atmospheric CO₂ and global temperature. Explain why this alone does not prove that CO₂ causes warming, and state one other type of evidence for the causal link.

A correlation shows only that the two variables change together. It does not show which causes which — warming could release CO₂ (for example from oceans), and in past cycles changes in Earth’s orbit started warming that was then amplified by CO₂ — or a third factor could affect both. Other evidence: laboratory measurements showing that CO₂ absorbs infrared radiation; satellite observations of less outgoing infrared at the wavelengths absorbed by CO₂; climate models that reproduce observed warming only when increased greenhouse gases are included.

2. Explain how melting permafrost can cause a positive feedback in global warming.

Permafrost contains large amounts of organic matter that has been frozen and undecomposed for thousands of years. As global temperatures rise, permafrost thaws, and decomposers become active and break down the organic matter, releasing carbon dioxide; in waterlogged, anaerobic conditions they release methane. Both are greenhouse gases that increase warming, which thaws more permafrost, releasing more gas — so the effect reinforces itself.

3. Explain how warming could change boreal forests from carbon sinks to carbon sources.

Boreal forests are normally carbon sinks, taking up more CO₂ by photosynthesis than they release. Warmer temperatures and less winter snow increase drought, reducing primary production and causing trees to die (forest browning). Dead, dry trees make forest fires more frequent and intense, and intense fires burn legacy carbon stored in deep soil layers. When respiration, decomposition and combustion release more carbon than photosynthesis fixes, the forest becomes a net carbon source, adding to warming — a tipping point.

4. Explain how increasing atmospheric CO₂ and rising temperatures threaten coral reefs.

CO₂: the ocean absorbs CO₂, forming carbonic acid and lowering pH (ocean acidification). This reduces carbonate ion availability, suppressing calcification, so corals build their calcium carbonate skeletons more slowly and reef structures may dissolve. Temperature: prolonged high water temperatures cause corals to expel their zooxanthellae — bleaching — so they lose their main source of food from photosynthesis and may die. Loss of corals removes the structure and food base of the reef, leading to collapse of the whole ecosystem.

5. AHL Explain why climate change can disrupt the synchrony between reindeer migration and plant growth.

The two events are triggered by different cues. Spring growth of plants such as Arctic mouse-ear chickweed is cued by temperature, so as springs warm, plant growth starts earlier. Reindeer migration to calving grounds is cued by photoperiod (day length), which is unaffected by climate change, so migration timing stays the same. Reindeer therefore arrive after the peak of plant growth, when food quality and availability have declined, reducing calf survival.

6. AHL Explain how the proportion of brown tawny owls in Finland has increased as a result of climate change.

Tawny owls show heritable colour variation: grey and brown variants. In snowy winters, grey owls are better camouflaged and survive better, so brown owls have lower fitness. Climate change has made winters milder with less snow cover, removing this disadvantage, so brown owls survive and reproduce as well as or better than grey ones. The selection pressure has changed, the fitness of the brown variant has increased, and the frequency of the allele(s) for brown colouring has risen — evolution by natural selection in response to climate change.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

NASA Climate (Vital Signs) — data on CO₂, temperature, Arctic sea ice minimum and ice sheets, for calculating percentage change.
National Snow and Ice Data Center — sea ice extent data and analysis.
NOAA Coral Reef Watch — satellite monitoring of ocean temperatures and coral bleaching alerts.