IB Biology · Theme D Continuity and change · Ecosystems
D4.3 Climate change
Human activity has raised carbon dioxide and methane, and positive feedbacks are amplifying the warming. Ecosystems are shifting: boreal forests losing carbon, polar ice habitats melting, reefs bleaching, ranges moving. Forests and peatlands can pull carbon back; at HL, phenology and evolution show change under way.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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D4.3.1 What humans have added to the atmosphere
2028 guide: scope reduced — NOS paragraph on correlation vs causation (Antarctic ice cores) removed from this statement. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
Anthropogenic climate change is warming caused by human increases in greenhouse gases.
The causes studied are carbon dioxide and methane.
Carbon dioxide rose from about 280 ppm to over 420 ppm: fossil fuels, deforestation, cement.
Methane has more than doubled: ruminant livestock, rice paddies, landfill, fossil fuel leakage.
Both gases absorb long-wave radiation re-emitted by the Earth's surface, enhancing the greenhouse effect.
Students often blame the ozone hole for warming. In fact warming comes from greenhouse gases absorbing long-wave radiation.
Students often think methane comes from burning fuel. In fact combustion gives carbon dioxide; methane comes from livestock, rice, landfill and leaks.
D4.3.2 Feedbacks that make warming feed itself
A positive feedback cycle amplifies the original change instead of correcting it.
Warmer oceans hold less dissolved gas, so the deep ocean releases carbon dioxide.
Lost snow and ice lowers albedo: darker surfaces absorb more solar radiation.
Thawing permafrost and drying peat decompose, releasing carbon dioxide and, where waterlogged, methane.
More droughts and forest fires release further carbon dioxide from biomass and soil.
Students often think warm water dissolves more carbon dioxide. In fact gas solubility falls with temperature, so warming seas release it.
Students often think melting ice cools the planet like ice in a drink. In fact it exposes dark surfaces that absorb more radiation.
D4.3.3 Boreal forest: from carbon sink to carbon source
Boreal forest (taiga) has long accumulated carbon, much of it in thick soil organic layers.
Warmer temperatures and less winter snowfall bring drought, lower primary production and forest browning.
Fires grow more frequent and intense, burning deeper into legacy carbon stored over many fire cycles.
When losses outweigh uptake the forest tips from net accumulation to net loss: a tipping point.
Students often think warming makes forests grow faster and store more. In fact in the taiga drought and fire are turning it into a source.
Students often think fires only release carbon that regrowth will recapture. In fact legacy carbon from centuries of storage is not replaced before the next fire.
D4.3.4 Polar ice as habitat: emperor penguins and walruses
Landfast ice is fixed to the coast; emperor penguins breed on it through the Antarctic winter.
Early breakout before chicks fledge has caused breeding failure.
Arctic sea ice gives walruses a resting platform over shallow feeding grounds.
As summer ice retreats to deep water, walruses crowd onto land and travel further.
Students often think warmth itself harms polar animals. In fact it removes the ice platform they breed and rest on.
Students often think melting sea ice floods breeding grounds. In fact floating ice displaces its own mass; sea-level rise comes from land ice and expansion.
D4.3.5 Warm surface water blocks nutrient upwelling
Upwelling brings cold, nutrient-rich deep water to the sunlit surface.
Nitrate and phosphate from decomposition at depth then fuel phytoplankton production.
Warmer, less dense surface water forms a stable layer that can prevent upwelling.
Less upwelling means less ocean primary production and less energy through marine food chains.
Students often think warmer water boosts phytoplankton. In fact it starves them of nutrients by blocking upwelling.
Students often think plankton get nutrients from sunlight. In fact light is energy; nutrients come up from deep water.
D4.3.6 Species move poleward and upslope
A range shift moves the area a species occupies as it tracks tolerable temperatures.
Montane birds in New Guinea have shifted upslope.
North American tree species have spread north while contracting at their southern edge.
Trees shift across generations, by seed dispersal and survival, not by individuals moving.
Students often picture a range shift as migration. In fact for trees it is seedlings establishing beyond one edge and failing at the other.
Students often think a shifting species keeps the same range size. In fact mountain-top species run out of ground, so ranges often contract.
D4.3.7 Coral reefs: acidification and bleaching
Carbon dioxide dissolving in the sea causes ocean acidification: pH down from about 8.2 to 8.1.
Fewer carbonate ions suppress calcification, so corals build skeletons more slowly.
High water temperature causes bleaching: corals expel their zooxanthellae and start to starve.
Dead corals lose their structure, so the whole reef ecosystem can collapse.
Students often think acidification causes bleaching. In fact temperature does; acidification slows skeleton building.
Students often think bleached coral is dead. In fact it is alive and can recover if the water cools soon enough.
D4.3.8 Sequestering carbon in forests and peat
Carbon sequestration removes carbon dioxide from the air into long-term stores.
Afforestation plants trees on land not recently forested; forest regeneration regrows cleared forest.
Whether non-native plantations or rewilding with native species sequesters more is under active debate.
Peat forms where waterlogging excludes oxygen and blocks decomposition; re-wetting drained peatland restores the sink.
Peat forms in temperate and boreal zones and very rapidly in some tropical ecosystems.
Students often think peat is the product of fast rotting in wet ground. In fact waterlogging stops decomposition, so plant material builds up.
Students often think fast-growing plantations are certainly best. In fact scientists are still debating plantations versus native rewilding.
D4.3.9 Phenology: what sets the timing of the seasons' events HL
Phenology is research into the timing of events: flowering, budburst, bud set, migration, nesting.
Photoperiod and temperature patterns are cues that set that timing.
Day length is fixed by the Earth's orbit, so photoperiod cues do not shift with warming.
Temperature-cued events are advancing as the climate warms.
Students often think every seasonal event is triggered by warmth. In fact many are cued by day length.
Students often think days lengthen earlier in a warmer year. In fact photoperiod is unchanged by climate.
D4.3.10 When the cues disagree, synchrony breaks HL
2028 guide: scope reduced — Reported (unverified secondary source) that the 2028 guide removes the great tit (Parus major) / peak caterpillar biomass example from D4.3.10. The 2025 content is authored here because it is examined in 2026 and 2027: c26 and q21 depend on the great tit example, so q21 is tagged guideVersions ["2025"]. The Arctic mouse-ear chickweed / reindeer example (c25, m28, m29, q20) survives in both guides, so q20 remains ["2025","2028"]. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
In one population temperature is the cue; in another, photoperiod.
Arctic mouse-ear chickweed (Cerastium arcticum) grows earlier as springs warm.
Reindeer (Rangifer tarandus) migrate on a photoperiod cue, so calving has not advanced.
Calves now arrive after peak plant growth and get poorer food.
Students often think all species shift by the same amount. In fact cues differ, so events drift apart.
Students often think reindeer will just switch cue next year. In fact the cue is heritable; changing it takes selection over generations.
D4.3.11 More insect generations per year HL
Insects are ectotherms; development speeds up with temperature.
Warmer seasons let more generations fit into a year.
The spruce bark beetle has one generation in cool years, two or more in warm.
More generations mean larger populations and more damage to spruce forests.
Students often think generations per year is fixed by genes. In fact it depends on how fast development runs, which depends on temperature.
Students often think beetles live longer in warmth. In fact each generation is shorter, so more fit in.
D4.3.12 Tawny owls: evolution within decades HL
Fitness is a genotype's relative contribution to the next generation.
Tawny owls (Strix aluco) come in heritable grey and brown morphs.
Brown owls survived snowy winters less well; with less snow, brown fitness has risen.
In Finland brown owls rose from about 30% to 50% in three decades: allele frequencies changed.
Students often think the owls changed colour to match less snow. In fact colour is fixed for life; brown owls simply survived better.
Students often think evolution needs millions of years. In fact strong selection on a heritable trait shifts allele frequencies within generations.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which is an anthropogenic cause of the rise in the atmospheric concentration of methane since pre-industrial times?
Answer and reasoning
Exhalation of waste gases by the greatly increased human population — A student who thinks population growth raises greenhouse gases through breathing picks this. Exhaled gases return carbon recently fixed by photosynthesis and are not a source of methane.
Enteric fermentation in the growing numbers of ruminant farm livestock — Methanogenic archaea in the guts of cattle and other ruminants release methane, and livestock numbers have risen greatly. Rice paddies, landfill and leakage from fossil fuel extraction are other anthropogenic sources.
Complete combustion of coal and petroleum in vehicles and power stations — A student who assumes every greenhouse gas is a combustion product picks this. Complete combustion of fossil fuels releases carbon dioxide; methane is itself a fuel that combustion consumes.
Escape of ancient methane trapped in bubbles in melting polar ice — A student who pictures greenhouse gas as ancient bubbles trapped in ice picks this. Ice sheets are not a store of methane, and the anthropogenic rise comes from enteric fermentation in livestock, rice paddies, landfill and leakage of natural gas during fossil fuel extraction and transport.
2 Which process is part of a positive feedback cycle that increases global warming?
Answer and reasoning
More evaporation forms clouds that reflect sunlight and cool the surface — A student who thinks positive feedback restores the original state picks a stabilizing process. This would be negative feedback; positive feedback amplifies the warming.
Melting snow and ice absorb heat from their surroundings and cool the oceans — A student who thinks of ice cooling a drink picks this. The important effect of losing snow and ice is the loss of a reflective surface, which increases absorption of solar radiation.
Warming ocean water dissolves more carbon dioxide from the atmosphere — A student who generalizes from the solubility of solids picks this. Gases are less soluble in warmer water, so warming oceans release carbon dioxide.
Thawing permafrost releases methane gas into the atmosphere — Methane is released from melting permafrost as previously frozen organic matter decomposes anaerobically. Methane is a greenhouse gas, so this causes further warming and further thawing: a positive feedback cycle.
3 Which sequence of events explains how boreal forests in a warming climate change from net carbon accumulation to net carbon loss?
Answer and reasoning
Warmer conditions and more carbon dioxide speed up photosynthesis, producing more litter, so decomposition releases more carbon than the trees fix. — A student who expects warmth and carbon dioxide to boost growth picks this. In the taiga, warming brings drought, and primary production falls rather than rising.
More frequent fires burn the trees, but regrowth reabsorbs that carbon, so the net loss comes only from timber removed by logging. — A student who thinks fires are carbon neutral picks this. Intense fires combust legacy carbon from the soil organic layer, which regrowth does not replace before the next fire.
Warming and less winter snow cause drought, reduced primary production, forest browning and more fires that burn legacy carbon. — Reduced snowfall and warmer temperatures leave the soil drier, so drought reduces primary production, causes browning and makes fires more frequent and intense. Fires that combust legacy carbon release more carbon than the forest fixes.
The forest passes a tipping point at which warming stops, and its carbon balance then slowly returns to what it was before. — A student who thinks a tipping point is a gradual, reversible turning point picks this. A tipping point is a threshold beyond which the forest shifts to a new state, net carbon loss, that may not reverse.
4 Emperor penguins (Aptenodytes forsteri) breed on landfast ice during the Antarctic winter, and the chicks fledge in December and January. Why does the early breakout of landfast ice threaten breeding?
Answer and reasoning
If the ice breaks out before the chicks have grown waterproof adult plumage, they fall into the sea and drown or die of cold. — Landfast ice must persist until the chicks fledge. When it breaks out early, chicks that still have down rather than waterproof feathers are lost to the sea, and colonies can suffer near-total breeding failure.
The open water exposes the penguins to warmer air, which they cannot tolerate because they are adapted to cold. — A student who thinks polar animals are threatened by heat itself picks this. The threat is the loss of the ice platform on which the chicks are reared, not the temperature.
The melting ice raises the sea level and floods the colony on the shore where the eggs are being incubated. — A student who thinks melting sea ice raises sea level picks this. Floating ice does not raise sea level, and the eggs are incubated on the ice itself, not on the shore.
Walruses that share the ice platform crowd onto the remaining ice and trample the eggs and chicks. — A student who mixes up the faunas of the two poles picks this. Walruses live in the Arctic; emperor penguins breed in the Antarctic, where there are no walruses.
5 Off the coast of Peru, cold nutrient-rich water normally rises to the surface. In some years the surface water is unusually warm, and the catch of anchovies falls sharply. Which explanation is correct?
Answer and reasoning
Warm surface water raises the rate of enzyme activity in phytoplankton, which grow so fast that they use up all the oxygen the anchovies need. — A student who applies the enzyme–temperature rule to the whole ocean picks this. Production falls in warm years because nutrients, not temperature, limit phytoplankton, and fewer nutrients reach the surface.
Phytoplankton obtain their nutrients from sunlight, so the fall in the catch must be caused by the greater cloud cover in the warmer years. — A student who thinks sunlight supplies nutrients picks this. Light supplies energy; the mineral nutrients that limit phytoplankton come from below, and the warm water stops them rising.
Warm surface water is less dense and does not mix with the water below, so fewer nutrients reach the surface and primary production falls. — Warm surface water forms a stable layer that prevents nutrient upwelling. Ocean primary production falls, so less energy flows through the food chain to the anchovies and the catch collapses.
Warm water dissolves more carbon dioxide, which acidifies the sea and kills the eggs and larvae of the anchovies. — A student who thinks warm water dissolves more gas picks this. Warmer water dissolves less carbon dioxide; the collapse is due to the loss of nutrient upwelling and primary production.
6 Which statement correctly matches a threat to coral reefs with its cause?
Answer and reasoning
Coral bleaching is caused by ocean acidification, which dissolves the pigments out of the coral tissue. — A student who merges the two threats picks this. Bleaching is caused by increased water temperature, which makes corals expel their zooxanthellae; acidification suppresses calcification.
Suppression of calcification in corals is caused by the increased carbon dioxide concentration of seawater. — Extra dissolved carbon dioxide lowers the pH and the carbonate ion concentration of seawater, so corals build their calcium carbonate skeletons more slowly.
Ocean acidification lowers the pH of seawater below 7, so the calcium carbonate skeletons of corals dissolve away. — A student who reads acidification as 'becoming acidic' picks this. Seawater remains alkaline at about pH 8.1; the harm is a reduced carbonate concentration that suppresses calcification.
Coral bleaching kills the polyps immediately, so a reef that has bleached can no longer recover. — A student who thinks white coral is dead coral picks this. A bleached coral is alive; it dies only if the heat stress is prolonged, and can otherwise regain its algae.
7 A drained peatland that had been releasing carbon dioxide is re-wetted so that the soil becomes waterlogged again. Why does this restoration act as an approach to carbon sequestration?
Answer and reasoning
Waterlogging speeds up rotting, so plant remains turn into peat faster and store carbon — A student who expects wet things to rot faster picks this. Waterlogging excludes oxygen and so inhibits the saprotrophs; peat is plant material that has not fully decomposed, and it stores carbon because decomposition is slowed, not speeded up.
The cold of the water stops decomposition, so the carbon in plant remains stays stored — A student who thinks peat is a cold-climate product picks this. Peat forms very rapidly in some warm tropical ecosystems; it is the lack of oxygen in waterlogged soil, not cold, that inhibits decomposition and lets carbon accumulate.
The weight of the water compresses the dead plants into peat, locking the carbon in — A student who models peat on coal formation picks this. Peat accumulates at the surface of waterlogged soil because decomposition is inhibited; compression is not what forms it or what stores the carbon.
Without oxygen, saprotrophs cannot decompose plant remains, which build up as peat — Saprotrophs need oxygen for aerobic respiration. Re-wetting makes the soil anaerobic again, so the decomposition that was releasing carbon dioxide from the drained peat is inhibited and partially decomposed plant material once more accumulates as peat, storing carbon.
It investigates how the environment alters the phenotype of an organism during its lifetime. — A student who confuses phenology with phenotype picks this. Phenology is research into the timing of biological events, not into how characteristics develop.
It investigates the timing of biological events such as budburst, flowering and migration. — Phenology is research into the timing of biological events and the variables, such as photoperiod and temperature patterns, that influence that timing.
It shows that temperature is the cue for every seasonal event in every species. — A student who assumes warmth drives all of spring picks this. Photoperiod is the main cue for some events, such as bud set and the migration of many birds.
It shows that the photoperiod at a given latitude has changed as the climate warms. — A student who thinks longer days come earlier with warming picks this. Day length at a given latitude and date is fixed by the Earth's orbit and is unaffected by climate change.
9 In the Arctic, spring growth of the mouse-ear chickweed (Cerastium arcticum) is triggered by rising temperature, whereas the migration of reindeer (Rangifer tarandus) to their calving grounds is triggered by increasing day length. Springs are now warmer and earlier. What is the expected effect? HL
Answer and reasoning
Both events advance by the same number of days, so synchrony between them is maintained — A student who thinks all species shift together picks this. Only the temperature-cued event advances; the photoperiod-cued migration keeps to the same dates.
The reindeer switch to using temperature as their cue and arrive earlier to match the plants — A student who expects organisms to adjust at will picks this. The cue a population uses is heritable; it does not change within a generation.
Day length also increases earlier in a warm spring, so the reindeer migrate earlier as well — A student who thinks photoperiod shifts with the weather picks this. Day length on a given date is fixed by the Earth's orbit and does not change in a warm year.
Peak plant growth occurs before the calves are born, so they have less food — Plant growth, cued by temperature, now peaks earlier, but the reindeer, cued by photoperiod, arrive and calve on the same dates as before. The calves miss the most nutritious young growth, and calf survival falls: synchrony is disrupted.
10 The spruce bark beetle (Ips typographus) completes one generation per year in cool summers but two or three in warm summers. In a region where mean summer temperature has risen by 2 °C, the area of spruce forest killed by the beetle has increased greatly. Which explanation is correct? HL
Answer and reasoning
The number of generations per year is fixed genetically, so the extra damage must be due to beetles moving in from elsewhere. — A student who thinks life cycles per year are a fixed species characteristic picks this. Beetle development rate rises with temperature, so more generations are completed in warm years.
Faster development at higher temperatures allows more generations per year, so beetle numbers build up more rapidly. — Insects are ectotherms whose development speeds up as temperature rises. Extra generations within a year multiply the population, so more trees are attacked and killed.
Adult beetles live longer in the warmth, so each one attacks more trees over the course of its lifetime. — A student who confuses lifespan with generation time picks this. Warmth shortens the time from egg to adult; it is more generations, not longer-lived beetles, that raises numbers.
Warmer conditions increase the growth of the spruce trees, and the beetles feed on this extra wood. — A student who assumes warmth benefits trees picks this. Warm, dry conditions stress spruce and weaken their resin defences; the damage rises because beetle populations grow.
Read the ones marked not yet in Learn, then Verify.
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14 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 Antarctic ice cores show that, over the last 800 000 years, periods of high global temperature coincided with high atmospheric carbon dioxide concentrations and periods of low temperature with low concentrations. Which statement about this evidence is correct?
Answer and reasoning
It shows a positive correlation, but the causal link between carbon dioxide and temperature is established by other evidence. — The ice-core data show a positive correlation over hundreds of thousands of years. A correlation does not prove causation; the causal link is confirmed by other evidence, such as the measured absorption of infrared radiation by carbon dioxide.
It shows a positive correlation, which by itself proves that carbon dioxide raises global temperatures. — A student who takes a strong correlation as proof of cause picks this. Correlation alone cannot prove causation, since both variables could respond to a third factor; the causal link needs other evidence.
It shows a negative correlation in cold periods, when both variables fell, and a positive one in warm periods. — A student who thinks the sign of a correlation describes whether the variables are rising or falling picks this. High goes with high and low with low throughout, which is a single positive correlation.
It shows a correlation, but the warming was caused by thinning of the ozone layer rather than by carbon dioxide. — A student who merges ozone depletion with global warming picks this. Ozone depletion is a separate problem; the causal link between carbon dioxide and warming is established by other evidence.
2 In a data set of 40 countries, the higher the percentage of electricity generated by burning coal, the higher the carbon dioxide emissions per person. In the same countries, the greater the percentage of land covered by forest, the lower the net carbon dioxide emissions. Based on these data alone, how should the two relationships be described?
Answer and reasoning
The first is a positive correlation and the second is a positive correlation too — A student who thinks a correlation is positive whenever a variable increases picks this. In the second relationship higher forest cover goes with lower emissions, which is a negative correlation.
The first is a strong correlation and the second is a much weaker correlation — A student who reads 'negative' as 'weak' picks this. The sign of a correlation is its direction, not its strength; nothing in the data says the second relationship is weaker.
The first is a positive correlation and the second is a negative correlation — In the first, higher coal use goes with higher emissions: positive. In the second, higher forest cover goes with lower emissions: negative. Both are correlations; neither by itself proves causation.
The first is a causal relationship and the second is a negative correlation — A student who takes a correlation as proof of cause picks this. Coal burning is known from other evidence to release carbon dioxide, but these data alone show a correlation, not causation.
3 As the oceans warm, the amount of carbon dioxide that stays dissolved in the deep ocean changes. Why does this act as a positive feedback in global warming?
Answer and reasoning
Gases are less soluble in warmer water, so the warming ocean releases carbon dioxide, which enhances the greenhouse effect and warms the ocean further. — The solubility of a gas falls as temperature rises. Carbon dioxide released from the deep ocean adds to the atmospheric concentration, increasing the greenhouse effect and causing more warming, which releases more carbon dioxide.
Gases are more soluble in warmer water, so the ocean takes up carbon dioxide and this stored gas traps heat in the water, warming it further. — A student who assumes warm water dissolves more, as it does for sugar, picks this. Gases become less soluble as temperature rises, so the ocean releases carbon dioxide rather than absorbing more.
The carbon dioxide released returns the atmospheric concentration to its earlier value, which brings the temperature back to normal. — A student who thinks positive feedback restores a system picks this. That would be negative feedback; here the release adds carbon dioxide and pushes the temperature further from its original value.
Warming melts the sea ice, which releases ancient carbon dioxide that had been trapped as bubbles inside the ice since it formed. — A student who pictures trapped bubbles of ancient gas picks this. The feedback comes from the reduced solubility of carbon dioxide in warmer water, not from gas trapped in ice.
4 Fresh snow reflects about 85% of the solar radiation reaching it, whereas open ocean water reflects about 7%. Since 1980 the area of the Arctic Ocean covered by sea ice in September has fallen by more than 40%. What is the most likely consequence for the Arctic climate?
Answer and reasoning
Melting ice takes in heat from the water, so the Arctic Ocean is cooled and further loss of ice is slowed down. — A student who applies the ice-in-a-drink idea picks this. The dominant effect is that the exposed dark water absorbs far more solar radiation than the ice did, warming the region.
The exposed dark water absorbs far more of the solar radiation, warming the region and melting still more ice. — Open water reflects only about 7% of solar radiation and absorbs the rest, whereas snow-covered ice reflects most of it. Loss of ice therefore increases absorption and warming, which melts more ice: a positive feedback cycle.
The extra open water evaporates, and the cloud that forms reflects radiation and restores the ice cover. — A student who expects feedback to restore the original state picks this. The loss of a reflective surface amplifies warming; it is a positive feedback, not a self-correcting one.
Sea level rises as the ice melts and floods the coasts, but the amount of radiation absorbed is unchanged. — A student who thinks melting sea ice raises sea level picks this. Floating ice displaces its own mass of water, so sea level hardly changes, and the absorption of radiation changes greatly.
5 Researchers measured the carbon in the soil organic layer of a boreal forest after fires. In stands older than 70 years, the carbon that remained was older than the trees and had survived earlier fires. In stands younger than 60 years on drier sites, the fire had burnt through this old carbon. What do these results show?
Answer and reasoning
Fires release only carbon fixed since the previous fire, so regrowth of the trees will restore the carbon balance. — A student who thinks fires are carbon neutral picks this. In the young stands the fire burnt carbon older than the trees, so it released carbon that regrowth cannot replace before the next fire.
The forest has crossed a tipping point, so its carbon stock will recover gradually once the fires are out. — A student who thinks a tipping point is followed by gradual recovery picks this. A tipping point marks a shift to a new state, net carbon loss, which may not be reversible.
The old carbon lost from the young stands was ancient gas trapped in frozen soil that escaped as it thawed. — A student who pictures trapped gas picks this. The old carbon was stored organic matter in the soil organic layer that was combusted by the fire, not gas released by thawing.
Fires in young stands on dry sites combust legacy carbon that older stands protect from burning. — Legacy carbon is carbon in the soil organic layer that survived previous fires. In older stands a thick organic layer protected it, but in young stands on dry sites the fire burnt into it, releasing carbon stored over many fire cycles.
6 Walruses (Odobenus rosmarus) feed on molluscs on the floor of the shallow Arctic continental shelf and rest on sea ice between dives. Summer sea ice now retreats into deep water beyond the shelf. What is the consequence for the walruses?
Answer and reasoning
They overheat in the warmer water, because their thick blubber prevents them from losing heat quickly enough to survive. — A student who thinks the threat from warming is heat stress picks this. The problem is the loss of a resting platform near the feeding grounds, not the water temperature.
They must haul out on land in large crowded groups and travel much further to reach their feeding grounds. — With no sea ice over the shallow shelf, walruses rest on beaches in crowds of thousands, where calves can be trampled, and must swim long distances to feed. The sea ice was their habitat and it has been lost.
They move to the Antarctic, where the landfast ice still provides a resting platform close to the shelf. — A student who treats the two poles as one place picks this. Walruses are Arctic animals and do not occur in the Antarctic.
Sea level rises as the ice melts, so the shelf becomes too deep for them to reach the sea floor. — A student who thinks melting sea ice raises sea level picks this. Floating ice does not raise sea level; the problem is that the ice has moved away from the shallow shelf.
7 The bird species of a mountain in New Guinea were surveyed in 1965 and again in 2012. For most species both the upper and the lower limits of the altitudinal range had moved upslope, by about 100 m on average, over a period in which the region warmed. What is the best interpretation?
Answer and reasoning
The species have expanded their ranges, since they now occupy about 100 m more of the mountain. — A student who equates a range shift with expansion picks this. Both limits moved up by a similar amount, so the range moved rather than growing.
Because both limits moved equally, the species have lost none of their habitat area. — A student who thinks a range shift is cost-free picks this. A mountain narrows towards its summit, so a band of the same height contains less area higher up, and species near the top have nowhere left to go.
The correlation between warming and the shift proves that no other factor was involved. — A student who treats a correlation as proof of cause picks this. The pattern is consistent with warming, but a correlation alone cannot rule out other factors.
The ranges of the species shifted upslope, tracking the rise in temperature. — Both range limits moving upslope as the region warmed is the pattern expected when a species tracks the band of temperatures it can tolerate. Upslope shifts of montane birds in New Guinea are evidence of range shifts driven by climate change.
8 In a survey of forests in the eastern United States, seedlings of several northern tree species were found on average further north than mature trees of the same species, while at the southern edge of the range there were mature trees but few seedlings. What does this indicate?
Answer and reasoning
The ranges are shifting northward, with seedlings establishing beyond the northern edge and the range contracting at the southern edge. — Trees shift range across generations. Seeds that establish north of the current adults extend the range northward, and failure of seedlings in the south means the southern edge will retreat as the mature trees die: northward spread with range contraction.
The mature trees are themselves migrating northward as individuals, leaving the southern part of their range behind them. — A student who pictures a range shift as individuals moving picks this. Rooted trees cannot move; the shift happens through seed dispersal and the survival of seedlings over generations.
The species are expanding their ranges, since seedlings are appearing in the north while mature trees remain in the south. — A student who reads any shift as expansion picks this. Few seedlings in the south means that edge will not be replaced when the adults die, so the range contracts there.
The species are unaffected by warming, because trees of some age are still present right across the whole of the range. — A student who thinks species simply move with the climate unharmed picks this. The lack of regeneration in the south shows the range is being lost there.
9 During a marine heatwave, the corals on a reef turned white. When the water cooled four weeks later, most of the corals regained their colour. Which explanation is correct?
Answer and reasoning
The corals died when they turned white, and the colour came from new corals that settled on the skeletons within four weeks. — A student who thinks bleached coral is dead picks this. New corals could not grow to cover a reef in four weeks; the original corals were alive and recovered their algae.
The warm water became acidic and bleached the pigments, which re-formed once the pH returned to normal. — A student who thinks acidification causes bleaching picks this. Bleaching is a response to heat stress; the colour is lost with the expelled algae, not chemically bleached.
Heat stress made the corals expel their zooxanthellae, and they took the algae up again once the stress had ended. — High temperature causes corals to expel their mutualistic algae, leaving the white skeleton visible through the tissue. The corals stayed alive, and once the water cooled they regained their algae and their colour.
The pH of the seawater fell below 7 in the heat and dissolved the skeletons, which were rebuilt when it rose again. — A student who thinks the sea becomes acidic picks this. Seawater stays alkaline, and skeletons could not be dissolved and rebuilt in weeks; the corals had lost and regained their algae.
10 A government wants to sequester carbon on degraded land. Option A is a plantation of a fast-growing non-native conifer, harvested every 40 years. Option B is to allow native woodland to regenerate naturally. Scientists disagree about which is better. Which statement about this debate is justified?
Answer and reasoning
The plantation sequesters carbon faster at first, but native woodland may store more in the long term, so the evidence does not settle the question. — This is the active scientific debate the IB refers to. Fast-growing plantations give rapid early sequestration; native regeneration is slower but may store more carbon over the long term, including in soil, and has other benefits. Neither is established as best.
The plantation is certainly better, because the fastest rate of tree growth gives the greatest amount of carbon sequestered. — A student who reasons from growth rate alone, and assumes the question is settled, picks this. Harvesting, soil carbon and long-term storage all matter, and scientists actively disagree.
Both options lock the carbon away permanently once the trees grow, so the only real difference between them is the cost. — A student who thinks carbon taken up by trees is stored for ever picks this. Harvested timber, decomposition and fire return carbon to the atmosphere, which is central to the debate.
Both options are afforestation and involve growing trees on the land, so they will sequester the same amount of carbon. — A student who treats all tree-based approaches as the same picks this. Option A is afforestation with a plantation; option B is forest regeneration, and the two differ in species, harvesting and long-term storage.
11 A migratory bird species arrives at its northern breeding grounds on almost the same date every year, whether the spring is warm or cold. In the same region, the date of budburst in birch trees varies by up to three weeks between years. What can be concluded about the cues? HL
Answer and reasoning
Both are cued by temperature, but the birds respond to it much less strongly than the trees — A student who assumes temperature cues everything picks this. A date that does not vary with the warmth of the spring points to a cue that does not vary between years: photoperiod.
Both are cued by photoperiod, which varies between years according to the spring weather — A student who thinks day length changes with the weather picks this. Photoperiod on a given date is the same every year, which is why it cannot explain the varying budburst date.
Migration is cued mainly by photoperiod, whereas budburst is cued mainly by temperature — Photoperiod is identical on the same date each year, so an event on a fixed date is consistent with a photoperiod cue. Budburst varies with the warmth of the spring, consistent with a temperature cue.
Both respond to the same cue, so the timing of the two events will remain synchronized — A student who thinks all species in an ecosystem shift together picks this. The two events already vary differently between years, so they are responding to different cues.
12 In a north European oak forest, the date of peak caterpillar biomass advanced by about nine days over 23 years as springs warmed. Over the same period the mean date on which great tits (Parus major) laid their eggs did not change significantly. Which conclusion is supported? HL
Answer and reasoning
The chicks now hatch after the food peak, so the two events are no longer synchronized and the breeding success of the great tits is reduced. — Great tits time their breeding so that nestlings are fed during the caterpillar peak. The peak has moved earlier but laying has not, so nestlings hatch when caterpillars are declining and fewer survive: the synchrony has been disrupted.
Both populations experienced exactly the same warming, so the small difference in response cannot harm the birds. — A student who thinks all species shift together picks this. The data show the two responses differ, and a nine-day gap between hatching and the food peak reduces nestling survival.
The great tits will adjust their laying date to match the caterpillars within a generation, so no mismatch persists. — A student who expects organisms to correct a mismatch at will picks this. The timing response is heritable and changes only through selection over generations.
Because both events are cued by spring temperature, the caterpillars cannot have advanced more than the birds did. — A student who assumes temperature cues everything equally picks this. The data show the caterpillars advanced by nine days while laying did not, so the two responded differently.
13 In Finland, tawny owls (Strix aluco) occur as a grey morph and a brown morph, and the colour is inherited. Brown owls survived snowy winters less well than grey owls. Between 1981 and 2008 winters became milder with less snow, and the proportion of brown owls rose from about 30% to about 50%. What best explains the change? HL
Answer and reasoning
Individual grey owls gradually became browner over the years, as their plumage no longer needed to match snow in winter. — A student who thinks individuals adjust their colour to the environment picks this. Colour morph is inherited and fixed for life; the change is in the proportions of the two morphs in the population.
Mutations producing brown plumage occurred more often once the winter snow cover had gone from the region. — A student who thinks mutation responds to need picks this. Both alleles already existed; what changed was their relative fitness, and selection altered their frequencies.
The fitness of the brown morph rose as snow cover fell, so brown alleles increased in frequency by natural selection. — With less snow, brown owls no longer suffered the higher winter mortality they had in snowy years, so they survived and reproduced better. Their allele became more common in the population: evolution as a consequence of climate change.
The change is far too rapid to be evolution, so it must be a chance fluctuation in the numbers of each morph from year to year. — A student who thinks evolution needs millions of years picks this. A consistent rise over three decades, matching a heritable trait to a changed environment, is natural selection observed in real time.
14 The proportion of brown tawny owls (Strix aluco) in a population has risen over three decades as winter snow cover has decreased. Which statement about this change is correct? HL
Answer and reasoning
It is not evolution, because evolution takes many thousands of years to produce any change. — A student who links evolution only to the geological timescale picks this. Strong selection on a heritable trait changes allele frequencies within a few generations.
It is not evolution, because the individual owls changed their colour to suit the milder conditions. — A student who attributes the change to individuals adapting picks this. Colour morph is inherited and does not change during an owl's life.
It is evolution, because the reduced snow cover caused new mutations for brown plumage to arise. — A student who thinks environmental change directs mutation picks this. The brown allele was already present; selection changed its frequency.
It is evolution, because the frequency of the brown allele in the population has changed. — Evolution is a change in the heritable characteristics of a population. The rise of the brown morph is a change in allele frequency caused by natural selection under changed snow cover.
That was your twenty minutes. Real practice on D4.3 is past-paper questions marked against the mark scheme.
What the exam asks of D4.3
Paper 1A asks you to match a feedback, a habitat change or a named species to its cause. Paper 1B gives time series of carbon dioxide, ice extent, phenological dates or owl morph frequencies and asks you to describe and explain the trend. Paper 2 uses *outline* and *explain*: state the cause, the mechanism, then the consequence for the ecosystem, using the named examples. HL questions use *discuss* for phenological mismatch and *explain* for evolution: name the cue or the selection pressure, the heritable trait, and the change in frequency.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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