IB Biology · Theme D: Continuity and change · SL and HL
D4.3 Climate change
A one-page summary of D4.3 Climate change, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What are the drivers of climate change?
What are the impacts of climate change on ecosystems?
What D4.3 covers
Drivers, feedbacks and a tipping point
- D4.3.1Two gases, two rising curves
- D4.3.2Melting ice makes more melting
- D4.3.2Warmer carbon stores release more
- D4.3.2Drought and fire close the loop
- D4.3.3The boreal forest flips
Impacts on polar, ocean and reef ecosystems
- D4.3.4When the ice goes out too early
- D4.3.4A walrus needs its floating platform
- D4.3.5A warm lid on the ocean
- D4.3.6Birds climbing the mountain
- D4.3.6Trees shifting north
- D4.3.7Acid oceans, thinner skeletons
- D4.3.7Heat drives the colour out
- D4.3.7When the reef collapses
Removing carbon: sequestration
- D4.3.8Three ways to store carbon
- D4.3.8Plantation or rewilding?
HL — timing, synchrony and evolution
- D4.3.9 · HLReading the calendar of life
- D4.3.10 · HLOut of step
- D4.3.11 · HLOne extra generation, a much bigger outbreak
- D4.3.12 · HLLess snow, more brown owls
D4.3 Climate change: summary
Drivers
- Human-caused rises in CO2 (about 280 to 420 ppm; fossil fuels, deforestation) and methane (about 720 to 1,920 ppb; livestock, rice, landfill, fossil fuel extraction).
- Both absorb and re-emit infrared radiation, retaining heat.
Positive feedbacks
- Warming causes a change that causes more warming: ice-albedo loss, CO2 from the deep ocean, faster decomposition of peat and permafrost, methane from permafrost, more drought and fire.
- Boreal forest tipping point: drought, browning and fire burn legacy carbon, so net accumulation becomes net loss.
Polar and ocean
- Landfast ice breakout kills emperor penguin chicks; sea ice loss forces walruses onto crowded land.
- Warm surface water suppresses upwelling, cutting nutrients, phytoplankton and food-chain energy.
Ranges and reefs
- Range shifts upslope (New Guinea birds, 113 m) and poleward (North American trees); species at summits have nowhere to go.
- Reefs: acidification suppresses calcification, bleaching expels zooxanthellae; together they risk collapse.
Sequestration
- Afforestation, forest regeneration and peat-forming wetland restoration store carbon.
- Debate: fast non-native plantations vs slower but more diverse, resilient native rewilding.
HL · Timing and evolution
- Phenology cues (photoperiod, temperature) can drift apart: chickweed vs reindeer mismatch; an extra spruce bark beetle generation per year.
- Tawny owls: less snow raised the fitness of brown morphs, so the morph frequency changed.
Key terms
- Positive feedback cycle
- A self-reinforcing loop in which warming causes a change that itself causes further warming.
- Albedo
- The fraction of incoming sunlight that a surface reflects; snow and ice have a high albedo, open ocean a low one.
- Legacy carbon
- Carbon accumulated slowly in biomass, litter and soil or peat over decades to centuries, which fire can release within days.
- Landfast ice
- Sea ice that stays attached to the coast, an ice shelf or grounded icebergs rather than drifting.
- Upwelling
- The rise of cold, nutrient-rich deep water to the surface, fuelling high primary production.
- Ocean acidification
- A fall in ocean pH caused by dissolved CO2, which reduces carbonate ions and suppresses calcification.
- Coral bleaching
- The expulsion of zooxanthellae by heat-stressed corals, leaving a white skeleton.
- Carbon sequestration
- The removal of atmospheric CO2 into a longer-term store, such as growing forest or peat.
- Phenology
- The study of the timing of recurring biological events, such as flowering, budburst, migration and nesting.
- Photoperiod
- Day length: set by latitude and Earth's tilt and unaffected by climate change.
- Bud set
- The formation of dormant buds in deciduous trees at the end of the growing season.
- Phenological mismatch
- Loss of synchrony between interacting species whose timing is cued differently.
- Voltinism
- The number of generations an insect completes in a year; warming can increase it.
- Colour morph
- One of several genetically determined colour forms in a species, such as grey and brown tawny owls.
Sample exam questions
Three of the 37 multiple-choice questions for D4.3. Try each one before opening the answer.
Question 1. Which two greenhouse gases does the syllabus specify as the anthropogenic drivers of climate change?
- Ozone and nitrogen
- Carbon dioxide and methane
- Carbon dioxide and oxygen
- Methane and hydrogen
Show the answer
Answer: B. The syllabus explicitly limits anthropogenic causes of climate change to increases in atmospheric carbon dioxide and methane — not the full set of greenhouse gases (e.g. N2O, halocarbons) that a broader environmental science course might cover.
Question 2. Anthropogenic methane (CH4) emissions come mainly from:
- Livestock digestion (enteric fermentation), rice paddy cultivation, landfill waste, and fossil fuel extraction — all involving anaerobic decomposition or fossil methane release
- Direct combustion of methane gas in industry
- Photosynthesis in agricultural crops
- Ocean acidification
Show the answer
Answer: A. Methane is produced by methanogenic archaea under anaerobic conditions (ruminant guts, flooded rice paddies, landfill) or released directly during fossil fuel extraction and transport (leaks from gas wells and pipelines).
Question 3. An additional generation of spruce bark beetle per year is ecologically significant because:
- It has no effect on tree mortality
- Bark beetles only attack already-dead trees
- Population growth compounds with each generation, so an extra generation per year can massively increase total beetle numbers and the area of tree mortality within an outbreak, particularly in drought-weakened trees with reduced resin defences
- It reduces the total beetle population
Show the answer
Answer: C. Because insect populations grow multiplicatively across generations, even one additional generation per year can produce a disproportionate outbreak. This interacts with the boreal forest tipping point (D4.3.3): drought-stressed trees have reduced resin flow (their main defence against boring beetles), making outbreaks more severe and contributing further to forest dieback and legacy carbon combustion via increased fire risk in dead standing timber.
Linking questions
Questions that connect D4.3 to other parts of the course, the kind that come up in Paper 2.
- D4.2 defines a tipping point using Amazon rainforest dieback (positive rainfall-forest feedback pushing towards a drier, savanna-like state). Using the boreal forest example (D4.3.3), explain how a different positive feedback mechanism (drought and fire, rather than reduced transpiration) can push another biome past a comparable tipping point. (see D4.2)
- Natural selection changes allele frequencies in response to a changing selection pressure (D4.1). Using the tawny owl colour-morph example (D4.3.12), explain how declining snow cover has altered the relative fitness of the grey and brown morphs, and why this counts as ongoing evolution rather than simply a change in individual owls' appearance. (see D4.1)
- The carbon cycle (C4.2) includes both natural sinks (oceans, forests, soils/peat) and natural sources. Explain, using carbon sequestration approaches (D4.3.8), how afforestation and peat-forming wetland restoration are attempts to re-strengthen natural carbon sinks that anthropogenic activity has weakened. (see C4.2)
- Photosynthesis (C1.3) is the process by which forests and wetlands remove atmospheric CO2. Discuss why the rate of carbon capture achievable by afforestation is limited compared with the rate at which fossil fuel combustion releases CO2 (D4.3.1, D4.3.8). (see C1.3)
- Eutrophication (D4.2.8) and ocean acidification (D4.3.7) are both chemical disruptions of aquatic ecosystems caused by human activity, but through different mechanisms. Compare how each process ultimately reduces biodiversity or ecosystem function. (see D4.2)
Practise D4.3
Study notes, every question and full markschemes for D4.3 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.