Climate change — IB Diploma Biology HL
IB Biology climate change: CO₂ and methane, positive feedback, tipping points, polar ice, coral reefs, range shifts and phenology (HL).
Revision notes
7 short notes, in the order of the specification. Each one in short:
Anthropogenic climate change is caused by human increases in atmospheric concentrations of carbon dioxide and methane, which enhance the greenhouse effect. Positive feedback cycles then accelerate warming: release of carbon dioxide from the deep ocean, loss of reflective snow and ice, faster decomposition and methane release in thawing permafrost, and more droughts and forest fires.
Boreal forests are an example of a tipping point: warmer temperatures and less winter snowfall cause drought, lower primary production, forest browning and more intense fires, which burn legacy carbon, turning net carbon accumulation into net loss. Melting of landfast ice threatens emperor penguin breeding grounds, and loss of Arctic sea ice removes walrus habitat.
Changes in ocean currents alter the timing and extent of nutrient upwelling: warmer surface water can prevent nutrients rising, decreasing ocean primary production and energy flow through food chains. Coral reefs are threatened with ecosystem collapse because increased carbon dioxide causes ocean acidification and suppresses calcification, and higher water temperatures cause coral bleaching.
Poleward and upslope range shifts happen when warming makes the edges of a species' range too hot, while cooler areas become suitable. Tropical-zone montane bird species in New Guinea have shifted their ranges upslope. Some North American tree species show range contraction at their warm edge and northward spread at their cool edge.
Afforestation, forest regeneration and restoration of peat-forming wetlands are approaches to carbon sequestration: removing carbon dioxide from the atmosphere and storing it as organic carbon. Scientists debate whether plantations of non-native trees or rewilding with native species is best. Peat forms in waterlogged soils, where decomposition is slow.
Phenology is research into the timing of biological events such as flowering, budburst, bird migration and nesting. Photoperiod and temperature patterns influence this timing. Climate change disrupts synchrony when one population uses temperature as its cue and another uses photoperiod, as with Arctic mouse-ear chickweed and migrating reindeer, or great tits and caterpillars.
Climate change increases the number of insect life cycles within a year: warmer, longer summers let the spruce bark beetle complete two or more generations instead of one, causing outbreaks that kill spruce trees. Climate change also causes evolution: with less snow cover, brown tawny owls have gained fitness relative to grey ones.
Exam questions
8 exam-style questions (26 marks), each with its mark scheme.
Answer the questionsFlashcards
21 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of ecosystems (continuity and change) on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
Which two greenhouse gases does the guide limit anthropogenic causes to?
Show answer
Carbon dioxide and methane.
What is legacy carbon?
Show answer
Old organic carbon in deep soil layers of boreal forest that survived earlier fires.
What causes coral bleaching?
Show answer
Increases in water temperature cause corals to expel their symbiotic algae.
HL only What is phenology?
Show answer
Research into the timing of biological events.
HL only Why are spruce bark beetle outbreaks increasing?
Show answer
Warming lets them complete more life cycles each year.
Frequently asked questions
What are positive feedback loops in climate change?
Positive feedback loops are cycles in which warming causes changes that cause more warming. Examples are loss of reflective snow and ice, so more solar radiation is absorbed; thawing permafrost releasing CO₂ and methane; CO₂ released from the deep ocean; and more droughts and forest fires releasing carbon.
How does climate change affect coral reefs?
Climate change threatens coral reefs in two ways. Increased carbon dioxide causes ocean acidification, which suppresses calcification so corals build skeletons more slowly. Higher water temperatures cause coral bleaching, when corals expel their symbiotic algae and may die. Loss of corals causes collapse of the whole reef ecosystem.
Why are species moving to higher altitudes and towards the poles?
Species are moving because warming makes the warm edge of their range too hot, while cooler places further upslope or poleward become suitable. Tropical montane birds in New Guinea have shifted upslope, and some North American trees show contraction at their warm edge and northward spread.
How does climate change disrupt the timing of biological events?
HL only Climate change disrupts timing when interacting species use different cues. Temperature cues shift with warming but photoperiod does not. Arctic mouse-ear chickweed now grows earlier in spring, but reindeer still arrive by photoperiod, so calves are born after peak food. Similarly great tit chicks hatch after the caterpillar peak.
Written and checked against the IB Biology HL specification · Updated October 2026