IB DP Biology: Climate change: causes, feedbacks, ecological impacts and phenology (DP Biology) – Revision Notes

These revision notes condense IB Diploma Programme Biology section D4.3 (Climate change) from the IB Biology guide, first assessment 2025. D4.3.1 to D4.3.8 are SL and HL. D4.3.9 to D4.3.12 are HL only. For full explanations see the study guide, then test yourself with the practice questions. Course pages: IB DP Biology hub, checklist and free diagnostics.

Key terms

Term	Meaning
Anthropogenic	Caused by human activity
Greenhouse gas	Gas that absorbs and re-emits infrared radiation, reducing heat loss to space
Positive feedback	A change that causes further change in the same direction
Tipping point	Threshold beyond which a system shifts to a new state that is hard to reverse
Albedo	Fraction of incoming sunlight a surface reflects
Upwelling	Rise of cold, nutrient-rich deep water to the surface
Calcification	Deposition of calcium carbonate to build shells and skeletons
Bleaching	Loss of symbiotic algae (zooxanthellae) from coral
Carbon sequestration	Long-term removal of carbon dioxide from the air into stores
Phenology (HL only)	Study of the timing of biological events
Synchrony (HL only)	Matching in time of linked events, such as food supply and offspring demand

D4.3.1 Causes

• Carbon dioxide: burning of coal, oil and gas; clearing of forest.
• Methane: cattle and sheep, flooded rice fields, waste tips, gas leaks.
• Ice cores show a positive correlation between temperature and carbon dioxide. A pattern of shared change is not proof of cause; the causal link needs other evidence.

Method: evaluating a correlation claim

1. State the pattern (positive or negative).
2. Say what is not shown (cause and direction).
3. Name what extra evidence would help (laboratory absorption data, known emissions, a mechanism).

D4.3.2 Positive feedback loops

Trigger	Loop (ends by increasing warming again)
Ice and snow melt	Darker surface absorbs more sunlight, so more melting
Warmer ocean	Less gas stays dissolved, so more carbon dioxide released
Peat and permafrost carbon warm up	Faster decomposition releases carbon dioxide
Permafrost melts	Anaerobic decay releases methane
Drought and fire	Carbon released and trees lost, so less carbon dioxide taken up

D4.3.3 to D4.3.5 Ecosystem impacts

Boreal forest tipping point: warmer temperatures and less winter snow lead to drought, lower primary production, browning, and more frequent and intense fires. Fires burn legacy carbon in old litter and soil. Net carbon accumulation turns into net loss.

Polar habitats:

• Emperor penguin (Aptenodytes forsteri): early breakout of landfast ice removes the breeding platform before chicks are ready.
• Walrus: loss of Arctic sea ice removes resting and nursing platforms.

Upwelling chain: warmer, less dense surface water stratifies, so deep nutrients do not rise, so phytoplankton production falls, so less energy reaches higher trophic levels.

Linking tip: the polar, ocean and forest changes all affect energy flow and population size, so connect each to a food chain or to carbon storage when a question asks about impact on an ecosystem.

D4.3.6 Range shifts

• Direction: poleward and upslope.
• Evidence: upslope shifts of montane birds in New Guinea; North American trees contracting in range and spreading north.
• Limits: summits, slow dispersal, fragmentation, loss of linked species.

Why the shift is predictable: species have a tolerable temperature range. As the local climate warms, the cooler edge of that range lies higher up the mountain or farther from the equator, and populations survive and reproduce better there. The warm edge of the old range becomes unsuitable, so the range contracts.

D4.3.7 Coral reefs

Cause	Effect
Carbon dioxide dissolves, pH falls (ocean acidification)	Calcification suppressed
Water temperature rises	Bleaching: algae expelled, coral starves
Corals lost	Habitat and food web lost, so reef ecosystem collapses

Why collapse rather than gradual decline: corals build the physical structure and supply food through their algae. Remove them and shelter, nursery habitat and the base of the reef food web go together.

D4.3.8 Sequestration approaches

Approach	Strength	Limitation
Afforestation	Fast carbon fixation	Plantations may have low biodiversity
Forest regeneration	Native species and structure	Can be slower
Peat-forming wetland restoration	Waterlogging slows decomposition and protects stored carbon	Low annual uptake; needs water

The debate on non-native plantations versus native rewilding is a nature of science point: both views use evidence, and the answer depends on the aim and the site.

HL only: D4.3.9 to D4.3.12

• Phenology cues: photoperiod and temperature patterns; events include flowering, budburst, bud set, migration and nesting.
• Synchrony loss: one population uses temperature, another photoperiod. Examples: Arctic mouse-ear chickweed (Cerastium arcticum) and reindeer (Rangifer tarandus) arrival; great tit (Parus major) breeding and peak caterpillar biomass.
• Spruce bark beetle (Ips typographus): warmth speeds development, so more generations a year and larger outbreaks.
• Tawny owl (Strix aluco): less snow changes the relative fitness of colour variants; selection changes allele frequencies.

Worked reminders

Percentage change = (new − old) ÷ old × 100. Carbon dioxide from 280 to 420 ppm (supplied): 140 ÷ 280 × 100 = 50%.

pH scale: each unit is a tenfold change. A fall of 0.2 unit raises hydrogen ion concentration by 10^0.2 = 1.58 times.

Albedo: 150 W m⁻² arrives. Ice reflecting 90% absorbs 15 W m⁻²; dark water reflecting 10% absorbs 135 W m⁻², which is 9 times as much.

Elevation shift: 3 m per year for 30 years = 90 m. With a lapse rate of 6.5 °C per 1,000 m (supplied), 90 m is about 0.59 °C of temperature change.

Mismatch gap (HL only): food peak day 150, demand peak day 152, so the gap is 2 days. If the food peak moves to day 138 and demand to day 144, the gap becomes 6 days. Report the direction as well as the size: the birds moved less than their food.

Relative fitness (HL only): divide each survival value by the highest one. Grey 0.74 and brown 0.58 give brown a relative fitness of 0.58 ÷ 0.74 = 0.78.

Answer frames

Explain a feedback loop (3 to 4 marks). Name the trigger, state the physical or biological change, state the effect on greenhouse gas or absorbed heat, then say that this increases warming further.

Explain an ecological impact. Link three things: the climate variable (temperature, pH, ice cover), the physiological or ecological effect (breeding fails, calcification slows, primary production falls), and the consequence for the population or ecosystem.

Evaluate a carbon sequestration approach. Give the mechanism, one strength, one limit, and a judgement that depends on the site and the aim.

Interpret a graph or table.

1. Give the start and end values, with units.
2. Calculate the change, and the percentage change if useful.
3. Describe the trend, then give the biological reason.
4. State a limit of the data, such as a short time period or a correlation only.

Must-know distinctions

• Acidification (carbon dioxide, pH) versus bleaching (temperature, algae lost).
• Landfast ice (attached to the coast) versus sea ice (floating pack).
• Afforestation (new forest) versus regeneration (regrowth of cleared land).
• Cue (what triggers timing) versus resource peak (what the timing must match). HL only.
• Adaptation of individuals versus evolution of a population over generations. HL only.

Quick self-test

1. Name two anthropogenic sources of methane.
2. Why does an ice-core correlation not prove causation?
3. Complete the loop: permafrost thaws, then what?
4. What is legacy carbon combustion?
5. Primary production falls from 8.0 to 6.4 units. Calculate the percentage change.
6. Why does a warmer ocean surface reduce primary production?
7. Why can a summit species not shift upslope?
8. Which gas causes ocean acidification, and what does it suppress in corals?
9. State two benefits of restoring peat-forming wetlands.
10. (HL only) Give one cue used for the timing of budburst.
11. (HL only) Why can warming increase the number of bark beetle generations?
12. (HL only) How does less snow affect tawny owl variants?

Answers

1. Livestock digestion, rice paddies, landfill or fossil fuel leaks (any two).
2. Two variables can change together because of a third factor, or in the reverse direction; extra evidence is needed.
3. Anaerobic decomposers release methane, which warms the air and melts more permafrost.
4. Burning of old carbon stored in deep litter and organic soil during fires.
5. (6.4 − 8.0) ÷ 8.0 × 100 = −20%.
6. Stratified warm water blocks upwelling, so surface nutrients run out.
7. There is no higher habitat, so the population contracts or is lost.
8. Carbon dioxide; calcification.
9. Carbon stored in peat stays protected, and waterlogging slows decomposition; the habitat also supports wetland species.
10. Temperature pattern (photoperiod is another).
11. Faster development at higher temperature lets an extra life cycle fit into the season.
12. The brown variant’s fitness rises relative to the grey, so it becomes more common.

Where marks are usually lost

• Describing a feedback without showing that it reinforces warming.
• Saying “carbon dioxide traps heat” without absorption and re-emission of infrared radiation.
• Treating a link between two variables as proof of cause.
• Mixing up acidification and bleaching.
• Giving only the cause of a range shift, not the evidence asked for.
• Working out percentage change from the end value rather than the start value.
• Naming an organism but not linking its response to the specific change.
• (HL only) Saying owls or insects “adapt” individually instead of describing selection across generations.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, International Baccalaureate Organization (section D4.3).