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

This study guide covers IB Diploma Programme Biology, section D4.3 (Climate change), as set out in the IB Biology guide for first assessment in 2025. Statements D4.3.1 to D4.3.8 are for both SL and HL. Statements D4.3.9 to D4.3.12 are additional higher level, so they are HL only. Use it with the revision notes and the practice questions. See also the IB DP Biology hub, the checklist and the free diagnostics.

What this unit covers

Guide reference	What you must be able to do	Level
D4.3.1	Human causes of rising carbon dioxide and methane; correlation versus causation	SL and HL
D4.3.2	Positive feedback cycles in warming	SL and HL
D4.3.3	Boreal forest switch from carbon sink to source (tipping point)	SL and HL
D4.3.4	Landfast ice and sea ice loss as polar habitat change	SL and HL
D4.3.5	Ocean currents, upwelling and marine production	SL and HL
D4.3.6	Poleward and upslope range shifts, with evidence	SL and HL
D4.3.7	Threats to coral reefs and reef collapse	SL and HL
D4.3.8	Afforestation, regeneration and peatland restoration for sequestration	SL and HL
D4.3.9	Phenology and its cues	HL only
D4.3.10	Loss of synchrony between populations	HL only
D4.3.11	Extra insect life cycles per year (spruce bark beetle)	HL only
D4.3.12	Evolution as a consequence of climate change (tawny owl)	HL only

D4.3.1 Anthropogenic causes of climate change

The guide limits this to human-driven rises in carbon dioxide and methane. Both are greenhouse gases: they absorb and re-emit some of the infrared radiation the warmed surface gives off, so less escapes to space.

• Carbon dioxide rises through burning fossil fuels and deforestation, which removes photosynthesizing biomass and releases stored carbon.
• Methane rises through livestock digestion, flooded rice paddies, landfill and fossil fuel leaks. Methanogens (anaerobic microorganisms) produce the methane from livestock, paddies and landfill.

Nature of science: correlation and causation. Ice cores from Antarctica show that temperature and carbon dioxide concentration rose and fell together over hundreds of thousands of years. That is a positive correlation. Two variables moving together does not by itself show that one drives the other: a third factor could drive both, or the direction could be reversed. The causal link rests on independent evidence, such as laboratory measurements of the gas absorbing infrared radiation.

Worked example 1. A monitoring station records carbon dioxide at 354 ppm in 1990 and 414 ppm in 2020. Methane rises from 1,715 ppb to 1,880 ppb over the same period. Compare the changes.

1. Carbon dioxide: 414 − 354 = 60 ppm. Percentage change = 60 ÷ 354 × 100 = 16.9%. Mean rate = 60 ÷ 30 = 2.0 ppm per year.
2. Methane: 1,880 − 1,715 = 165 ppb. Percentage change = 165 ÷ 1,715 × 100 = 9.6%.
3. The units differ (ppm and ppb), so compare percentage change: carbon dioxide rose by more.

D4.3.2 Positive feedback cycles in global warming

A positive feedback amplifies the original change. The guide lists these cycles:

1. Loss of reflective snow and ice. Warming melts ice and snow. Darker land or open water absorbs more solar radiation, so more heat is absorbed, which causes more melting.
2. Carbon dioxide from the deep ocean. Warmer water holds less dissolved gas, so more carbon dioxide escapes to the atmosphere.
3. Faster decomposition of peat and permafrost organic matter. Warming speeds up decomposers acting on organic matter that was previously frozen or waterlogged, releasing carbon dioxide.
4. Methane from melting permafrost. Thawed, waterlogged ground allows anaerobic breakdown that releases methane.
5. More droughts and forest fires. Fires release stored carbon and remove trees, so less carbon dioxide is taken in by photosynthesis.

Write each loop in order and return to the start: more warming, more of the process, more heat retained, more warming.

Worked example 2. Sunlight arriving at a polar surface is 200 W m⁻² (a figure supplied for the question). Fresh snow reflects 85% of it. Dark open sea reflects 10%. Compare the energy absorbed.

1. Snow absorbs 15% of 200 = 30 W m⁻².
2. Sea absorbs 90% of 200 = 180 W m⁻².
3. 180 ÷ 30 = 6 times as much. Replacing snow with sea increases heating, a positive feedback.

D4.3.3 Boreal forests and a tipping point

A tipping point is a threshold beyond which a system changes state and does not easily return. Boreal forest (taiga) has long stored more carbon than it released. Warmer temperatures and less winter snowfall increase drought. Drought lowers primary production and causes forest browning (leaf and needle loss). Fires then become more frequent and more intense. Fire also burns old carbon in deep litter and organic soil (legacy carbon combustion). When release exceeds uptake the forest becomes a net source of carbon.

Worked example 3. Net carbon balance of a boreal region (data supplied, g C m⁻² year⁻¹; positive means net gain): 1990s +45, 2000s +20, 2010s −15. Mean area burned rose from 1.2 to 2.7 million hectares per year.

1. Change in balance = −15 − 45 = −60 g C m⁻² year⁻¹.
2. The sign changed to negative: the region became a source.
3. Burned area increase = (2.7 − 1.2) ÷ 1.2 × 100 = 125%.

D4.3.4 Polar habitat change

Landfast ice is sea ice attached to the coast or sea floor. Emperor penguins (Aptenodytes forsteri) breed on it through the Antarctic winter. If it breaks out early, before chicks have waterproof plumage, chicks are lost. Sea ice in the Arctic is used by walruses as a platform for resting between dives and for nursing young. With less sea ice, walruses crowd onto land and must swim farther to feed, raising energy cost.

Worked example 4. A colony falls from 2,400 breeding pairs to 1,560 after early breakouts. Change = (1,560 − 2,400) ÷ 2,400 × 100 = −35%.

D4.3.5 Ocean currents and nutrient upwelling

In upwelling zones, currents bring cold, nutrient-rich deep water to the lit surface layer, where phytoplankton use the nutrients. Warmer surface water is less dense and forms a stable layer, which prevents the deep water rising. Surface nutrients run out, primary production falls, and so does energy flow to zooplankton, fish and their predators. Changed currents can also shift the timing and extent of upwelling away from when and where species depend on it.

Worked example 5. Primary production in an upwelling zone falls from 310 to 245 g C m⁻² year⁻¹ (supplied data). Change = (245 − 310) ÷ 310 × 100 = −21.0%. Less energy enters the food chain, and every higher trophic level receives less.

D4.3.6 Range shifts of temperate species

As climate warms, species track their preferred conditions. They move poleward (towards cooler latitudes) or upslope (towards cooler altitudes). Evidence: tropical montane birds in New Guinea have moved upslope, and North American trees show range contraction and northward spread.

Limits: summit species have nowhere higher to go, slow-dispersing trees may lag behind the climate, and fragmented habitat blocks movement.

Worked example 6. Mean elevation of a bird community is 1,850 m in 1965 and 2,010 m in 2015. A lapse rate of 6.5 °C per 1,000 m is supplied.

1. Shift = 2,010 − 1,850 = 160 m.
2. Rate = 160 ÷ 50 = 3.2 m per year.
3. Temperature change tracked = 160 ÷ 1,000 × 6.5 = 1.04 °C.

D4.3.7 Coral reefs and ecosystem collapse

Dissolved carbon dioxide forms carbonic acid, lowering ocean pH (ocean acidification). Lower carbonate availability suppresses calcification, so corals build skeletons more slowly. Raised water temperature causes bleaching: the coral expels its symbiotic zooxanthellae (algae), loses its main source of photosynthetic products and its colour, and may die. Reefs shelter many species, so coral loss causes collapse of the community.

Worked example 7. Surface pH falls from 8.20 to 8.10, and calcification falls from 1.8 to 1.3 mg CaCO₃ cm⁻² day⁻¹ (supplied data).

1. pH is logarithmic. A fall of 0.10 unit means hydrogen ion concentration rises by a factor of 10^0.10 = 1.26, an increase of about 26%.
2. Calcification change = (1.3 − 1.8) ÷ 1.8 × 100 = −27.8%.

D4.3.8 Carbon sequestration approaches

• Afforestation: planting trees where there was no recent forest, fixing carbon in wood and soil.
• Forest regeneration: letting cleared land regrow naturally.
• Peat-forming wetland restoration: rewetting drained peatland. Waterlogging limits oxygen, so decomposers work slowly and plant remains accumulate as peat. This happens in temperate and boreal zones and, very quickly, in some tropical ecosystems.

Nature of science: scientists debate whether plantations of non-native trees or rewilding with native species is better. Plantations may sequester quickly but support fewer species. Native regeneration supports more species but may be slower.

Worked example 8. Supplied rates, t C ha⁻¹ year⁻¹: plantation 3.2, native regeneration 2.4, restored peat 0.5. Over 30 years on 50 ha: plantation 3.2 × 50 × 30 = 4,800 t C; native 2.4 × 50 × 30 = 3,600 t C; peat 0.5 × 50 × 30 = 750 t C. Peat gains are small per year, but rewetting also protects the carbon already stored, so rate alone does not settle the evaluation.

D4.3.9 Phenology (HL only)

Phenology is the study of when biological events happen. Examples are flowering, buds bursting and bud set in deciduous trees, migration of birds, and nest building. Cues include photoperiod (day length) and temperature patterns. Photoperiod at a place does not change with climate; temperature does.

Worked example 9. Budburst advances by 4 days per °C of spring warming (supplied). Spring warms by 1.5 °C: 4 × 1.5 = 6 days earlier.

D4.3.10 Disruption to synchrony (HL only)

One population may use temperature as its cue while another uses photoperiod. When temperature changes, only the first shifts its timing, so interdependent events separate. Two examples in the guide: Arctic mouse-ear chickweed (Cerastium arcticum) growth in spring and the arrival of migrating reindeer (Rangifer tarandus), and great tit (Parus major) breeding against peak caterpillar biomass in north European forests (or a suitable local example).

Worked example 10. Caterpillar biomass peaks on day 150 in the 1980s and day 138 in the 2010s. Chick demand peaks on day 152, then day 144.

1. Gap, 1980s: 152 − 150 = 2 days. Gap, 2010s: 144 − 138 = 6 days.
2. Food moved 12 days earlier, birds 8, so synchrony weakened by 4 days, and chicks hatch after the food peak.

D4.3.11 More insect life cycles in a year (HL only)

Insects are ectotherms, so development speeds up with temperature, and a longer warm season can allow an extra generation each year. In the spruce bark beetle (Ips typographus), an extra generation multiplies numbers again, so outbreaks and damage to spruce become more likely.

Worked example 11. Simplified model: each female gives 20 surviving daughters per generation. One generation: ×20. Two generations: 20 × 20 = 400. The extra generation raises the yearly increase 20 times.

D4.3.12 Evolution as a consequence of climate change (HL only)

Climate change alters selection pressures. Tawny owls (Strix aluco) have grey and brown colour variants. Snow cover affects how well each is camouflaged and how well each survives. With less snow, the fitness of the brown variant rises relative to the grey, and the brown allele becomes more common in the gene pool: evolution by natural selection.

Worked example 12. Supplied winter survival, snowy winters: grey 0.74, brown 0.58. Relative fitness: grey 1.00; brown 0.58 ÷ 0.74 = 0.78. Mild winters: grey 0.70, brown 0.72, so grey = 0.70 ÷ 0.72 = 0.97 and brown 1.00. The advantage shifts to brown as snow declines.

Common errors

• Calling a correlation proof of causation, or the reverse.
• Describing a feedback without closing the loop.
• Saying individuals “adapt” to warming. Populations change over generations through selection.
• Mixing up acidification (carbon dioxide) with bleaching (temperature).

Official syllabus

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