D4.3 Climate changeIB Biology SL: Revision notes

Human (anthropogenic) activities are increasing atmospheric carbon dioxide (burning fossil fuels, deforestation) and methane (ruminant livestock, rice paddies, landfill, leaks from gas extraction). Both are greenhouse gases, and limiting their rise is the main way to limit warming.

Antarctic ice cores show a positive correlation between carbon dioxide and global temperature over hundreds of thousands of years: both rise and fall together. A negative correlation would mean one falls as the other rises. Correlation is not causation: the ice-core data alone do not prove that carbon dioxide raises temperature, although other evidence (carbon dioxide absorbs infrared radiation) confirms the causal link.

A positive feedback reinforces the change that caused it. Examples in global warming:

• release of carbon dioxide from the deep ocean as it warms;
• loss of reflective snow and ice, so more solar radiation is absorbed;
• faster decomposition of peat and of organic matter in thawing permafrost;
• release of methane from melting permafrost;
• more droughts and forest fires.

A tipping point is reached when a system changes to a new state that sustains itself. In boreal forest (taiga), warmer temperatures and less winter snow cause drought, reduced primary production, forest browning and more frequent, intense fires that burn legacy carbon stored for centuries. The forest switches from net carbon accumulation to net carbon loss.

Melting of landfast ice and sea ice destroys polar habitats. Emperor penguins (Aptenodytes forsteri) breed on landfast ice; early break-out before chicks have waterproof feathers causes breeding failure. Walruses in the Arctic rest on sea ice above feeding grounds; as it retreats they must haul out on crowded beaches far from their food.

Changes in ocean currents alter the timing and extent of nutrient upwelling. Warmer surface water forms a stable layer that prevents nutrient-rich deep water rising, which decreases primary production by phytoplankton and so reduces energy flow through marine food chains.

As temperatures rise, temperate species show poleward and upslope range shifts. Tropical montane birds in New Guinea have moved upslope; many North American tree species show range contraction and northward spread.

Higher carbon dioxide dissolves in sea water, causing ocean acidification, which suppresses calcification (deposition of calcium carbonate) by corals. Higher water temperatures cause coral bleaching: corals expel their symbiotic algae (zooxanthellae) and may starve. Corals build the reef structure that other species depend on, so loss of corals causes collapse of the reef ecosystem.

Carbon can be removed from the atmosphere and stored by afforestation (planting trees on land that was not forest), forest regeneration and restoring peat-forming wetlands. Peat forms in waterlogged soils where decomposition is slow, in temperate and boreal zones and very rapidly in some tropical ecosystems.

There is active scientific debate over whether plantations of non-native trees (fast uptake, but low biodiversity and possible vulnerability) or rewilding with native species (slower, but more diverse and resilient) is the better approach.