Anthropogenic causes of climate change

• Climate change in this topic is limited to anthropogenic increases in atmospheric carbon dioxide (CO₂) and methane (CH₄).

• These gases increase the warming effect of the atmosphere, so the key exam idea is that humans are driving the rise in these gases.

• Exam skills / NOS: distinguish positive correlation from negative correlation.

• Exam skills / NOS: distinguish correlation from causation.

• Example: Antarctic ice-core data shows a positive correlation between global temperature and atmospheric CO₂ over very long timescales, but correlation alone does not prove causation.

This diagram shows how warming can trigger further warming through linked feedback loops. It is ideal for revising positive feedback cycles, especially the idea that climate change can become self-amplifying. Source

Positive feedback and tipping points

• Positive feedback = a change causes further changes that amplify the original warming.

• Examples of positive feedback cycles in global warming:

  • release of CO₂ from the deep ocean

  • increased absorption of solar radiation after loss of reflective snow and ice

  • faster decomposition of peat and previously undecomposed organic matter in permafrost

  • release of methane from melting permafrost

  • increases in droughts and forest fires

• Tipping point = a threshold after which change becomes rapid or difficult to reverse.

• Boreal forests / taiga are a key tipping-point example.

• Warmer temperatures and decreased winter snowfall can increase drought and reduce primary production.

• This can cause forest browning, more frequent and intense fires, and legacy carbon combustion.

• Result: boreal forests may shift from net carbon accumulation to net carbon loss.

Habitat and ecosystem impacts

• Polar habitat change includes melting of landfast ice and sea ice.

• Emperor penguins may lose breeding grounds if landfast ice breaks out too early in the Antarctic.

• Walruses may lose important sea-ice habitat in the Arctic.

• Climate warming can alter ocean currents and reduce the timing/extent of nutrient upwelling.

• Warmer surface water can prevent nutrients reaching the surface, decreasing ocean primary production.

• Lower primary production means less energy flow through marine food chains.

• Climate change can also cause poleward and upslope range shifts.

• Required examples:

  • upslope range shifts in tropical montane birds in New Guinea

  • range contraction and northward spread in North American tree species

This image shows the strong decline in Arctic sea-ice extent, helping explain polar habitat change. It links directly to the syllabus examples of habitat loss affecting ice-dependent species. Source

Coral reefs and ecosystem collapse

• Coral reefs are a key example of possible ecosystem collapse.

• Increased CO₂ causes ocean acidification.

• Ocean acidification suppresses calcification in corals, so reef-building becomes harder.

• Increased water temperature causes coral bleaching.

• Loss of corals can lead to the collapse of entire reef ecosystems.

• Important responses that increase carbon sequestration:

  • afforestation

  • forest regeneration

  • restoration of peat-forming wetlands

• Peat-forming wetlands are important because peat stores carbon in waterlogged soils.

• NOS / debate: scientists debate whether plantations of non-native tree species or rewilding with native species is the better sequestration strategy.

This infographic summarizes the two core reef threats in the syllabus: ocean acidification and warming-driven bleaching. It is useful for linking abiotic change to loss of coral structure and then to reef ecosystem collapse. Source

This image represents the tawny owl, the syllabus example for evolution as a consequence of climate change. In exam answers, connect this organism to changing snow cover and shifts in the fitness of colour variants. Source