6.2 Climate change — causes and impactsIB Environmental Systems and Societies SL: Revision notes

Climate describes the typical conditions (temperature, rainfall, wind) produced by physical processes in the atmosphere, averaged over a long period (usually 30 years or more). Weather is the state of the atmosphere at a particular time and place.

Anthropogenic carbon dioxide emissions, mainly from burning fossil fuels and from deforestation, have raised atmospheric CO2 from about 280 ppm before industrialisation to about 420 ppm today. The global rate of emission has accelerated, particularly since 1950.

Evidence for the link with temperature comes from ice cores (trapped air bubbles and isotopes), tree rings and deposited sediments. These show a positive correlation between CO2 concentration and global temperature. Correlation alone does not prove cause, but combined with the physics of the greenhouse effect it is strong evidence.

The natural greenhouse effect has been enhanced by anthropogenic emissions of GHGs (CO2, methane, N2O). More infrared radiation is trapped, which causes global warming and so climate change.

Systems diagrams and models represent cause and effect with feedback loops and changes to the global energy balance:

• Positive feedback amplifies change: melting ice lowers albedo, more energy is absorbed, more ice melts; thawing permafrost releases methane and CO2.
• Negative feedback reduces change: more CO2 can increase photosynthesis, removing some CO2 (limited by water and nutrients).

Climate change affects ecosystems from the local to the global scale. It changes temperature, rainfall and seasons, so species must adapt, move or die out. Effects include:

• Biome shifts: boreal forest and shrubs spread north into Arctic tundra, squeezing tundra specialists such as the Arctic fox.
• Coral bleaching: warmer seas cause corals to expel their algae; mass bleaching events have repeatedly struck the Great Barrier Reef.
• Changes to the resilience of ecosystems (their ability to withstand or recover from disturbance). Stress from warming reduces resilience, and thresholds can be crossed.

Impacts occur at a variety of scales (household, local, national, global) and depend on socio-economic conditions, which affect the resilience of societies.

Impacts include sea-level rise and flooding, droughts and crop failure, extreme weather, spread of disease and loss of livelihoods and migration. Tuvalu, with a highest point of about 4.6 m, faces flooding and saltwater intrusion and has little money to adapt. Wealthier countries such as the Netherlands can afford extensive sea defences, so their societies are more resilient.

The planetary boundaries framework identifies limits within which humanity can operate safely. For climate, the proposed boundary is 350 ppm atmospheric CO2. Evidence suggests that the Earth has already passed the planetary boundary for climate change.

Perspectives on climate change differ among individuals and societies because of values, beliefs, education, culture, economic interests, political views and direct experience of impacts:

• Technocentric: technology and innovation can solve the problem.
• Ecocentric: nature has intrinsic value; cut consumption and emissions strongly.
• Anthropocentric: manage impacts to protect human welfare, balancing costs.