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

Climate is the typical conditions that result from physical processes in the atmosphere (averaged over decades), unlike weather. Anthropogenic CO2 emissions have raised atmospheric CO2 significantly, with the global rate accelerating particularly since 1950.

Ice cores, tree rings and sediments show a positive correlation between CO2 and global temperature. The enhanced greenhouse effect from human emissions has led to global warming and therefore climate change.

Systems diagrams and models represent cause and effect, with positive and negative feedback loops and changes in the global energy balance.

Climate change affects ecosystems at local to global scales, changes their resilience and causes biome shifts. On the Great Barrier Reef, marine heatwaves cause mass bleaching (2016, 2017, 2020, 2022); repeated events reduce resilience.

Societies are affected at a range of scales and socio-economic conditions, which influence their resilience: the 2022 Pakistan floods hit poorer rural households hardest.

Evidence suggests the Earth has already passed the planetary boundary for climate change (proposed 350 ppm CO2; concentrations are above 420 ppm). This increases risk but does not make action pointless: impacts scale with warming.

Perspectives on climate change differ between individuals and societies because of values, beliefs, culture, education, economic interests and direct experience. A technocentric view relies on innovation; an ecocentric view stresses limits and the intrinsic value of nature.

(HL) Long-term data come from weather stations, observatories, radar and satellites, including records of temperature and GHG concentrations (direct measurements). Indirect (proxy) measurements include isotope ratios in ice cores, dendrochronology (tree rings) and pollen in peat cores.

(HL) Global climate models use equations to represent the processes and interactions that drive climate; inputs are manipulated to predict outputs. Their validity is tested by hindcasting: running them with past conditions and comparing results with records. Models use different scenarios of future emissions and socio-economic change to predict possible impacts, so results show a range of outcomes.

(HL) Models show the Earth may approach a critical threshold and move to a new equilibrium; local systems also have tipping points (for example the Amazon turning to savanna, a reef shifting to algal dominance). Individual tipping points may interact to create tipping cascades.

(HL) Countries vary in their responsibility for climate change and in their vulnerability: the least responsible are often the most vulnerable (for example Pakistan against the United States). This has political and economic implications and raises issues of equity, such as demands for loss and damage finance.