Understanding climate change projections

As outlined in the Intergovernmental Panel on Climate Change's Fifth Assessment Report, greenhouse gases, such as carbon dioxide, have a warming effect on global climate.

These gases absorb heat that would otherwise be lost to space, and re-radiate it back into the atmosphere and to the Earth’s surface. The IPCC concluded that it was extremely likely that more than half of the observed increase in global average surface air temperature from 1951–2010 has been caused by the anthropogenic increase in greenhouse gas emissions and other anthropogenic forcings. Further increases in greenhouse gas concentrations resulting primarily from burning fossil fuel will lead to further warming, as well as other physical and chemical changes in the atmosphere, ocean and land surface.

Climate change projections give the climate response to a set of greenhouse gas, aerosol and land-use scenarios that are consistent with socio-economic assumptions of how the future may evolve. These scenarios are known as the Representative Concentration Pathways (RCPs).

In the Fifth Assessment Report, the IPCC concluded that global mean surface air temperatures for 2081–2100 relative to 1986–2005 are likely to be in the following ranges: 0.3 to 1.7 °C warmer for RCP2.6 (representing low emissions); 1.1 to 2.6 °C and 1.4 to 3.1 °C warmer for RCP4.5 and RCP6.0 respectively (representing intermediate emissions); and 2.6 to 4.8 °C warmer for RCP8.5 (representing high emissions).

Regional climate change projections, including those presented on this website, are based on up to 40 CMIP5 global climate model simulations driven by four emission scenarios. However, the projections should be viewed in the context of the confidence ratings that are provided, which consider a broader range of evidence than just the model outputs.

Ranges of projected climate change and confidence in projections

Quantitative projections of future climate change presented as ranges. This allows for differences in how future climate may evolve due to three factors – greenhouse gas and aerosol emissions, the climate response and natural variability:

Future emissions during the 21st century cannot be known precisely and are dealt with here by examining several different scenarios. The choice of how many and which scenarios to examine is dependent on the decision-making context.

The response of the climate system to past and future forcing is underpinned by observational and model studies of temperature change, climate feedbacks and changes in the Earth’s energy budget. The IPCC concluded that the net feedback from the combined effect of changes in water vapour, and differences between atmospheric and surface warming is extremely likely positive and therefore amplifies changes in climate. The net radiative feedback due to all cloud types combined is likely positive. The equilibrium climate sensitivity quantifies the response of the climate system to constant radiative forcing on multi-century time scales. It is defined as the change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO2 concentration. Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C. Changes to atmospheric circulation in a warmer climate are one of the biggest uncertainties regarding the regional climate response. Climate models have some known regional biases that affect confidence.

Natural variability (or ‘internal variability’ within the climate system) can dominate over the ‘forced’ climate change in some instances, particularly over shorter time frames and smaller geographic areas. The precise evolution of climate due to natural variability (e.g. the sequence of wet years and dry years) cannot be predicted (IPCC, 2013, Chapter 11). However, the projections presented here allow for a range of outcomes due to natural variability, based on the different evolutions of natural climatic variability contained within each of the climate model simulations.

The relative importance of each of these three factors differs for each variable, different timeframes and spatial scale. For some variables with large natural variability, such as rainfall, the main source of differing projections in the next 2-3 decades is likely to be natural variability rather than differences in emission scenarios (the influence of which becomes more important as greenhouse gas concentrations increase). In addition, unpredictable events, such as large volcanic eruptions, could influence climate over the century. See the Fifth Assessment Report (IPCC, 2013) Chapter 11 for further discussion of these issues.

Confidence in a projected change is assessed based on the type, amount, quality and consistency of evidence (which can be process understanding, theory, model output, or expert judgment) and the extent of agreement amongst the different lines of evidence. Note that although confidence may be high in the direction of change, in some cases confidence in magnitude of change may be medium or low (e.g. due to some known model deficiency). More information on the method used to assess confidence in the projections presented on this website is provided in Section 6.4 of the Climate Change in Australia Technical Report.

Further information (external links)