Climate Change Modelling and Projections

Researchers use general circulation models (otherwise known as global climate models or GCMs) to examine future global and regional climate change., GCMs are built using mathematical representations of the dynamic Earth system and are based on the laws of physics. These models represent, in three dimensions, the large-scale circulations of the atmosphere and ocean, such as the progression of high and low pressure systems and large-scale oceanic currents. Models also include the cryosphere (snow and sea ice) as well as the land surface.

Climate models help with understanding our present weather and climate, and also allow us to consider plausible future scenarios of how the climate might change. They generate simulations to tell us what happened or what might happen under a range of different scenarios, such as for greenhouse gas concentrations .

Although models used for weather prediction and climate applications share the same fundamentals, climate models predict the “statistics” of the weather, such as the average conditions, over a season or trends over decades, rather than the exact “weather” on any particular day.

There are now over 40 GCMs run around the world. Modelling groups use a common set of GHG and aerosol scenarios, called Representative Concentration Pathways (RCPs). This co-ordinated approach permits ready comparison of projections across the many model simulations for which data are available.

To account for future anthropogenic greenhouse gas and aerosol emissions, modellers have developed a number of possible emission futures called Representative Concentration Pathways (RCPs). These RCPs predict the enhanced greenhouse effect – dependant on how people, business and countries change their emission practices.

  • RCP 8.5 (high) – There is little curbing of emissions, i.e. people continue business as normal and greenhouse gas emissions continue to rise over the 21st century without abatement, with a decline in aerosols.
  • RCP 6  and 4.5 (intermediate) – Some activities, strategies and technologies have been utilised to reduce emissions. Greenhouse gas emissions peak then decline.
  • RCP 2.6 (low) – This is the most ambitious mitigation scenario. Such a pathway would require early participation from all emitters, including developing countries, as well as the application of technologies for actively removing carbon dioxide from the atmosphere Greenhouse gas emissions peak quickly and then decline rapidly to very low values.

The estimated carbon emissions and the corresponding radiative forcing for the four RCPs are shown in the figure below:

"(A)

(A) Emissions of carbon in gigatoms for the different RCP scenarios. *numbers show the atmospheric CO2 equivalent levels in parts per million (PPM). (B) Radiative forcing. The numbers on the right hand axis show the final forcing (W/m2). Drawn from data available at http://pik-potsdam.de/~mmalte/rcps/, cited in CSIRO and Bureau of Meteorology (2015).

Climate models are then run for each for these different RCP  scenarios to give the range of predictions for things like average rainfall, temperate and sea level rise. Planning needs to be adaptable to take into account future variability rather than just concentrating on best and worst case scenarios.

This video, produced by CSIRO, explains how GCMs are used to forecast weather and project future climate:

Watch this CSIRO video to find out why GHG emissions are a major contributor to climate change and how different emission scenarios are used to take this into consideration for projecting future climate: