The IPCC’s Representative Concentration Pathways

For the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), new development scenarios were created. These “Representative Concentration Pathways” (RCPs) describe anticipated radiative forcings, i.e. the sum of the climate-related disturbances in the atmosphere, in watts per square meter. Model calculations translate the forcing into climatic changes evoked by them and reveal which greenhouse gas emissions they are based on.

Four of these RCPs are taken into account in the Fifth Progress Report: RCP2.6 (relatively low radiative forcing) RCP4.5 (mean radiative forcing) RCP6.0 (high radiative forcing) and RCP8.5 (very high radiative forcing).

The German Climate Computing Centre’s earth system model MPI-ESM helped to investigate how these scenarios affect global temperatures, sea ice melt, the sea level or acidification and carbonate saturation.

According to these calculations, the global mean temperature would increase by about 4.8 degrees Celsius compared to pre-industrial conditions by the year 2100 in the scenario RCP8.5. Only the scenario RCP2.6 would keep the average global temperature rise below the 2 degrees Celsius. To achieve this goal, global carbon dioxide emissions would have to remain at current levels until 2020 and decline to about 400 ppm (parts per million) by 2100. The particular sensitivity of tropical coral reefs requires such drastic measures: About fifty per cent of coral reefs can be obtained if the temperature rise would be limited to about 1.2 degrees Celsius.

Animation: Temperature changes in the RCP8.5 scenario:

Ocean acidification could be limited considerably with the medium scenario RCP4.5 and stabilize at a pH of slightly more than 7.9 – which still would be about 0.15 units more acidic than today. RCP2.6 suggests a decrease after 2050. However, the ocean reacts so slowly that 200 additional years would pass until today’s conditions would be restored.

Animation: Ocean acidification in the RCP8.5 scenario: