Chapter 12 of the 5th Assessment Report of IPCC (Intergovernmental Panel on Climate Change) deals with future projections of climate change in the long-term. Long-term in this case means beyond the middle of the 21st century when the projections start to depend more strongly on the pathway or scenario of emissions of greenhouse gases, principally CO2. We rely extensively on computer models of the climate system for this as, obviously, there are no observations of the future. While computer models are not completely accurate in their ability to project future climate change, it is possible to use our understanding of the climate system to assess uncertainties in the projections.
The new IPCC report (AR5) uses a new set of emissions scenarios called the Representative Concentration Pathways (RCPs). The scenario with the lowest amount of change, RCP2.6, assumes significant mitigation actions by the world to reduce CO2 emissions. Under this scenario, there is a chance for the change in global mean surface temperatures to remain below 2°C warming since pre-industrial times. We only have medium confidence in this statement as some model simulations do show warming above 2°C for even this aggressive mitigation scenario.
The scenario with the most warming is the ‘business-as-usual’ RCP8.5, in which global mean temperature could be 4°C or more above pre-industrial times. Perhaps these don’t sound like big numbers, but the regional changes which go with the global warming could be very much greater. For this highest scenario, it is likely that we will see an Arctic Ocean that is virtually free of sea-ice in summer by the middle of the 21st century (a headline result of the Summary for Policy Makers – figure SPM.7(b)). We will see more warm days and fewer cold nights with a change in frequency that depends on the global mean temperature change. There will be melting of snow, a loss of up to 25% by the end of the century under RCP8.5, and of mountain glaciers, up to 85% mass loss. Sea level rise under this scenario will be between a half and one meter with an acceleration of the rate of rise at the end of the century.
Global rainfall patterns are likely to be affected by climate change but in a highly non-uniform way. Some regions will experience an increase in rainfall, some a decrease and some regions will experience no change at all. Approximately speaking, the contrast between regions of high rainfall and regions of low rainfall will increase, as will the contrast between wet and dry seasons. But there will be regional exceptions. Rainfall patterns will be affected by changes in atmospheric circulation, such as the weakening and poleward expansion of the Hadley Circulation.
The regional impacts of climate change depend, to first order, on the amount of global warming. That amount of warming depends on the amount of CO2 that is emitted. For the first time the IPCC has made an assessment of the carbon budget that would be required to stay below, for example, a 2°C target level of global warming. This amounts to a total emission of around 800 Giga Tonnes (Gt) of carbon since the beginning of the industrial revolution when taking into account all greenhouse gases (a Gt expressed in grams is 1 followed by 15 noughts). This sounds like a lot, until you realise that about 500 Gt of carbon has already been emitted today.