As discussed in Chapter 5, the changes in external forcing have to a large extent driven past climate variations. In order to “predict” the climate of the 21st century and beyond, it is thus necessary to estimate future changes in the forcing. This is achieved by the development of scenarios for the emission of greenhouse gases, aerosols, various pollutants in the atmosphere, land use, etc. These scenarios depend on many uncertain factors (as discussed below) and some of the uncertainties in the estimates of future climate changes are related to these factors (see Fig. 6.8). This is the reason why, in the scientific literature, the term climate projection is generally preferred to the term climate prediction, as it emphasises the fact that the results depend on the scenarios chosen and the hypothesis made in those scenarios. The scenarios are also used for analysing impact, adaptation and vulnerability, thus providing a consistent approach for socio-economic and climatic issues.
Various types of scenarios have been proposed in recent years and decades. In the forth assessment report of the IPCC, the climate projections were based on the SRES scenarios (Special Report on Emission Scenarios, see Section 6.1.2) which cover the whole of the 21st century. Those scenarios were derived in a sequential form (Fig.6.1). First, the main driving forces influencing the emissions from demographic, social and economic development have to be identified. This implies estimating population growth, future levels of economic activity, the way exchanges between different countries will be organised, the technology choices/opportunities of the countries, etc. On the basis of these estimates, some models produce scenarios for future emissions of greenhouse gases and aerosols, and for land-use changes. Different combinations of demographic and socio-economic change can lead to similar emission paths. For instance, large population growth combined with efficient technologies and renewable energy can lead to similar emissions to a smaller increase in the Earth’s population with less efficient and more energy-demanding technologies. In addition to the emission scenarios, the concentrations of greenhouse gases and aerosols in the atmosphere are also provided for models that do not include a representation of the carbon and/or aerosols cycle.
For the next IPCC assessment report (IPCC AR5), a slightly different approach was followed. Four representative concentration pathways (RCPs) were selected, covering a wide range of future changes in radiative forcing (see Section 6.1.3). The emissions (and concentrations) of greenhouse gases corresponding to these four RCPs were then provided to the climate-modelling community, so that they could perform climate projections. In parallel, possible socio-economic scenarios compatible with those RCPs were developed, providing different socio-economic alternatives for the same RCP. If needed, the information provided by the climate-model projections can be used in the socio-economic scenarios to assess the impact of climate change on society. Such a parallel approach strengthens the collaboration between the different communities, while ensuring that the climate-modelling groups only have to run a small set of well contrasted emission scenarios with their models (which are very demanding of computer time). Another advantage of the new scenarios is that they include both more detailed short term estimates (to about 2035) and stylised estimates to about 2300, in addition to the classical, long-term estimates up to 2100 provided by the SRES scenarios.
Neither the SRES nor the RCPs made any attempt to provide a best guess or to assess the likelihood of the various scenarios. Many elements of the scenarios are too unpredictable for this to be feasible. As a consequence, all the scenarios should be considered as reasonable possible and equally probable.
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