Box 28 CO2 stabilisation and global mean temperature response

Global mean annual temperature (GMAT) is the metric most commonly employed by the IPCC and adopted in the international policy arena to summarise future changes in global climate and their likely impacts (see Chapter 19, Box 19.2). Projections of global mean warming during the 21st century for the six SRES illustrative scenarios are presented by WG I (Meehl et al., 2007) and summarised in Figure 2.8. These are baseline scenarios assuming no explicit climate policy (see Box 2.2). A large number of impact studies reported by WG II have been conducted for projection periods centred on the 2020s, 2050s and 2080s10, but only best estimates of GMAT change for these periods were available for three SRES scenarios based on AOGCMs (coloured dots in the middle panel of Figure 2.8). Best estimates (red dots) and likely ranges (red bars) for all six SRES scenarios are reported only for the period 2090-2099. Ranges are based on a hierarchy of models, observational constraints and expert judgement (Meehl et al., 2007).

A more comprehensive set of projections for these earlier time periods as well as the 2090s is presented in the lower panel of Figure 2.8. These are based on a simple climate model (SCM) and are also reported in WG I (Meehl et al., 2007, Figure 10.26). Although SCM projections for 2090-2099 contributed to the composite information used to construct the likely ranges shown in the middle panel, the projections shown in the middle and lower panels should not be compared directly as they were constructed using different approaches. The SCM projections are included to assist the reader in interpreting how the timing and range of uncertainty in projections of warming can vary according to emissions scenario. They indicate that the rate of warming in the early 21st century is affected little by different emissions scenarios (brown bars in Figure 2.8), but by mid-century the choice of emissions scenario becomes more important for the magnitude of warming (blue bars). By late century, differences between scenarios are large (e.g. red bars in middle panel; orange and red bars in lower panel), and multi-model mean warming for the lowest emissions scenario (B1) is more than 2°C lower than for the highest (A1FI).

GHG mitigation is expected to reduce GMAT change relative to baseline emissions, which in turn could avoid some adverse impacts of climate change. To indicate the projected effect of mitigation on temperature during the 21st century, and in the

10 30-year averaging periods for model projections held at the IPCC Data Distribution Centre.

absence of more recent, comparable estimates in the WG I report, results from the Third Assessment Report based on an earlier version of the SCM are reproduced in the upper panel of Figure 2.8 from the Third Assessment Report. These portray the GMAT response for four CO2-stabilisation scenarios by three dates in the early (2025), mid (2055), and late (2085) 21st century. WG I does report estimates of equilibrium warming for CO2-equivalent stabilisation (Meehl et al., 2007)11. Note that equilibrium temperatures would not be reached until decades or centuries after greenhouse gas stabilisation.

Global mean annual temperature change relative to 1980-1999 (°C) 12 3 4

COg stabilisation: TAR

450 ppm

550 ppm

650 ppm

750 ppm

SRES: AR4 WG1 multiple sources B1 •

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