Observed changes in global mean surface air temperature since 1950 (from three major databases, as anomalies relative to 1961–1990) are shown in Figure 1.4. As in the prior assessments, global climate models generally simulate global temperatures that compare well with observations over climate timescales (Section 9.4). Even though the projections from the models were never intended to be predictions over such a short timescale, the observations through 2012 generally fall within the projections made in all past assessments. The 1990– 2012 data have been shown to be consistent with the FAR projections (IPCC, 1990), and not consistent with zero trend from 1990, even in the presence of substantial natural variability (Frame and Stone, 2013).
The scenarios were designed to span a broad range of plausible futures, but are not aimed at predicting the most likely outcome. The scenarios considered for the projections from the earlier reports (FAR, SAR) had a much simpler basis than those of the Special Report on Emission Scenarios (SRES) (IPCC, 2000) used in the later assessments. For example, the FAR scenarios did not specify future aerosol distributions. AR4 presented a multiple set of projections that were simulated using comprehensive ocean–atmosphere models provided by CMIP3 and these projections are continuations of transient simulations of the 20th century climate. These projections of temperature provide in addition a measure of the natural variability that could not be obtained from the earlier projections based on models of intermediate complexity (Cubasch et al., 2001).
Note that before TAR the climate models did not include natural forcing (such as volcanic activity and solar variability). Even in AR4 not all models included natural forcing and some also did not include aerosols. Those models that allowed for aerosol effects presented in the AR4 simulated, for example, the cooling effects of the 1991 Mt Pinatubo eruption and agree better with the observed temperatures than the previous assessments that did not include those effects.
The bars on the side for FAR, SAR and TAR represent the range of results for the scenarios at the end of the time period and are not error bars. In contrast to the previous reports, the AR4 gave an assessment of the individual scenarios with a mean estimate (cross bar; ensemble mean of the CMIP3 simulations) and a likely range (full bar; –40% to +60% of the mean estimate) (Meehl et al., 2007).
In summary, the trend in globally averaged surface temperatures falls within the range of the previous IPCC projections. During the last decade the trend in the observations is smaller than the mean of the projections of AR4 (see Section 9.4.1, Box 9.2 for a detailed assessment of the hiatus in global mean surface warming in the last 15 years). As shown by Hawkins and Sutton (2009), trends in the observations during short-timescale periods (decades) can be dominated by natural variability in the Earth’s climate system. Similar episodes are also seen in climate model experiments (Easterling and Wehner, 2009). Due to their experimental design these episodes cannot be duplicated with the same timing as the observed episodes in most of the model simulations; this affects the interpretation of recent trends in the scenario evaluations (Section 11.2). Notwithstanding these points, there is evidence that early forecasts that carried formal estimates of uncertainty have proved highly consistent with subsequent observations (Allen et al., 2013). If the contributions of solar variability, volcanic activity and ENSO are removed from the observations the remaining trend of surface air temperature agree better with the modelling studies (Rahmstorf et al., 2012).
- Cubasch, U., et al., 2001: Projections of future climate change. In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [J. T. Houghton, Y. Ding, D. J. Griggs, M. Noquer, P. J. van der Linden, X. Dai, K. Maskell and C. A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 527–582.
- Meehl, G. A., et al., 2007: Global climate projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 749–845.
- Frame, D. J., and D. A. Stone, 2013: Assessment of the first consensus prediction on climate change. Nature Clim. Change, 3, 357–359.
- Hawkins, E., and R. Sutton, 2009: The potential to narrow uncertainty in regional climate predictions. Bull. Am. Meteorol. Soc., 90, 1095–1107.
- Easterling, D. R., and M. F. Wehner, 2009: Is the climate warming or cooling? Geophys. Res. Lett., 36, L08706.
- Allen, M. R., J. F. B. Mitchell, and P. A. Stott, 2013: Test of a decadal climate forecast. Nature Geosci., 6, 243–244.
- Rahmstorf, S., G. Foster, and A. Cazenave, 2012: Comparing climate projections to observations up to 2011. Environ. Res. Lett., 7, 044035.
ES 1.1 1.2.1 1.2.2 1.2.3 1.3 1.3.1 1.3.2 1.3.3 1.3.4 22.214.171.124 126.96.36.199 188.8.131.52 1.4.1 1.4.2 1.4.3 1.4.4 1.5 1.5.1 1.5.2 1.6 Box 1 FAQ Refs