9.4.1.2 Simulations of the 20th Century

There are now a greater number of climate simulations from AOGCMs for the period of the global surface instrumental record than were available for the TAR, including a greater variety of forcings in a greater variety of combinations. These simulations used models with different climate sensitivities, rates of ocean heat uptake and magnitudes and types of forcings (Supplementary Material, Table S9.1). Figure 9.5 shows that simulations that incorporate anthropogenic forcings, including increasing greenhouse gas concentrations and the effects of aerosols, and that also incorporate natural external forcings provide a consistent explanation of the observed temperature record, whereas simulations that include only natural forcings do not simulate the warming observed over the last three decades. A variety of different forcings is used in these simulations. For example, some anthropogenically forced simulations include both the direct and indirect effects of sulphate aerosols whereas others include just the direct effect, and the aerosol forcing that is calculated within models differs due to differences in the representation of physics. Similarly, the effects of tropospheric and stratospheric ozone changes are included in some simulations but not others, and a few simulations include the effects of carbonaceous aerosols and land use changes, while the naturally forced simulations include different representations of changing solar and volcanic forcing. Despite this additional uncertainty, there is a clear separation in Figure 9.5 between the simulations with anthropogenic forcings and those without.

Figure 9.5. Comparison between global mean surface temperature anomalies (°C) from observations (black) and AOGCM simulations forced with (a) both anthropogenic and natural forcings and (b) natural forcings only. All data are shown as global mean temperature anomalies relative to the period 1901 to 1950, as observed (black, Hadley Centre/Climatic Research Unit gridded surface temperature data set (HadCRUT3); Brohan et al., 2006) and, in (a) as obtained from 58 simulations produced by 14 models with both anthropogenic and natural forcings. The multi-model ensemble mean is shown as a thick red curve and individual simulations are shown as thin yellow curves. Vertical grey lines indicate the timing of major volcanic events. Those simulations that ended before 2005 were extended to 2005 by using the first few years of the IPCC Special Report on Emission Scenarios (SRES) A1B scenario simulations that continued from the respective 20th-century simulations, where available. The simulated global mean temperature anomalies in (b) are from 19 simulations produced by five models with natural forcings only. The multi-model ensemble mean is shown as a thick blue curve and individual simulations are shown as thin blue curves. Simulations are selected that do not exhibit excessive drift in their control simulations (no more than 0.2°C per century). Each simulation was sampled so that coverage corresponds to that of the observations. Further details of the models included and the methodology for producing this figure are given in the Supplementary Material, Appendix 9.C. After Stott et al. (2006b).

Global mean and hemispheric-scale temperatures on multi-decadal time scales are largely controlled by external forcings (Stott et al., 2000). This external control is demonstrated by ensembles of model simulations with identical forcings (whether anthropogenic or natural) whose members exhibit very similar simulations of global mean temperature on multi-decadal time scales (e.g., Stott et al., 2000; Broccoli et al., 2003; Meehl et al., 2004). Larger interannual variations are seen in the observations than in the ensemble mean model simulation of the 20th century because the ensemble averaging process filters out much of the natural internal interannual variability that is simulated by the models. The interannual variability in the individual simulations that is evident in Figure 9.5 suggests that current models generally simulate large-scale natural internal variability quite well, and also capture the cooling associated with volcanic eruptions on shorter time scales. Section 9.4.1.3 assesses the variability of near surface temperature observations and simulations.

The fact that climate models are only able to reproduce observed global mean temperature changes over the 20th century when they include anthropogenic forcings, and that they fail to do so when they exclude anthropogenic forcings, is evidence for the influence of humans on global climate. Further evidence is provided by spatial patterns of temperature change. Figure 9.6 compares observed near-surface temperature trends over the globe (top row) with those simulated by climate models when they include anthropogenic and natural forcing (second row) and the same trends simulated by climate models when only natural forcings are included (third row). The observed trend over the entire 20th century (Figure 9.6, top left panel) shows warming almost everywhere with the exception of the southeastern USA, northern North Atlantic, and isolated grid boxes in Africa and South America (see also Figure 3.9). Such a pattern of warming is not associated with known modes of internal climate variability. For example, while El Niño or El Niño-like decadal variability results in unusually warm annual temperatures, the spatial pattern associated with such a warming is more structured, with cooling in the North Pacific and South Pacific (see, e.g., Zhang et al., 1997). In contrast, the trends in climate model simulations that include anthropogenic and natural forcing (Figure 9.6, second row) show a pattern of spatially near-uniform warming similar to that observed. There is much greater similarity between the general evolution of the warming in observations and that simulated by models when anthropogenic and natural forcings are included than when only natural forcing is included (Figure 9.6, third row). Figure 9.6 (fourth row) shows that climate models are only able to reproduce the observed patterns of zonal mean near-surface temperature trends over the 1901 to 2005 and 1979 to 2005 periods when they include anthropogenic forcings and fail to do so when they exclude anthropogenic forcings. Although there is less warming at low latitudes than at high northern latitudes, there is also less internal variability at low latitudes, which results in a greater separation of the climate simulations with and without anthropogenic forcings.

Figure 9.6. Trends in observed and simulated temperature changes (°C) over the 1901 to 2005 (left column) and 1979 to 2005 (right column) periods. First row: trends in observed temperature changes (Hadley Centre/Climatic Research Unit gridded surface temperature data set (HadCRUT3), Brohan et al., 2006). Second row: average trends in 58 historical simulations from 14 climate models including both anthropogenic and natural forcings. Third row: average trends in 19 historical simulations from five climate models including natural forcings only. Grey shading in top three rows indicates regions where there are insufficient observed data to calculate a trend for that grid box (see Supplementary Material, Appendix 9.C for further details of data exclusion criteria). Fourth row: average trends for each latitude; observed trends are indicated by solid black curves. Red shading indicates the middle 90% range of trend estimates from the 58 simulations including both anthropogenic and natural forcings (estimated as the range between 4th and 55th of the 58 ranked simulations); blue shading indicates the middle 90% range of trend estimates from the 19 simulations with natural forcings only (estimated as the range between 2nd and 18th of the 19 ranked simulations); for comparison, the dotted black curve in the right-hand plot shows the observed 1901 to 2005 trend. Note that scales are different between columns. The ‘ALL’ simulations were extended to 2005 by adding their IPCC Special Report on Emission Scenarios (SRES) A1B continuation runs where available. Where not available, and in the case of the ‘NAT’ simulations, the mean for the 1996 to 2005 decade was estimated using model output from 1996 to the end of the available runs. In all plots, each climate simulation was sampled so that coverage corresponds to that of the observations. Further details of the models included and the methodology for producing this figure are given in the Supplementary Material, Appendix 9.C.

Climate simulations are consistent in showing that the global mean warming observed since 1970 can only be reproduced when models are forced with combinations of external forcings that include anthropogenic forcings (Figure 9.5). This conclusion holds despite a variety of different anthropogenic forcings and processes being included in these models (e.g., Tett et al., 2002; Broccoli et al., 2003; Meehl et al., 2004; Knutson et al., 2006). In all cases, the response to forcing from well-mixed greenhouse gases dominates the anthropogenic warming in the model. No climate model using natural forcings alone has reproduced the observed global warming trend in the second half of the 20th century. Therefore, modelling studies suggest that late 20th-century warming is much more likely to be anthropogenic than natural in origin, a finding which is confirmed by studies relying on formal detection and attribution methods (Section 9.4.1.4).

Modelling studies are also in moderately good agreement with observations during the first half of the 20th century when both anthropogenic and natural forcings are considered, although assessments of which forcings are important differ, with some studies finding that solar forcing is more important (Meehl et al., 2004) while other studies find that volcanic forcing (Broccoli et al., 2003) or internal variability (Delworth and Knutson, 2000) could be more important. Differences between simulations including greenhouse gas forcing only and those that also include the cooling effects of sulphate aerosols (e.g., Tett et al., 2002) indicate that the cooling effects of sulphate aerosols may account for some of the lack of observational warming between 1950 and 1970, despite increasing greenhouse gas concentrations, as was proposed by Schwartz (1993). In contrast, Nagashima et al. (2006) find that carbonaceous aerosols are required for the MIROC model (see Table 8.1 for a description) to provide a statistically consistent representation of observed changes in near-surface temperature in the middle part of the 20th century. The mid-century cooling that the model simulates in some regions is also observed, and is caused in the model by regional negative surface forcing from organic and black carbon associated with biomass burning. Variations in the Atlantic Multi-decadal Oscillation (see Section 3.6.6 for a more detailed discussion) could account for some of the evolution of global and hemispheric mean temperatures during the instrumental period (Schlesinger and Ramankutty, 1994; Andronova and Schlesinger, 2000; Delworth and Mann, 2000); Knight et al. (2005) estimate that variations in the Atlantic Multi-decadal Oscillation could account for up to 0.2°C peak-to-trough variability in NH mean decadal temperatures.