A probabilistic quantification of the anthropogenic component of twentieth century global warming

Abstract

This paper examines in detail the statement in the 2007 IPCC Fourth Assessment Report that “Most of the observed increase in global average temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations”. We use a quantitative probabilistic analysis to evaluate this IPCC statement, and discuss the value of the statement in the policy context. For forcing by greenhouse gases (GHGs) only, we show that there is a greater than 90 % probability that the expected warming over 1950–2005 is larger than the total amount (not just “most”) of the observed warming. This is because, following current best estimates, negative aerosol forcing has substantially offset the GHG-induced warming. We also consider the expected warming from all anthropogenic forcings using the same probabilistic framework. This requires a re-assessment of the range of possible values for aerosol forcing. We provide evidence that the IPCC estimate for the upper bound of indirect aerosol forcing is almost certainly too high. Our results show that the expected warming due to all human influences since 1950 (including aerosol effects) is very similar to the observed warming. Including the effects of natural external forcing factors has a relatively small impact on our 1950–2005 results, but improves the correspondence between model and observations over 1900–2005. Over the longer period, however, externally forced changes are insufficient to explain the early twentieth century warming. We suggest that changes in the formation rate of North Atlantic Deep Water may have been a significant contributing factor.

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Notes

  1. 1.

    P.J. Michaels: “A rational discussion of climate change: The science, the evidence, the response”. Hearing before the Subcommittee on Energy and Environment, Committee on Science and Technology, House of Representatives, 111th Congress, Second Session, November 17, 2010. Serial No. 111–114, pp. 85–99. Available at: http://frwebgate.access.gpo.gov/cgibin/getdoc.cgi?dbname=111_house_hearings&docid=f:62618.pdf.

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Appendices

Appendix 1: Aerosol forcing uncertainties

The 2005 reference-level aerosol forcings used in our analyses and their associated uncertainties are taken from the AR4. Table 4 summarizes this information. Although these values were based on both observational estimates and model results, there was no attempt by IPCC to assess their overall consistency (or lack thereof) with observed changes in climate. As noted in the main text, when the temperature implications of the indirect aerosol uncertainty range are considered, the very large 95th percentile value for indirect aerosol forcing (−1.8 W/m2) appears highly unlikely, unless compensated for by missing (or underestimated) positive forcings (or by an unusual manifestation of low-frequency internally generated variability).

Table 4 Summary of forcings from the IPCC AR4 (W/m2 in 2005)

The AR4 range for indirect aerosol forcing is essentially a compromise between observationally based studies (see main text), which invariably give a much lower value for the upper bound (e.g., around −1 W/m2 in Stott et al. 2006), and estimates derived from forward calculations, which range up to −4 W/m2 for total sulfate aerosol forcing (Anderson et al. 2003). Why, then, is −1.8 W/m2 for indirect aerosol forcing highly unlikely? We can answer this question by performing another set of sensitivity studies where we consider only the effects of indirect forcing uncertainties. To do so we assume that all anthropogenic forcings except indirect aerosol forcing are given their central values, and consider a range of values for the reference (2005) indirect forcing amount.

If the AR4 95th percentile value of −1.8 W/m2 is accepted rather than the best estimate (−0.7 W/m2), then the total anthropogenic forcing in 2005 would be reduced by 1.1 W/m2 (i.e., 1.8 minus 0.7) relative to the estimates of total forcing in Table 4 (rows 13 or 14). In other words, total anthropogenic forcing in 2005 would be reduced to 0.62 W/m2 using the row-13 result, or to only 0.5 W/m2 if we use the central AR4 total forcing value (row 14). (The MAGICC estimate for this case is 0.52 W/m2, see Fig. 7). The implied historical development of total anthropogenic forcing then would show negative forcing until the late twentieth century, and the implied global-mean temperature record would show no net warming at all over the twentieth century (see Fig. 8)—indeed, it would show a considerable cooling from 1860 until the early 1970s. In the absence of large, currently unknown positive forcings, or a very large contribution from internally generated variability to observed warming, these are extremely unlikely results, so one must conclude that a negative indirect aerosol forcing as large as 1.8 W/m2 is also extremely unlikely (i.e., probability much less than 1 %).

Fig. 7
figure7

The effect on radiative forcing of assuming different values for indirect aerosol forcing. The top three curves show total anthropogenic forcing assuming central values for all components other than indirect aerosol forcing. The bottom three curves show the history of indirect aerosol forcing used in the top three curves. These curves are based on the best-estimate of indirect aerosol forcing in 2005 (−0.7 W/m2), and on assumed 2005 forcings of −1.1 and −1.8 W/m2. All three curves use the recently derived history of SO2 emissions from Smith et al. (2011). SO2 emissions changes prior to 1860 are assumed to be negligible

Fig. 8
figure8

The effect on global-mean temperature of assuming a large value for indirect aerosol forcing (viz. −1.8 W/m2 in 2005, the 95th percentile value according to the IPCC AR4) compared with temperatures for the central indirect forcing estimate (−0.7 W/m2) and a less extreme maximum of −1.1 W/m2. Results assume a central value for the climate sensitivity (3.0 °C). Temperature changes are relative to 1765

If the 95th percentile for indirect aerosol forcing given in the AR4 is so improbable, then we need to produce an improved estimate of the uncertainty in aerosol forcing to use in our probabilistic calculations. For total aerosol forcing using the AR4 uncertainty ranges for individual components, the 90 % confidence range, by quadrature, is ±0.86 W/m2 (see Table 4). This is based on an uncertainty range of ±0.75 W/m2 for indirect forcing (i.e., half the 90 % confidence interval range). Changing the 95th percentile value for indirect forcing from −1.8 to −1.1 W/m2 produces more realistic histories of past forcing and temperature change (see Figs. 7, 8). Note, however, that even −1.1 W/m2 would require high end values for the positive forcing components. If the indirect aerosol forcing uncertainty is reduced to ±0.4 W/m2 (i.e., a 90 % range of −0.3 to −1.1 W/m2) then the overall aerosol forcing uncertainty (i.e., the 90 % confidence interval) is reduced to ±0.57 W/m2. This is what we assume for our probabilistic calculations.

Appendix 2: Linear trend results

In the main text we have presented results using the robust trend as the metric for change. Here we give results using the linear trend. Figure 9, the equivalent of Fig. 4, shows cdfs for temperature changes over 1900–2005 and 1950–2005 for (a) GHG forcing only and (b) full anthropogenic forcing. Figure 10, the equivalent of Fig. 6, compares results for anthropogenic forcing with those for anthropogenic-plus-natural forcing over (a) 1900–2005 and (b) 1950–2005.

Fig. 9
figure9

Cumulative distribution functions for temperature changes over 1900–2005 and 1950–2005 for a GHG-only and b all anthropogenic forcings. Observed data are from NOAA/NCDC, with ENSO effects removed. All changes are estimated using the total linear trend. This Figure should be compared with Fig. 4 in the main text

Fig. 10
figure10

Cumulative distribution functions for global-mean temperature changes over 1900–2005 and 1950–2005. Results for anthropogenic forcing only (c.f. Fig. 9) are compared with those for combined anthropogenic and natural (solar plus volcanic) forcing. The observed trend is from the NOAA/NCDC data set with ENSO effects removed. All changes are estimated using the total linear trend. This Figure should be compared with Fig. 6 in the main text

Consider Fig. 9 first. For GHG forcing only, the linear trend results are very similar to those using the robust trend. They show, as before, that the observed warming over either period is very likely less than that expected from GHG forcing. The probability that this is so is about 93 %.

For full anthropogenic forcing, the linear trend results are qualitatively the same as the robust trend results. Over 1950–2005, the observed warming trend is slightly greater than the model expectation: a probability of 57 % for the linear trend compared with 61 % for the robust trend. Over 1900–2005 the observed trend is substantially greater than the model expectation: a probability of 87 % for the linear trend compared with 78 % for the robust trend. Compared with the 1950–2005 results, the larger model versus observed difference over 1900–2005 is due to a much larger observed warming in the early twentieth century than the model expectation. We have attributed this additional warming, at least in part, to an increase in the AMOC.

That the model-observed discrepancy over 1900–2005 cannot be attributed to natural (solar plus volcanic) external forcing is demonstrated in Fig. 10a. If the linear trend is used as the change metric, the cdf for anthropogenic forcing is virtually the same as that for anthropogenic-plus-natural forcing: adding in natural forcing has almost no effect on model estimates of linear trend over 1900–2005. In both the anthropogenic only and anthropogenic-plus-natural forcing cases, the probability that the observed warming is greater than the model expectation is high, about 87 %. This is in contrast to the robust trend result where adding in natural forcing improved (slightly) the trend fits between model and observations. These results are further illustrated in Fig. 11.

Fig. 11
figure11

Median model results for anthropogenic forcing only and anthropogenic-plus-natural forcing, compared with observed (NOAA/NCDC) data. Model results are relative to 1950. Observations have been zeroed to have the same mean as the “anthropogenic + natural forcings” result over 1950–2005. This Figure is similar to Fig. 5 in the main text, but includes the linear trend lines

Over 1950–2005 (Fig. 10b), observed warming is also greater than the model expectation, both for anthropogenic forcing only (probability of 57 %) and for anthropogenic-plus-natural forcing (79 %)—but note that neither of these model-observed differences is statistically significant. Adding in the effects of natural forcing increases the trend discrepancy between model and observations. This is the case for both the linear trend and robust trend change metrics. This does not, however, mean that the overall fit is degraded by adding in the effects of natural forcing. Table 5 shows correlations between median model results for both forcing cases and the four observed temperature time series. Over both 1900–2005 and 1950–2005 the correlations are improved by the addition of natural forcing.

Table 5 Correlations between median model results and observed annual-mean temperatures (with ENSO removed)

Figure 11 complements Fig. 10 and shows the median model time series for the responses to anthropogenic and anthropogenic-plus-natural forcing, together with the observed (NOAA/NCDC) data (with ENSO removed). It can be seen that the model trends over 2000–2005 are very similar for both forcing cases, and that the observed trend is substantially greater.

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Wigley, T.M.L., Santer, B.D. A probabilistic quantification of the anthropogenic component of twentieth century global warming. Clim Dyn 40, 1087–1102 (2013). https://doi.org/10.1007/s00382-012-1585-8

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Keywords

  • Global warming
  • Probabilistic calculations
  • Climate
  • Human influences
  • Solar forcing
  • NADW
  • Aerosol forcing