This is a quick post. It reinforces one of the points made repeatedly in recent comparisons of the observed rise in Global Surface Temperatures during the 20thCentury and of those hindcast by the Multi-Model Mean of the models used by the IPCC in AR4. And I also want to be able to reference one of the following graphs (Figure 3) in an upcoming summary of the recent AR4 model versus observations posts. (I have at least one more post on these comparisons before the summary.)
For the post The IPCC Says… – The Video – Part 1 (A Discussion About Attribution), I replicated the data that represented the Multi-Model Mean hindcast of Global Surface Temperature anomalies presented by the IPCC in Figure 9.5 of AR4. And in that post, it was cell a of Figure 9.5, which represented the models forced with natural and anthropogenic forcings. The post It Really Should Go Without Saying, BUT…presented the replicated Multi-Model Mean of the models forced by only natural (solar and volcanic aerosols) forcings. For both, I created the replica data using the X-Y coordinates of Microsoft Paint. So consider that when viewing the following. The other point to consider is that not all of the modelers presented the hindcast for natural forcings only, so there are differences in the numbers of models presented by the IPCC in the two datasets.
Figure 1 presents the replica of the Multi-Model Mean of the 20th Century Global Surface Temperature hindcasts presented by the IPCC in Figure 9.5 of AR4. It includes the data from cell a, which included natural and anthropogenic forcings, and the data from cell b, which showed the hindcast for natural (solar and volcanic aerosols) forcings only. The greatest divergence occurs during the late 20thCentury.
If we subtract the mean of the hindcasts with natural forcings only from the hindcasts that used natural and anthropogenic forcings, Figure 2, we’re left with what should be a rough approximation of the anthropogenic component of the models.
The hindcast difference (anthropogenic and natural forced minus natural forced) is again illustrated in Figure 3, but in Figure 3 the two warming periods of 1917-1944 and 1976-2000 are also shown, as are the trends for those two periods. The linear trend of the anthropogenic component during the late warming period of 1976-2000 is more than 8 times higher than the trend of the early warming period.
One would think that, if anthropogenic forcings were the dominant cause of the rise in surface temperatures during the late warming period, the observed rate at which surface temperatures rose during the late warming period would be much higher than the rate during the early warming period. But it’s not, as shown in Figure 4.
(Keep in mind: we’ve already established in the earlier posts that the observed early warming period cannot be simulated by the models that also include natural forcings.)