We discussed Steinman et al. (2015) and Michael Mann’s post about it at RealClimate in the article On Steinman et al. (2015) – Michael Mann and Company Redefine Multidecadal Variability And Wind Up Illustrating Climate Model Failings. This brief post adds to that discussion.
THE PAUSE, HIATUS, SLOWDOWN (WHATEVER) IN GLOBAL WARMING IS MORE PREVALENT IN THE SOUTHERN HEMISPHERE, YET MANN AND STEINMAN ET AL. FOCUSED ON THE NORTHERN HEMISPHERE
The opening paragraph in Michael Mann’s post at RealClimate reads:
No, climate change is not experiencing a hiatus. No, there is not currently a “pause” in global warming.
The initial topic of discussion is GLOBAL warming…and Mann’s perception that there is no pause in the surface temperatures GLOBALLY.
Yet Mann in his blog post and Steinman et al. have redirected the discussion from global warming to Northern Hemisphere warming. That is, Steinman et al. (2015) was not an examination of the modeled and observed surface temperatures globally. Their paper only looked at the surface temperatures of the Northern Hemisphere, and the sea surface temperatures of the North Atlantic and North Pacific. Mann even notes this a few paragraphs later, where he opens the paragraph with:
We focused on the Northern Hemisphere…
But it’s well known that the hiatus, the pause, the slowdown, etc., in surface warming is more prevalent in the Southern Hemisphere than in the Northern Hemisphere. If you aren’t aware of that fact, I’ll show you.
The year 1998 is commonly used for the start of the global warming slowdown, and Kevin Trenberth used 2001 in his article Has Global Warming Stalled? for the Royal Meteorological Society. (Also see my post Open Letter to the Royal Meteorological Society Regarding Dr. Trenberth’s Article “Has Global Warming Stalled?”.) So we’ll present the data with both start years. For the data, we’ll present the widely referenced, spatially complete GISS Land-Ocean Temperature Index (LOTI) data.
Figure 1 compares the Northern and Southern Hemisphere surface temperature anomalies during the hiatus. In the top cell, the data start in 1998, and in the bottom cell, the data start in 2001. As noted earlier, the slowdown in surface warming is more prevalent in the Southern Hemisphere.
And for those interested, Figures 2 and 3 present model-data comparisons for the Northern (Figure 2) and Southern (Figure 3) Hemispheres for the two slowdown periods. The models are represented by the multi-model mean of all of the climate models stored in the CMIP5 archive, with historic forcings through 2005 and RCP8.5 scenario forcings thereafter. The data and the model outputs are available through the KNMI Climate Explorer. We present the multi-model mean because it best represents the consensus, the groupthink, on how surface temperatures should have warmed if they were warmed by manmade greenhouse gases and the other forcings. In other words, the model mean represents the forced component of the models. For additional information on the use of the model mean, see the post here. (Also see the discussion of Estimates of the forced component under the heading of Materials and Methods in the Supplementary Material furnished with Steinman et al.)
# # #
It appears that Michael Mann may have biased his opinions about the slowdown in global surface warming by looking only at data for the Northern Hemisphere, which shows higher warming rates than the Southern Hemisphere. One only has to look at trend maps of the global warming slowdown periods in Figure 4 to see that there is less warming in the Southern Hemisphere. (The trend maps are available from the GISS map-making webpage.)
The Steinman et al. (2015) analyses do not explain the slowdown and stoppage of surface warming in the Southern Hemisphere…and, as noted in the earlier post, they had to redefine multidecadal temperature variability to try to explain it in the Northern Hemisphere.
The GISS LOTI data and the climate model outputs are available from the KNMI Climate Explorer.