Kracked Up Over Krakatoa: Only unrealistic models gave the desired Warmist conclusions
It was all the rage a few years back to claim that long ago volcanic eruptions—for instance Krakatoa in 1883—were still acting to mask a large fraction of the oceanic warming that should have occurred because of anthropogenic carbon dioxide emissions. The epitome of this argument was published in Nature magazine, by an all-star cast of scientists ever-eager to suggest that it is all our fault and then some. The authors included Tom Wigley, Ben Santer, Karl Taylor, Krishna AchutaRao, Jonathan Gregory, and lead author Peter Gleckler.
The accompanying Editors’ Summary of the 2006 Nature article by Gleckler et al. provides the gist:
"The 1883 eruption of the volcano Krakatoa in Indonesia has echoed down the centuries in art and in legend. Now an analysis of a suite of 12 climate models shows that Krakatoa also made its presence felt well into the twentieth century in the form of reduced ocean warming and sea-level rise. The changes lasted much longer than was previously suspected and were sufficient to offset much of the ocean warming and sea-level rise caused by more recent human activities".
The IPCC incorporated this finding into their Fourth Assessment Report (AR4) to show that models better match the observed history of the increase in oceanic heat content and sea level rise from thermal expansion when modern (since 1880) volcanic eruptions were included along with anthropogenic forcings. The implication was two-fold; 1) the climate models were now able to closely match reality (so they should be considered reliable), and 2) the cooling from volcanoes was offsetting a large fraction of the influence of anthropogenic global warming (i.e. our influence was even worse than we thought).
Now, a new study comes along, performed by one of the et al.’s of the Gleckler study, that basically shows that the conclusions of that original paper were quite likely incorrect, because the climate models examined had been equilibrated to an improper set of “background” conditions—conditions unnaturally free of any and all volcanic eruptions.
Had the climate models been equilibrated to more realistic conditions—after all, big climatologically important volcanic eruption are a fairly common part of the earth’s natural environment and not just a phenomenon of the pat 120 years—Krakatoa and subsequent volcanoes would not have induced a large, long-term warming-offsetting cooling tendency. And in that case, the apparent match between climate models and reality would fall apart as models would be warming their oceans far more rapidly than observations show the real oceans are (i.e., the models don’t work so well after all).
Jonathan Gregory writing in Geophysical Research Letters put it this way: "This artefact [the model response to improper equilibration] could be misleading in comparisons and attribution of observed and simulated changes in ocean heat content".