I was sent an informative e-mail by Robert Pollock on the global climate response to volacanic eruptions which I am presenting below with permission. Robert is a retired physicist with training in radiation dosimetry. He started a company to measure radon in the environment and sold it a few years ago.
His excellent set of information is presented below
On Tue, 11 Sep 2012, Robert Pollock wrote:
Roger, I don’t know if you have an interest in volcanic eruptions, but they are often cited as an example of the efficacy of GCMs and are very important when looking at ocean heat content.
Gleckler et al. modeled the effect of volcanic eruptions on ocean heat content. Using 12 climate models they showed that Krakatoa in 1883 made its presence felt well into the 20th century in the form of reduced sea level rise and less ocean warming (both on the surface and at depth). As stated in the AR4, including volcanic eruptions improved the model’s match to reality, and the cooling from volcanoes was offsetting a considerable fraction of anthropogenic ocean warming.
Figure 1 from Gleckler shows the difference with and without volcanic forcing between 1880 and 2000: www.nature.com/nature/journal/v439/n7077/fig_tab/439675a_F1.html
At the end of the 20th century simulations with (blue) and without (green) volcanic forcings have a difference of some 70% (18/60 10^22 J).
The authors wrote
“Inclusion of volcanic forcing from Krakatoa (and, by implication, from even earlier eruptions) is important for a reliable simulation of historical increases in ocean heat content and sea-level change due to thermal expansion.”
However, in a 2010 paper Gregory notes that ‘even earlier eruptions’ were not included in the Gleckler modeling work and if they had been, the conclusion would have been quite different. If an eruption produces a cooling and a drop of sea level rise that lasts decades (if not centuries) then each new eruption would lead to further decreases indefinitely.
Such is not the case, and Gregory modeled a steady-state condition resulting from earlier eruptions before Krakatoa. With other climate model the background natural conditions do not include volcanic eruptions. The impact of a new eruption (as part of a series) then becomes less and doesn’t lead to a long-term trend in ocean heat content.
Most GCMs overestimate the (depressive) effect of volcanoes and thus also overestimate the forcing from greenhouse gases to reproduce the climate and ocean heat content of the 20th century.
Gleckler et al. Volcanoes and climate: Krakatoa’s signature persists in the ocean www.nature.com/nature/journal/v439/n7077/abs/43975a.html
Gregory Long-term effect of volcanic forcing on ocean heat content www.agu.org/pubs/crossref/2010/2010GL045507.shtml
Driscoll et al. now have a paper in press that looks at the current generation of models used for the AR5 (13 CHIMP5 models) and their ability to model large tropical eruptions. The abstract lists a number of problems and
“raises concern for the ability of current climate models to simulate the response of a major mode of global circulation variability to external forcings. This is also of concern for the accuracy of geoengineering modeling studies that asses the atmospheric response to stratosphere-injection particles.”
Coupled Model Intercomparison Project 5 (CMIP5) simulations of climate following volcanic eruptions www.agu.org/pubs/crossref/pip/2012JD017607.shtml