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Bet you haven’t seen this one on TV: A newer, more sophisticated climate model has lost more than 25% of its predicted warming! You can bet that if it had predicted that much more warming it would have made the local paper.
The change resulted from a more realistic simulation of the way clouds work, resulting in a major reduction in the model’s “climate sensitivity,” which is the amount of warming predicted for a doubling of the concentration of atmospheric carbon dioxide over what it was prior to the industrial revolution.
Prior to the modern era, atmospheric carbon dioxide concentrations, as measured in air trapped in ice in the high latitudes (which can be dated year‐by‐year) was pretty constant, around 280 parts per million (ppm). No wonder CO2 is called a “trace gas” — there really is not much of it around.
The current concentration is pushing about 390 ppm, an increase of about 40% in 250 years. This is a pretty good indicator of the amount of “forcing” or warming pressure that we are exerting on the atmosphere. Yes, there are other global warming gases going up, like the chlorofluorocarbons (refrigerants now banned by treaty), but the modern climate religion is that these are pretty much being cancelled by reflective “aerosol” compounds that go in the air along with the combustion of fossil fuels, mainly coal.
Most projections have carbon dioxide doubling to a nominal 600 ppm somewhere in the second half of this century, absent no major technological changes (which history tells us is a very shaky assumption). But the “sensitivity” is not reached as soon as we hit the doubling, thanks to the fact that it takes a lot of time to warm the ocean (like it takes a lot of time to warm up a big pot of water with a small burner).
So the “sensitivity” is much closer to the temperature rise that a model projects about 100 years from now — assuming (again, shakily) that we ultimately switch to power sources that don’t release dreaded CO2 into the atmosphere somewhere around the time its concentration doubles.
The bottom line is that lower sensitivity means less future warming as a result of anthropogenic greenhouse gas emissions. So our advice… keep on working on the models, eventually, they may actually arrive at something close puny rate of warming that is being observed
At any rate, improvements to the Japanese‐developed Model for Interdisciplinary Research on Climate (MIROC) are the topic of a new paper by Masahiro Watanabe and colleagues in the current issue of the Journal of Climate. This modeling group has been working on a new version of their model (MIROC5) to be used in the upcoming 5th Assessment Report of the United Nations’ Intergovernmental Panel on Climate Change, due in late 2013. Two incarnations of the previous version (MIROC3.2) were included in the IPCC’s 4th Assessment Report (2007) and contribute to the IPCC “consensus” of global warming projections.
The high resolution version (MIROC3.2(hires)) was quite a doozy — responsible for far and away the greatest projected global temperature rise (see Figure 1). And the medium resolution model (MIROC3.2(medres)) is among the Top 5 warmest models. Together, the two MIROC models undoubtedly act to increase the overall model ensemble mean warming projection and expand the top end of the “likely” range of temperature rise.
Global temperature projections under the “midrange” scenario for greenhouse‐gas emissions produced by the IPCC’s collection of climate models. The MIROC high resolution model (MIROC3.2(hires)) is clearly the hottest one, and the medium range one isn’t very far behind.
The reason that the MIROC3.2 versions produce so much warming is that their sensitivity is very high, with the high‐resolution at 4.3°C (7.7°F) and the medium‐resolution at 4.0°C (7.2°F). These sensitivities are very near the high end of the distribution of climate sensitivities from the IPCC’s collection of models (see Figure 2).
Equilibrium climate sensitivities of the models used in the IPCC AR4 (with the exception of the MIROC5). The MIROC3.2 sensitivities are highlighted in red and lie near the upper und of the collection of model sensitivities. The new, improved, MIROC5, which was not included in the IPCC AR4, is highlighted in magenta, and lies near the low end of the model climate sensitivities (data from IPCC Fourth Assessment Report, Table 8.2 and Watanabe et al., 2010).
Note that the highest sensitivity is not necessarily in the hottest model, as observed warming is dependent upon how the model deals with the slowness of the oceans to warm.
The situation is vastly different in the new MIROC5 model. Watanabe et al. report that the climate sensitivity is now 2.6°C (4.7°F) — more than 25% less than in the previous version on the model. If the MIROC5 had been included in the IPCC’s AR4 collection of models, its climate sensitivity of 2.6°C would have been found near the low end of the distribution (see Figure 2), rather than pushing the high extreme as MIROC3.2 did.
And to what do we owe this large decline in the modeled climate sensitivity? According to Watanabe et al., a vastly improved handling of cloud processes involving “a prognostic treatment for the cloud water and ice mixing ratio, as well as the cloud fraction, considering both warm and cold rain processes.” In fact, the improved cloud scheme — which produces clouds which compare more favorably with satellite observations — projects that under a warming climate low altitude clouds become a negative feedback rather than acting as positive feedback as the old version of the model projected. Instead of enhancing the CO2‐induced warming, low clouds are now projected to retard it.
Here is how Watanabe et al. describe their results: