11ºC warming, climate crisis in 10 years?

What about paleo-climate? An early attempt to use the Vostok ice core data in a regression analysis (Lorius et al., 1990) resulted in a climate sensitivity of 3-4ºC. The best period for these purposes is the last glacial maximum. This was a relatively stable climate (for several thousand years, 20,000 years ago), and a period where we have reasonable estimates of the radiative forcing (albedo changes from ice sheets and vegetation changes, greenhouse gas concentrations (derived from ice cores) and an increase in the atmospheric dust load) and temperature changes. A reasonable estimate of the forcings is 6.6+/-1.5 W/m2 (roughly half from albedo changes, slightly less than half from greenhouse gases – CO2, CH4, N2O). The global temperature changes were around 5.5 +/-0.5°C (compared to pre-industrial climate). This estimate then gives 0.8 +/- 0.2°C/(W/m2), or ~3+/-1°C for 2xCO2. This is actually quite a strong constraint, as we will see.

With this background, what should one make of the climateprediction.net results? They show that the sensitivity to 2xCO2 of a large multi-model ensemble with different parameters ranges from 2 to 11°C. This shows that it is possible to construct models with rather extreme behavior – whether these are realistic is another matter. To test for this, the models must be compared with data. Stainforth et al. subject their resulting models only to very weak data constraints, namely only to data for the annual-mean present-day climate. Since this does not include any climatic variations (not even the seasonal cycle), let alone a test period with a different CO2 level, this data test is unable to constrain the upper limit of the climate sensitivity range. The fact that even model versions with very high climate sensitivities pass their test does not show that the real world could have such high climate sensitivity; it merely shows that the test they use is not very selective. Our feeling is that once the validation becomes more comprehensive, most of the extremely high sensitivity examples will fail (particularly on the seasonal cycle, which tests for variations rather than just a mean).

A yet more stringent test for realistic climate sensitivity is the application of a model to a climate with different CO2 levels. Consider the implications for glacial climate of a sensitivity of twice the most likely value of 3°C, i.e. 6°C. This would imply that either the glacial forcings were only half what we thought, or that the temperature changes were twice what we infer. This would be extremely difficult to square with the paleo-data. Obviously the situation becomes even more untenable for the larger values (>6°C). Hence, we feel that the most important result of the study of Stainforth et al. is that by far most of the models had climate sensitivities between 2ºC and 4ºC, giving additional support to the widely accepted range (Update: As mentioned in the follow up post, this clustering is mainly a function of the sensitivity of the original model and the random nature of the perturbations). The fact that some of the models had much higher sensitivities should not be over-interpreted.

The ‘Meeting the Climate Challenge’ report tried to quantify what is meant by ‘dangerous’ interference in climate. All countries including the US and Australia have signed the Framework Convention on Climate Change which obligates them to prevent ‘dangerous’ interference with the climate system. Actually quantifying what this means is rather tricky. For various reasons (although some are subjective) they suggest that any global warming above 2°C (above the pre-industrial) is likely to be increasingly dangerous. The issue is how one prevents such an outcome given the uncertainty in the climate sensitivity.

The analysis used in this report is based on a study by Baer and Athanasiou. They perform a probability calculation assuming that any of the climate sensitivities in the IPCC range are equally likely. This is a relatively conservative assumption (since it does not include the really high sensitivities that we argued above are ruled out by paleo-data). The results suggest that in order to avoid ‘dangerous’ climate change with a reasonable probability (>90%), the maximum forcing that could be allowed is around 2 W/m2 over pre-industrial levels. This corresponds to a CO2 level of around 400 ppm, assuming all other forcings were at pre-industrial levels. This limit is to some extent subjective, but it is similar (though a little lower) than the level proposed by Jim Hansen.

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