11ºC warming, climate crisis in 10 years?

Attempts have also been made to constrain climate sensitivity from observations. Ideally, we would need a time when the climate was at an equilibrium state, and with good estimates of the forcings that maintained that state, and good data for the global mean temperature change. The 20th Century has the best estimates of the global mean temperature changes but the climate has not been in equilibrium (as shown by the increasing heat content of the oceans). Also, due to the multiplicity of anthropogenic and natural effects on the climate over this time (i.e. aerosols, land-use change, greenhouse gases, ozone changes, solar, volcanic etc.) it is difficult to accurately define the forcings. Thus estimates based purely on the modern period do not have enough precision to be useful. For instance, total forcings since 1850 are around 1.6+/-1 W/m2, the temperature change is around 0.7+/-0.1 °C and the current rate of warming of the ocean (to correct for the non-equilibrium conditions) is around ~0.75 W/m2. Together, that implies a sensitivity of 0.8 +/- 1 °C/W/m2 or 3.2+/-4°C for 2xCO2). More sophisticated methods of looking at the modern data don’t provide more of a constraint either (i.e. Forest et al., 2002; Knutti et al. 2002). (This large uncertainty essentially due to the uncertainty in the aerosol forcing; it is also the main reason why the magnitude of global dimming has little or no implication for climate sensitivity).

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).

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