11ºC warming, climate crisis in 10 years?

by Gavin Schmidt and Stefan Rahmstorf

Two stories this week, a paper in Nature (Stainforth et al, 2005) describing preliminary results of the climateprediction.net experiments, and the Meeting the Climate Challenge report from a high level political group have lead to dramatic headlines. On the Nature paper, BBC online reported that “temperatures around the world could rise by as much as 11ºC “; on the latter report it headlined: “Climate crisis near ‘in 10 years’”. Does this mean there is new evidence that climate change is more serious than previously thought? We think not.

Both issues touch on the issue of uncertainty, in particular, the uncertainty in the global climate sensitivity.

It is important to know roughly what the climate sensitivity of the planet is. There are a number of ways to do this, using either climate models or data or a combination of both. From the earliest experiments model estimates have ranged from around 2 to 5°C (for 2xCO2). The most quoted range comes from the 1979 Charney report. There, two models were looked at (from Suki Manabe and Jim Hansen) which had a 2 and 4°C sensitivity, respectively. Jule Charney added a half a degree uncertainty at the low and high end and thus the range became 1.5 to 4.5°C. Thus, this early range stood on rather shaky grounds. It has lasted for a surprisingly long time, with subsequent results neither challenging it, nor being able to narrow it down further. Subsequent model estimates have pretty much fallen within those limits, though the actual range for the state-of-the-art models being analysed for the next IPCC report is 2.6 to 4.1°C. (Note that the range of climate sensitivity is not the same as the temperature range projected for 2100 (1.4 to 5.8°C), which also includes uncertainty in projected emissions. The uncertainty due purely to the climate sensitivity for any one scenario is around half that range.)

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).

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