Tuesday, January 19, 2010

Climate sensitivity, part 4

To quantify the reduction in forcings caused by effects other than greenhouse gases, we can take a ratio of these forcings to the forcings caused by greenhouse gases. If we exclude extremely negative forcing ratios, which mainly occur at the start of the period when variability was high, we end up with an average that is around -0.6. This means that forcings for greenhouse gases have been effectively reduced by an average of around 60 per cent.

Does this mean that my initial estimate of climate sensitivity of 2 plus or minus 1 degree celsius needs to be increased? Perhaps. It first needs to be recognised that the well-mixed greenhouse gases include more than just CO2. CO2 makes up around 60 per cent of the forcings here. So, if we take our initial number 2 degrees and mulitply it by 2.5 (we need to do this because only 40 per cent of the forcings from greenhouse gases is not countered by other negative forcings) that becomes 5 degrees. If we then take 60 per cent of that, we get a climate sensitivity of 3 degrees.

That for me is scary. If the observed non-equilibrium climate sensitivity is that high, then the equlibrium sensitivity must be at towards the high end of the IPCC range.

However, it must be recognised that there is a significant range in the observed sensitivity here - it ranges from 1.5 to 4.5, like the IPCC figure, and there may be a larger error margin in there simply because of the deviation in the reduction in forcings over time. By this I mean that while the average is 0.6, it ranges from around 0.3 to around 0.9.

Now, I am new at this so there may be some fundamental mistake I am making here in increasing the value. Any help would be appreciated.

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