Climate sensitivity, part 2

In a previous post, http://evilreductionist.blogspot.com/2010/01/climate-sensitivity.html, I showed a graph of the natural logarithm of atmospheric CO2 concentrations versus yearly temperature. This was in an attempt to work out the sensitivity of the climate to increases in atmospheric CO2. I came up with a figure of 2 degrees celsius per doubling of CO2 concentrations.

The question is: is this a good way of determining climate sensitivity?

The first issue is that there are many things that affect global temperature: the solar cycle, ENSO variations, atmospheric aerosols, ozone, orbital variations, other cycles et cetera. So maybe the variation that we see in temperature over time can be explained by things other than CO2, and thus a linear graph of CO2 v temperature is not a good way of working out the climate sensitivity.

However, in response to this point one of the benefits of doing this graph over a relatively long period - 130 years - is that many of these variations will have been included. There will have been about a dozen or so solar cycles over that time. ENSO will have gone from El Nino to La Nina on numerous occassions. Atmospheric aerosols will have risen and fallen in concentration, along with ozone. Cycles of length shorter than 130 years will have had all their various stages included.

The argument here is that all of these things will have averaged out over the 130 year period and that the only thing not taken into account will have been increases in CO2 concentrations. Thus, the climate sensitivity derived will be a reasonable estimate.

We can test this assumption by examining changes in forcings over this period. Carrick posted a link in the previous thread to data on forcings over this period. It is here: http://data.giss.nasa.gov/modelforce/RadF.txt.

What do I mean by 'forcings'? A forcing is the energy received by the earth from some particular source. They are measured in watts per square metre. As an example, the forcing from CO2 and other greenhouse gases in 2003 was 2.7487 watts per square metre. Totalled over the entire surface of the earth, this is a fair bit of energy.

Using the data, we can create a graph of total forcings versus temperature. This is what I have done above.

The graph has a slope of 21.682. This means that for every full point of increase in forcings, the earth increases in temperature by .21682 degrees celsisus.

I will examine what this means for my estimate of climate sensitivity to changes in CO2 in my next post.