Real Climate has a roundup of the latest scientific papers dealing with sensitivity of our climate to CO2 increases with Part 1 of a planned series of posts.
I will leave the detail up to you to read if you wish at http://www.realclimate.org/index.php/archives/2013/01/on-sensitivity-part-i/ . I particularly like the graphic for its clarity in showing the various timescales for feedback systems:
It sets out in simple terms the time scale of the different methods of looking at climate sensitivity.
For example, the difference between the sensitivity often mentioned in the media of around 2 degrees and that of the longer term or so-called ‘slow feedbacks’ is clearly highly significant. It is a policy critical issue whether to regard the longer term as a consideration when making decisions today.
This figure makes it clear that if we are concerned about our grandchildren and their decendants, we should be looking at the “Earth System Sensitivity” and not just the short term influences (sensitivity in decadal time scales). The estimate of sensitivity is significantly higher for this case.
One huge uncertainty (in the upward direction in my view) is associated with the rate of change. We are currently increasing the climate forcing much faster than can be seen in the geological record. Doubling of the CO2 concentration generally has occurred on thousand year or million year time scales. We are applying this change over a few decades. This rapid prodding of the climate system is likely, in my opinion, to have destabilizing effects not seen in the geological record.
One example of the vulnerability to upward uncertainties is the raft of potential positive feedbacks. There is currently much discussion of the methane threat from melting permafrost and arctic sea floor clathrates. Other threats such as changes to major biological systems (e.g. drying of the Amazon, loss of coral reefs, etc) and destabilization of ice sheets (West Antarctica most importantly) clearly have potential in the upwards direction.
The most important point to keep in mind is that at current rates of emission and in the light of the political and business intention to continue without pause, we are heading for at least a trippling of CO2 levels. Current reserves of fossil fuels are 5 times the amount budgetted to keep us below the doubling limit (approx. 500Gt). This means the temperature for a doubling is not so relevant as the temperature for a trippling or quadrupling, particularly if positive feedbacks such as permafrost and methane emissions prove to be as suggested by the trends.
I note the Carbon Cycle feedbacks have been left completely out of the sensitivities in the graph.
I look forward to part 2 of the discussion from Real Climate.