I nominate 2.5C as a working definition of sensitivity until we get better data and start making plans accordingly.
He also aired it over at WUWT in slightly different terms:
So, I call for all those involved in the climate debate to throw down their weapons, embrace this practical solution [presumably referring to climate sensitivity being 2.5] as being of use to the rest of the world, climb aboard the Peace Train and sing Kumbaya. Right.
Ok, all together now: Kumbaya!!
I interpret his ‘motion’ as suggesting: Let’s assume that climate sensitivity is 2.5 deg C per doubling of CO2 and move the discussion to how we’re going to deal with that in terms of mitigation and adaptation.
This movement of the debate to the policy sphere is close to what I suggested to Fuller over a year ago, when I wrote:
Let’s distinguish the following main issues:
- To what extent is climate change occurring, and to what extent is it man-made?
- To what extent is that (going to be) a problem?
- What can or should we do about it?
The ‘next generation questions’ in my view relate to the last one: How are we going to deal with this?
Any realistic change in our scientific understanding is not going to change the needed policy response, at least not in the short to medium term (~decades). As Herman Daly noted: “If you jump out of an airplane you need a crude parachute more than an accurate altimeter.”
Tom gets at the 2.5 number by using the Mauna Loa time series for CO2 and comparing its 19% increase with the 0.5 deg C temperature increase over the same time period. Obviously there are some major issues with this approach:
- The net climate forcing is only poorly known because of large uncertainties in aerosol forcing. (Aerosol forcing is close, but opposite in sign, to that of the other greenhouse gases, so accidentally the net forcing is close to the CO2 forcing, though with a very large uncertainty dominated by the aerosol effects.)
- The climate hasn’t fully responded to the current climate forcing yet, as it takes time to equilibrate (mainly due to ocean thermal inertia). This reflects the warming in the pipeline.
- Temperature depends logarithmically on CO2; not linearly.
Nevertheless, his approach more or less accidentally arrives at a very decent estimate, which is right in the ballpark of more accurate estimates of climate sensitivity (3 +/- 1 degree C per doubling of CO2).
The warming we can expect in the future depends not only on the climate sensitivity (which we have just tentatively agreed to put at 2.5 for sake of the argument), but also on future emissions: How much greenhouse gases and aerosols are we going to dump in the atmosphere together? That depends on the choices we make, obviously. But it helps to move the debate forward by assuming a reasonable estimate of the former so we concentrate the debate on the latter, which is where it really gets difficult, because this is where tough choices have to be made and where values clash.
An example of the latter can be seen in the comments to Fuller’s WUWT post. But his message gets lost on most of them: Instead of discussing their values and political wishes, they attack the premise of Fuller’s climate sensitivity or climate science as a whole and ignore the difficult questions. A missed chance.