Tuesday, October 02, 2007

Climate sensitivity is 6C?

Belette alerted me to this article discussing what appears to be a new manuscript by Jim Hansen (which I have not seen, I'm basing this post purely on the quoted excerpt). In the manuscript, Hansen claimes that the "long term" climate sensitivity is 6C. Hansen has long been of the mainstream "climate sensitivity is about 3C" school of thought so it's interesting to see what appears at first glimpse to be a startling u-turn.

But things are not quite what they seem. Generally, when people talk about sensitivity, they mean the sensitivity of the atmosphere/ocean/sea-ice system to changes in boundary conditions such as (especially) CO2 and also other forcings such as the minor greenhouse gases and changes in solar forcing. The relatively slow-moving ice sheets, which can significantly affect the planetary albedo, also are generally placed in this "boundary condition" category.

In this new manuscript, Hansen simply considers the ice sheets as part of the interactive system - which of course they are on a long enough time scale. For colder climates, the ice sheet albedo change roughly matches the GHG forcing, so if we regress temperature against GHG forcing alone (rather than the conventional approach of adding the GHG and ice sheet forcing together) we get a result of about 6C per doubling of CO2, double the conventional figure. (There is no need to get into the question of lags vs leads and cause vs effect here, it is just a matter of what a cold climate state looks like compared to a warmer one.)

On a long enough time scale and looking backwards in time, this approach is not unreasonable. However, looking forward to a warmer climate, there is no significant ice sheet left still to melt (significant in terms of global temperature, that is - of course if the remaining ice sheets melt then sea level rise could be important for other reasons). So therefore there is no obvious reason to think this 6C value is the appropriate one to use in the context of ~100 years or more of future warming. Hansen appears to admit as much in his text, so I'm not really sure what the point of his article is. There's a vague claim that future feedbacks from vegetation will just happen to make up for the fact that the ice sheet feedback disappears, but there is no evidence shown for this and it seems nothing more than anthropomorphism to just say: "The real world will be aiming on the longer run at a warming corresponding to the higher climate sensitivity." Although some people expect a positive effect from the carbon cycle, expecting it to double the warming seems a stretch, to put it mildly.

[Jules spotted Harry Elderfield taking a similar approach in analysing paleoclimate data a few years ago in a poster at the EGU, but I can't find a ref to it - perhaps we (and/or others) persuaded him it was an inappropriate angle to pursue.]

1 comment:

EliRabett said...

The original 1988 GISS papers had climate sensitivity higher, ~4.5C as I recall, but over 30 years or less with the growth of GHG that has been observed this makes little difference. The point is made in the paper (Hmm it was 4.2)

"The climate model we employ has a global mean surface air equilibrium sensitivity of 4.2 C for doubled CO2. Other recent GCMs yield equilibrium sensitivities of 2.5-5.5 C.....

Forecast temperature trends for time scales of a few decades or less are not very sensitive to the model's equilibrium climate sensitivity (reference provided). Therefore climate sensitivity would have to be much smaller than 4.2 C, say 1.5 to 2 C, in order for us to modify our conclusions significantly."