Robbo wrote:
> Let me hark back to GCM for a minute. The models all are tuned to
> 20th century temps - that is they all reproduce temps pretty well and
> would generally be considered a failure if they couldn't be validated
> in this way. The projections to 2100 however have temperature
> increasing from 1 to 10 degrees (or more in some very extreme cases).
Um, it's more like 1.5 to 6, and that range covers the different
emissions scenarios, which are not really anything to do with climate
science. There has been a bit of wild stuff from MIT suggesting more
rapid warming, but this does not use GCMs and I for one don't believe it
(and have sound scientific reasons to back that up, based on what I know
of their methods).
> The method is unexceptional but a 'reality check' is commonly applied
> to heuristic solutions. In this case we have a simple check already
> alluded to. For an increase of 80 ppmv of CO2 since the mid 1940's
> there is an associated 0.6 degree temperature rise. We are at about
> 386 ppmv and expected (BAU) to increase to 590 ppmv by 2100. So an
> additional 200 ppmv giving a 1.5 degrees rise? Within the solution
> space. Assuming there is a little more heating of 0.5 degrees 'in the
> pipeline' (according to Jim Hanson) - say a maximum of 3 degrees by
> 2100.
This would be reasonable if it was known that CO2 was the only
significant forcing. However, sulphate aerosols have very likely offset
a significant proportion of the warming that would otherwise have
occurred. Hence the upper limit of plausible future warming is rather
higher than your value.
There is some recent evidence that the aerosol effect is towards the low
end of previous estimates, which would be good news, but I don't think
it is yet universally accepted. I do think it's plausible we would see
only about 2C more warming in this century even if emissions continue to
rise steadily (eg linearly extrapolating the recent rise in forcing),
but I wouldn't want to bet the farm on it.
James
--
You received this message because you are subscribed to the Google Groups
Global Change ("globalchange") newsgroup. Global Change is a public, moderated
venue for discussion of science, technology, economics and policy dimensions of
global environmental change.
Posts will be admitted to the list if and only if any moderator finds the
submission to be constructive and/or interesting, on topic, and not
gratuitously rude.
To post to this group, send email to [email protected]
To unsubscribe from this group, send email to
[email protected]
For more options, visit this group at
http://groups.google.com/group/globalchange
