[EMAIL PROTECTED] wrote:
> http://www.climateaudit.org/?p=1536
>
> We've talked about this subject before, but reading the above and
> comments, some further questions came to mind,
>
> 1. What do we know about aerosol trends? Is it really true that there
> is a 35% uncertainty even for emissions in 2000 compared to 1990?
>
The only place 35% appears on the linked page is with reference to China
(alone), and this seems unrealistic compared to what I remember reading
last time I looked (albeit this is from memory, I posted an Akimoto et
al ref here). Is someone actually claiming as much as 35% uncertainty in
_global_ emissions?
> 2. If aerosols may have a forcing of similar magnitude as CO2 at the
> moment, and may move up or down by 35% within a decade (and we don't
> really know whether they have or have not, let alone what they'll do
> between now and 2015), how can we be at all certain of short term
> temperature trends?
Standard estimate is about 1W for aerosols versus 2.4 for GHGs, which is
stretching "similar magnitude" rather (both numbers a bit out of date,
there must be something newer in AR4). Even assuming 35% uncertainty is
credible, 35% of 1W is something like 0.3C at equilibrium, less than
that 0.1C per decade. The IPCC forecasts are clearly predicated on the
SRES scenarios so it is no secret that something well outside
their range could give different results. We've already discussed how a
complete cessation of all aerosols could make an observable difference.
> 3. Specifically, a commenter at Climate Audit wonders whether Hansen
> really has been spectacularly right with his prediction, or whether it
> might just be that aerosol forcing went down unexpectedly and it was
> therefore just happy coincidence that Hansen was as close to the mark
> as he was?
As per the above, I don't think it is plausible that the change in
aerosols had an overwhelming effect on the net radiation balance. Note
that even as much as say 30% change in the overall forcing might not
have seriously contradicted his forecast, which was really a statement
of the fact "there is a substantial net radiative imbalance, sufficient
to dominate natural variability for the next couple of decades assuming
any plausible emissions scenario". With hindsight, not perhaps so
spectacular, but at its time...
Note that climate forecasts based on the full range of SRES emissions
barely diverge in ~30 years, even though the emissions themselves do.
James
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