Re: [asa] New Climate Model, Short-Term Cooling in a Warmer World

On 5/2/08, Randy Isaac <randyisaac@comcast.net> wrote:
>
> From science mag:
>
> "To take account of such ocean-driven natural variability, Keenlyside and
> his colleagues began their model's forecasting runs by giving the model's
> oceans the actual sea surface temperatures measured in the starting year of
> a simulation. Providing the initial state of the ocean doesn't make much
> difference when forecasting out a century, so long-range forecasters don't
> usually bother. But an initial state gives the model a starting point from
> which to calculate what the oceans will be doing a decade hence and
> therefore what future natural variability might be like."
> In other words, long-term predictions are relatively insensitive to
> initial conditions so there's no need to take the time to load them.
> Short-term fluctuations are, understandably, more sensitive to initial
> conditions. I think it does contribute to an increase in confidence of the
> models.
>
> Randy
>
> Randy's got that exactly right. The paper attempted to deal with
predicting at decadal time scales. For this you need initial conditions. The
difference between weather modelling and climate modelling is the whole
issue of initial conditions. Weather modelling is dominated by initial
conditions and climate by energy balance. This is the reason why most people
don't trust GCMs is because weather forecasts can be off with bad initial
conditions and they think climate modelling degrades with time like weather.
Rather, the accuracy of the climate model actually improves with longer time
scales. This paper synthesizes the two approaches to improve the accuracy
of the climate models at shorter time scales and smaller areas (North
America and Europe) *which counterintuitively are less accurate. *The
particular model in the paper was less successful in modelling the tropical
North Atlantic and central Africa. So, giving better initial conditions of
the Meridional Overturning Circulation is still not quite good enough. Some
may remember the "not going to happen" scenario of MOC completely shutting
down was the basis for the movie "The Day After Tomorrow". What is actually
happening however is that local cooling can be the result of global warming
-- just not nearly as catastrophic as our friends in Hollywood made it out
to be. Whether and how much the increased fresh water in the North Atlantic
that is the result of global warming will inhibit the Gulf Stream is still a
pretty open question. We need to have more monitoring here.

The News and Views article in May 1, 2008 Nature had the following caption
to a graph that had three lines that oscillated all over the place but
converged at 2050.

> These three possible trends of winter temperature in northern Europe from
> 1996 to 2050 were simulated by a climate model using three different (but
> plausible) initial states6<http://www.nature.com/nature/journal/v453/n7191/full/453043a.html#B6>.
> The choice of initial state crucially affects how natural climate variations
> evolve on a timescale of decades. But as we zoom out to longer timescales,
> the warming trend from greenhouse gases begins to dominate, and the initial
> state becomes less important. Keenlyside and colleagues2<http://www.nature.com/nature/journal/v453/n7191/full/453043a.html#B2>use observations of the sea surface temperature to set the initial state of
> their model. Their results indicate that, over the coming decade, natural
> climate variability may counteract the underlying warming trend in *some
> regions around the North Atlantic*. (Figure courtesy of A. Pardaens, Met
> Office Hadley Centre). [emphasis mine]
>
See my emphasis. This is not even a global cooling. So, far from being a
problem for climate change science it shows that the weaknesses of the
models, namely local effects and decadal time scales so we can do real,
usable, predictions are being addressed. I have noticed that now all of
sudden these models are being treated as "gospel" by the denialist community
on the web. How a relative minor (and preliminary) tweak makes the models
credible reveals how "outcome based" they are. Speaking of denialists, the
"Little Ice Age of January 2008" is over. March 2008 was the second warmest
March ever after 2002. The lower troposphere data for April shows 2008 for
part of the month even warmer than 2007 which was the warmest year in the
satellite record.
http://discover.itsc.uah.edu/amsutemps/execute.csh?amsutemps+001 GISS should
be posting the April info in about two weeks which should be very
interesting. The ENSO observations and forecasts are showing a moderation of
La Nina with most models showing neutral ENI by mid-year. Despite what the
Daily Telegraph may be telling you, global warming has not stopped.

Rich Blinne

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Received on Fri May 2 15:55:38 2008