Open Source Climate Modeling was Re: [asa] Crop Yields Face Non-Linear Effects Due to Climate Change

On Fri, Aug 28, 2009 at 7:13 AM, Bill Powers <wjp@swcp.com> wrote:

> As you say, it is good to see such model sensitivity studies using
> numerical models. Indeed, it is my view that this is the real value of such
> models, and not as reliable predictors of future weather.
>
> Regarding sensitivity, I hope that the numerical modelers have investigated
> the sensitivity of their own code to small variations, including numerical
> roundoff. Nonlinear codes are notorious for producing vastly different
> results just by changing the order in which factors are multiplied, etc.
>
> bill

The study used CCSM3 (Common Climate System Model 3). This is an open source
model so that differential studies can do true apples to apples comparisons.
As with all weather and climate circulation models CCSM3 solves
Navier-Stokes fluid flow equations which as you probably know is notoriously
sensitive to proper initial conditions. When used in climate modeling the
approach used by thermodynamics is used in order to get useful results far
into the future, averaging. The model is run multiple times, with slightly
different initial conditions, and then an average is computed. The spread of
the runs gives a sense of how accurate the results are. So, we have a good
idea what global average temperature will be but scientists are never, ever
satisfied. We want to know the local and regional effects of climate change
and we want greater and greater accuracy and we want more and more phenomena
accurately modeled. We also want to do climate simulation like weather
simulation where we know the specific sequence of events. Climate models are
counter-intuitive in that they are less accurate on shorter time scales than
on longer ones. We want them accurate at *all* time scales, short, medium,
and long.

The goals for improvement of CCSM3 include:

   - Increasing computer power, both in the U.S. and abroad, that can
   support more elaborate and more sophisticated models and modeling studies,
   using increased spatial resolution and covering longer intervals of
   simulated time;
   - improved understanding of many of the component processes represented
   in the CCSM, including cloud physics; radiative transfer; atmospheric
   chemistry, including aerosol chemistry, boundary-layer processes, polar
   processes, and biogeochemical processes; and the interactions of gravity
   waves with the large-scale circulation of the atmosphere;
   - improved understanding of how these component processes interact;
   - improved numerical methods for the simulation of geophysical fluid
   dynamics; and
   - improved observations of the atmosphere, including major advances in
   satellite observations.

Since this is open source, science is advanced by checking to see if adding
various hypotheses would make the model more accurate specifically in
regional simulations. I should note in passing that the main reason why
anthropogenic global warming is not mentioned as much any more in scientific
journals is that it is established science and the climatologists have moved
on to study local climate change. Leading journals only publish where there
are still questions to be answered.

Getting back to the study published today in *Science*. It looked at two
mechanisms: the top-down stratospheric response of ozone to
fluctuations of shortwave
solar forcing and the bottom-up coupled ocean-atmosphere surface response.
I've mentioned before that one of the problems of GCMs is modeling the
tropics. This study looked at very small inaccuracies in the model during
solar maxima with one local region that had* the precipitation
underestimated* and another region that had the *sea surface temperatures
overestimated*. Over the long term, the solar effects average out to zero
since they are on a roughly 11-year cycle. This is why the denialists who
are pushing solar irradiance as a cause of global warming are just plain
silly. Still, we are past average now and we want to accurately model the
local effects throughout the entire solar cycle.

In order to match observations during solar maxima we want to see the
following differences in the climate model output: enhance the climatological
off-equatorial tropical precipitation maxima in the Pacific, lower the
eastern equatorial Pacific sea surface temperatures, and reduce low-latitude
clouds to amplify the solar forcing at the surface.

When the two mechanisms above we're added in concert to the CCM3 model, it
produced the desired results. This is not the only study that used the CCM3
models to get greater local accuracy. This is how computer models -- far
from being completely inaccurate -- are an important tool in improving our
understanding of the climate. More accurate predictions of the future are
just a happy side effect. I've enclosed a link below with a bibliography of
CCM3-based studies. If the denialists think the GCMs are inaccurate they
have access to the code and they can do what many of the papers below did
and improve the models. By doing that, they would do the World a huge favor
so that we get the most accurate predictions of anthropogenic global warming
in order that the best public policy could be crafted.

Rich Blinne
Member ASA

http://www.ccsm.ucar.edu/publications/bibliography.html

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Received on Fri Aug 28 11:48:35 2009