Climate change -- the latest results

[Guest Baldin Lee Pramer]

Hi everyone. A distinguished panel of mathematicians recently met to

go over the models and methods of global climate modeling. Here is a

short article in the SIAM (Society for Industrial and Applied

Mathematics) news about their findings:

 

http://www.siam.org/news/news.php?id=1131

 

Mathematicians Confront Climate Change

June 12, 2007

Dana Mackenzie

 

This April, with headlines about climate change appearing almost

daily, climate scientists and mathematicians got together for a unique

workshop on how climate models might be improved. These models,

although highly mathematical, are often designed, interpreted, and

tweaked by people whose main expertise is not mathematics.

Participants agreed that mathematicians can and should play a more

active role in clarifying the meaning of climate models, figuring out

how to aggregate the results of different models, and adding new

components (such as models of sea ice) that are not treated adequately

in the existing models.

 

The Symposium on Climate Change, sponsored by the Mathematical

Sciences Research Institute in Berkeley, featured two separate but

intertwining events. On April 11, Congressman Jerry McNerney and

California state legislator Ira Ruskin joined six climate and energy

experts for a public forum in San Francisco that drew about 320

people. On the following two days, about 75 people met at MSRI for a

scientific symposium directed more specifically to problems of climate

modeling.

 

Climate change (which includes, but is not limited to, global warming)

has finally begun to enter public discourse not as a political slogan,

but as a reality that humans will have to deal with over the next

century. Last fall, the British government issued the Stern Review, a

first attempt to quantify the economic impact of climate change. This

spring, the Intergovernmental Panel on Climate Change (IPCC) issued a

series of assessments in which they concluded, among other things,

that the climate is warming and that it is "very likely" that most of

the warming is caused by humans. Even the U.S. Supreme Court got into

the act, with an April 2 ruling that carbon dioxide emissions are

subject to regulation as a pollutant.

 

Mathematicians have been largely absent from the public debate---even

though many policy decisions will hinge on the interpretation of

complicated, highly mathematical climate models. "I think that

mathematicians as a community are reluctant to get involved in

something that is politically so hot," says Christopher Jones of the

University of North Carolina, an organizer of MSRI's scientific

symposium. "But we can't afford not to get involved." The primary goal

of the symposium was to explore ways to attract mathematicians to what

is, for them, a largely unexplored branch of science. "We need a stamp

of approval that tells mathematicians, especially young

mathematicians, that this is an okay program of research," says Mary

Lou Zeeman of Bowdoin University, who helped to organize the

scientific symposium.

 

The Symposium on Climate Change was organized with unusual rapidity.

The seeds were planted in November, when McNerney, who has a PhD in

mathematics and worked for many years on alternative sources of

energy, won election to Congress in California's 11th district (south

of Berkeley). "We got interested in hosting an event with McNerney,

and then many things began suggesting climate change as a subject,"

says David Eisenbud, the departing director of MSRI. Eisenbud and

Jones, who was at MSRI for a semester-long program on dynamical

systems, began inviting speakers in February, only two months before

the event. "It was a miracle that it all came together, but that's a

sign of how much people care about this topic," Jones says.

 

Eisenbud invited Inez Fung, co-director of UC Berkeley's Institute for

the Environment, to be the keynote speaker at both the public forum

and the scientific symposium. Fung presented a compelling scientific

case that global warming is real and has been caused by human

activity. The dossier of evidence, she said, contains at least five

clues:

 

the extremely tight correlation between historical carbon

dioxide levels and historical climate, as determined from ice-core

samples;

the dramatic increase in measured concentrations of carbon

dioxide (and the even more pronounced increase in methane

concentrations, a greenhouse gas that is 20 times more potent than

carbon dioxide);

isotopic evidence that the additional carbon dioxide has come

from fossil-fuel burning;

an observed increase in temperature over recent years, with 11

of the 12 warmest years on record occurring since 1995; and

the unanimity of climate models in forecasting an increase in

temperature over the next century (with the increases ranging from 1

to 12 degrees Centigrade).

 

The clarity of global warming dissolves into frustrating ambiguity

when climate modelers are asked to predict the future, however. One

speaker after another at the symposium lamented the use of climate

models as crystal balls. "There are demands being made on these

climate models that the models weren't constructed for," says Doug

Nychka, a statistician at the National Center for Atmospheric

Research.

 

For example, the IPCC report relies on 24 climate models, most of

which were developed by various national weather services (NCAR being

one of them). These models all share some common physics: the

conservation of mass and energy in the atmosphere and in the ocean,

radiative forcing from the sun, and so on. Nevertheless, they are very

different in their detailed assumptions. Yet the IPCC report averages

them all---a process comparable to averaging apples and oranges to

determine what a generic fruit looks like.

 

To make things worse, every individual model contains vast

uncertainties. No model includes a realistic description of sea ice,

according to Cecilia Bitz of the University of Washington---a

particularly troublesome problem, because the ice in the Arctic Ocean

and Greenland is melting much more rapidly than the models predict.

The descriptions of the ocean in most climate models are highly

simplistic, and most ocean models treat the atmosphere equally

roughly. And then there is the modelers' dirty secret: Every model has

parameters that are "tuned" to make the output match observations more

closely. In some cases, as pointed out by William Collins of UC

Berkeley, the tuning causes the models to violate the very physics

that has been so painstakingly included.

 

Finally, even a perfect model may still produce a range of possible

answers, because of what Jim McWilliams of UCLA called "irreducible

uncertainty." Weather models can project wind speeds a few hours into

the future, but the wind speed over Scotland in 2050 may be

essentially unknowable.

 

Most of the speakers expressed their belief that these problems can be

addressed mathematically. Statisticians may turn up better ways to

combine models than simple brute-force averaging. Tuning should be

done openly, not clandestinely, with attention to defining the

parameter space in which the values are being chosen. Above all, the

uncertainties should be embraced, and not hidden. "Somehow, you have

to shed the idea that the models represent an exact state of the

environment," Jones says. "My suspicion," says Nychka, "is that there

will be a continuum of models, and a person will have to make a

choice, or else cast the results as a distribution."

 

The symposium also included some impressive examples of mathematical

climate analysis. Ben Santer of Lawrence Livermore National Laboratory

explained how to attribute climate changes to human or nonhuman causes

(such as solar fluctuations or volcanoes), by combining several

different climate variables into an "anthropogenic warming pattern."

Max Aufhammer of UC Berkeley merged economics and climate science to

quantify the effects of aerosols and temperature increases on rice

harvests in Asia. (Both had a negative effect; the atmospheric haze,

however, had a stronger effect than warming.)

 

Zeeman says that the symposium held two significant surprises for her.

One is that climate modelers already have models of "intermediate

complexity," which they use to build intuition but don't publish.

"Those are the models dynamical systems people would love to get their

hands on," she says. "Could we get the modelers to make them

available?"

 

The second surprise, Zeeman says, is "the role that mathematicians can

serve as translators." For example, mathematicians can help

atmospheric scientists talk with oceanographers, or can help formulate

climate models in terms of economic risk. According to Hans Kaper, a

program director for applied mathematics at the National Science

Foundation, economists and businesspeople must be brought into the

study of climate change. "Their input will give you credibility to the

outside world," Kaper says.

 

Readers who want to learn more about climate modeling won't have long

to wait. Zeeman is organizing a SIAM minisymposium on climate change

for next January's Joint Mathematics Meetings, where Fung is scheduled

to give a plenary talk. Other workshops and summer schools will surely

follow. (MSRI, the Institute for Mathematics and its Applications, and

the Newton Institute for Mathematical Sciences have all expressed

interest.) Says Nychka, "There's a lot of math to do here, and some of

it, I think, has not been invented yet."

 

Dana Mackenzie writes from Santa Cruz, California.

 

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Baldin Pramer