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Science Forum Index » Environment Forum » Medieval Global Warming
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| David Naugler |
 Posted: Wed Dec 17, 2003 3:16 pm |
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From:
http://www.technologyreview.com/articles/wo_muller121703.asp
Medieval Global Warming
A controversy over 14th century climate shows the peril of letting
politics shape the scientific debate.
By Richard Muller
Technology for Presidents
December 17, 2003
Six hundred years ago, the world was warm. Or maybe it wasn't. What's
the truth? Beware. This question has recently been elevated from a
mere scientific quandary to one of the hot (or cold) issues of modern
politics. Argue in favor of the wrong answer and you risk being
branded a liberal alarmist or a conservative Neanderthal. Or you might
lose your job.
Six editors recently resigned from the journal Climate Research
because of this issue. Their crime: publishing the article "Proxy
Climatic and Environmental Changes of the Past 1,000 Years," by W.
Soon and S. Baliunas of the Harvard-Smithsonian Center for
Astrophysics.
Without passing judgment on this particular paper, I can still point
out that our journals are full of poor papers. If editors were
dismissed every time they published one, they would all be out of work
within a month or two. What made the Soon and Baliunas situation
different is that their paper attracted enormous attention. And that's
because it threw doubt on the hockey stick.
If you don't know what the hockey stick is, do a Google search,
including the word "climate." You'll learn that it is the nickname for
a remarkable graph that has become a poster child for the
environmental movement. Published by M. Mann and colleagues in 1998
and 1999, the plot showed that the climate of the Northern Hemisphere
had been remarkably constant for 900 years until it suddenly began to
heat up about 100 years ago—right about the time that human use of
fossil fuels began to push up levels of atmospheric carbon dioxide.
The overall shape of the curve resembled a hockey stick laying on its
back—a straight part with a sudden bend upwards near the end.
The hockey stick was turned from a scientific plot into the most
widely reproduced picture of the global warming discussion. The
version below comes from the influential 2001 report of the
Intergovernmental Panel on Climate Change (IPCC). The hockey stick
figure appears five times in just the summary volume alone.
See: http://www.technologyreview.com/articles/wo_muller121703.asp
The "hockey stick"
(from the IPCC 2001 report)
Soon the graph acquired a very effective sound bite: 1998 was the
warmest year in the last thousand years. This carried a compelling
conclusion: global warming is real; humans are to blame; we must do
something—hurry and ratify the Kyoto treaty on limitations of fossil
fuel emissions. Yet some scientists urged caution, a go slow approach.
As a wise man once warned, "do not let the merely urgent interfere
with the truly important."
There was a minor scientific glitch. The hockey stick contradicted
previous work that had concluded that there had been a "medieval warm
period." In fact, it disagreed with a plot published by the IPCC
itself a decade earlier (in its 1990 report) that showed pronounced
warm temperatures from the years 1000 to 1400.
Such inconsistencies are common in science, and scientists love them.
They mean more work, maybe a little public attention (which can't hurt
funding), and the excitement that comes with the effort to resolve
uncertainty. The Soon and Baliunas paper was part of this process.
Their paper presented all the data in favor of the medieval warm
period.
The debate grew. Critics of Soon and Baliunas charged that their paper
wasn't balanced; because it consisted of a compilation of data showing
warming at different locations at different times, the criticism went,
the work was not a valid refutation of the hockey stick analysis,
which had combined a much larger set of data. That was a valid
concern, but it didn't necessarily mean that the Soon and Baliunas
results should be ignored. It simply meant that the issue was still
open.
Meanwhile, critics excoriated Climate Research for allegedly failing
to vet the Soon and Baliunas paper properly. The publisher, a German
company called Inter-Research, agreed, leading to the resignation of
the journal's editor-in-chief and, eventually, five other editors.
Then last month the situation became even more complex. S. McIntyre
and R. McKitrick published a paper in Energy and Environment with a
detailed critique of the original hockey stick work. They stated
bluntly that the original Mann papers contained "collation errors,
unjustifiable truncations of extrapolation of source data, obsolete
data, geographical location errors, incorrect calculations of
principle components, and other quality control defects." Moreover,
when they corrected these errors, the medieval warm period came
back—strongly. Mann, et al., disagreed. They immediately posted a
reply on the Web, with their criticism of McIntyre and McKitrick's
analysis.
The disagreement is not political; most of it arises from valid issues
involving physics and mathematics. First the physics. An accurate
thermometer wasn't invented until 1724 (by Fahrenheit), and good
worldwide records didn't exist prior to the 1900s. For earlier eras,
we depend on indirect estimates called proxies. These include the
widths of tree rings, the ratio of oxygen isotopes in glacial ice,
variations in species of microscopic animals trapped in sediment
(different kinds thrive at different temperatures), and even
historical records of harbor closures from ice. Of course, these
proxies also respond to other elements of weather, such as rainfall,
cloud cover, and storm patterns. Moreover, most proxies are sensitive
to local conditions, and extrapolating to global climate can be
hazardous. Chose the wrong proxies and you'll get the wrong answer.
The math questions involve the procedures for combining data sets.
Mann used a well-known approach called principle component analysis.
This method extracts from a set of proxy records the behavior that
they have in common. It can be more sensitive than simply averaging
data, since it typically suppresses nonglobal variations that appear
in only a few records. But to use it, the proxy records must be
sampled at the same times and have the same length. The data available
to Mann and his colleagues weren't, so they had to be averaged,
interpolated, and extrapolated. That required subjective judgments
which—unfortunately—could have biased the conclusions.
When I first read the Mann papers in 1998, I was disappointed that
they did not discuss such systematic biases in much detail,
particularly since their conclusions repealed the medieval warm
period. In most fields of science, researchers who express the most
self-doubt and who understate their conclusions are the ones that are
most respected. Scientists regard with disdain those who play their
conclusions to the press. I was worried about the hockey stick from
the beginning. When I wrote my book on paleoclimate (published in
2000), I initially included the hockey stick graph in the
introductory chapter. In the second draft, I cut the figure, although
I left a reference. I didn't trust it enough.
Last month's article by McIntyre and McKitrick raised pertinent
questions. They had been given access (by Mann) to details of the work
that were not publicly available. Independent analysis and (when
possible) independent data sets are ultimately the arbiter of truth.
This is precisely the way that science should, and usually does,
proceed. That's why Nobel Prizes are often awarded one to three
decades after the work was completed—to avoid mistakes. Truth is not
easy to find, but a slow process is the only one that works reliably.
It was unfortunate that many scientists endorsed the hockey stick
before it could be subjected to the tedious review of time.
Ironically, it appears that these scientists skipped the vetting
precisely because the results were so important.
Let me be clear. My own reading of the literature and study of
paleoclimate suggests strongly that carbon dioxide from burning of
fossil fuels will prove to be the greatest pollutant of human history.
It is likely to have severe and detrimental effects on global climate.
I would love to believe that the results of Mann et al. are correct,
and that the last few years have been the warmest in a millennium.
Love to believe? My own words make me shudder. They trigger my
scientist's instinct for caution. When a conclusion is attractive, I
am tempted to lower my standards, to do shoddy work. But that is not
the way to truth. When the conclusions are attractive, we must be
extra cautious.
The public debate does not make that easy. Political journalists have
jumped in, with discussion not only of the science, but of the
political backgrounds of the scientists and their potential biases
from funding sources. Scientists themselves are also at fault. Some
are finding fame and glory, and even a sense that they are important.
(That's remarkably rare in science.) We drift into ad hominem
counterattacks. Criticize the hockey stick and some colleagues seem to
think you have a political agenda—I've discovered this myself. Accept
the hockey stick, and others accuse you of uncritical thought.
There are also the valid concerns of politicians who have to make
decisions in a timely way. In 1947, Harry Truman grew so annoyed at
the prevarications of economists that he joked that he wanted a
one-armed advisor—who could not hedge his conclusions with the phrase
"on the other hand."
Some people think that science is served by open debate between
left-handed and right-handed advocates, just as in politics. But the
history of science shows it is best done by people who have two hands
each. Present results with caution, and insist on equivocating. Leave
it to the president and his advisors to make decisions based on
uncertain conclusions. Don't exaggerate the results. Use both hands.
We cannot afford to lower our standards merely because the problem is
so urgent.
Richard A. Muller, a 1982 MacArthur Fellow, is a physics professor at
the University of California, Berkeley, where he teaches a course
called "Physics for Future Presidents." Since 1972, he has been a
Jason consultant on U.S. national security. |
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