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.
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
principal 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.
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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.