Climate: Unraveling the borehole riddle
- Climate: Unraveling the borehole riddle
By Dan Whipple
Boulder (UPI) April 12, 2004
Progress in the genuine understanding of climate change, as with all
science, is achieved through the accumulation of detail and, in some
cases, excruciating detail.
Case in point: geologic boreholes.
In order to understand whether the modern climate warming is
exceptional, scientists must learn how the planet's atmosphere
behaved in the past. This is a complex task. Few temperature records
go back more than two centuries or so, and most of what does exist
tends to be concentrated in particular regions. For instance, there
are thermometer measurements for the 19th century for North America
and Europe, but virtually none for anywhere in the Southern
The problem of determining a global average temperature, even today --
never mind past millennia -- is further complicated by the fact that
two-thirds of the Earth is covered by oceans, which until very
recently had no current thermometry records and few ways of obtaining
Scientists from various disciplines have devised clever ways to
overcome this lack of data. They have relied on so-called proxy
records, which can provide a great deal of information, albeit
indirectly. As a result, each one, by definition, has limitations and
each one has generated a controversy about its interpretation.
To reconstruct Earth's climate, scientists have looked at a number of
proxy records, such as tree rings. They contain potential temperature
and precipitation data that can be preserved in fossilized remnants
as well as living trees.
Climate researchers also have examined coral reefs, whose growth
varies with water temperature. They have studied ocean sediments,
from which oxygen isotope contents can infer temperature. They have
extracted ice cores, which trap oxygen and hydrogen and can provide a
proxy for temperature records. They have calculated glacier
movements, which advance and retreat depending on temperature.
They also have drilled boreholes -- core samples extracted from soil
and rock instead of ice -- to attempt to determine how climatic
processes have transferred surface heat to the underground
Although all of these proxy records have drawn reactions ranging from
differing interpretations to outright disagreement, a recent exchange
over boreholes demonstrates how the details of climate science can
affect very large conclusions about the warming Earth.
Gavin Schmidt, of the NASA Goddard Institute for Space Studies and
Climate Systems Research at Columbia University in New York, and
Michael Mann, of the Department of Environmental Sciences at
University of Virginia in Charlottesville, published a paper recently
in the journal Geophysical Research Letters that argued snow cover
affected and perhaps skewed the borehole record.
"Our paper wasn't that fundamental," Schmidt told United Press
International. "I'm not going to claim that it was groundbreaking
work. Changes in snow cover can affect the temperatures that get
transmitted to deep boreholes. You're not going to find anybody who
is going to disagree with that."
As it turns out, some geologists are taking exception, including
David Chapman, Marshall Bartlett and Robert Harris, all of the
Department of Geology and Geophysics at the University of Utah in
Salt Lake City.
In a comment published in GRL, Chapman and the others argued: "We
have been working on the same question as Mann and Schmidt -- the
fidelity of ground surface temperature and surface air tracking, in
particular the effect of seasonal snow cover ... But our analysis,
based not on model simulation but analysis of measurements of GST and
SAT at observatories ... leads us to very different conclusions."
The most important point, Bartlett told UPI, "is that there is a
difference between the inferred amount of warming that has occurred.
We have confidence that the physics is really well understood. Any
change in the surface temperature gets conducted to the solid rock."
The interpretation resulting from these differences could be
substantial. Bartlett said the borehole records show warming to be
about twice as large since the advent of the industrial age as other
estimates. He also noted the trio's results "indicate that the
warming induced by industrialization since 1750 is more than we see
in the proxy records and agrees with the surface air temperature
This seemingly technical matter attracted the attention of Harvard
University astrophysicist Willie Soon, a controversial figure among
climate scientists, who argues that modern warming is not exceptional
in the historic period and the so-called medieval warm period -- from
about 800 to 1300 A.D. -- was warmer than today. Soon is also a
senior scientist at the George Marshall Institute in Washington,
D.C., one of the leading skeptical think tanks on climate.
Soon e-mailed the analyses by Schmidt and Mann and by Chapman,
Bartlett and Harris to journalists with a brief note: "Just in case
it is of interest." What apparently attracted his attention was not
the technical arguments so much as the fact Mann was one of the co-
authors. Mann's work on modern temperature increases was cited by the
Intergovernmental Panel on Climate Change's 2001 assessment on global
"Michael Mann has become a lightning rod for everybody and their
dog," Schmidt said. "It's a ridiculous tactic. Obviously, you know
why they do it. Mike has been getting the same thing ever since that
assessment report. He's a combative guy. He likes to carry ball to
"I work in a modeling group," Schmidt added. "I don't have any
particular axes to grind. General climate model experiments give you
a laboratory to test different things. You make things change with
respect to solar forcing, to greenhouse gases. To what extent does
this model reflect the real world?"
Schmidt explained that because snow cover in Eurasia actually has
increased over the last few decades.
"It's a very complicated story and involves four dynamics of climate.
There isn't going to be a 'gotcha' moment, when everything is going
to be clear, where you get the temperature for the middle of the
Little Ice Age," he said.
Dan Whipple covers the environment for UPI Science News. E-mail
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