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 new 300-site survey of borehole
temperatures spanning four conti- nents and five centuries has confirmed
what most scientists already believe the Earth is getting warmer and the
rate of warming has been accelerating rapidly since 1900.
 "In terms of climate change, the
20th century has not been just another century," said Henry N. Pollack,
University of Michigan professor of geological sciences. "Subsurface
rock temperatures confirm that the average global surface temperature has
increased about 1 degree C. (1.8 degrees F.) over the last five centuries
with one-half of that warming taking place in the last 100 years. The 20th
century is the warmest and has experienced the fastest rate of warming of
any of the five centuries in our study."
Pollack presented temperature readings
from 300 underground boreholes in Europe, North America, Australia and South
Africa at the American Geophysical Union meeting held last month. |
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Pollack is one of several geologists
who take the Earth's temperature by lowering sensitive thermometers into
boreholes drilled from the surface. Because subsurface rocks preserve a
record of actual surface temperature changes over time, boreholes are an
important data source for scientists studying global climate change. Short-term
changes, such as seasonal variations, penetrate only a few meters underground.
Long-term changes on scales of hundreds of years are preserved at greater
depths. Since meteorological data has been recorded globally only for the
last 100 years or so, borehole temperatures are especially important in
determining surface temperature for previous centuries.
Individual borehole temperatures can
be skewed by local topography or climate conditions, so Pollack and assistant
research scientist Shaopeng Huang merged the readings into continental data
ensembles to balance out local effects and let regional trends come through.
They then combined all four regions to get a global average. Because meteorologists
track long-term climate changes in 100-year intervals, Pollack and Huang
also looked for century-long trends in borehole data.
When they compared the average worldwide
borehole temperature change with global meteorological records over the
last century, they found both recorded a 0.5 degree C. average global temperature
increase since 1900. "The ground says the same thing the air says,"
Pollack explained. |
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According to Pollack, 80 percent of
the total 1 degree C. warming recorded in borehole readings from 1500 to
the present occurred after 1750 when people began large-scale burning of
coal, wood and other fossil fuels during the Industrial Revolution. Since
most warming has taken place after 1750, Pollack believes it is likely a
direct result of human activity, rather than a natural climate fluctuation.
"If the upward trend of greenhouse
gas emissions continues, we can expect another 1 degree C. increase in average
global temperature by 2050," Pollack said. "This estimate is not
based on model computations, but a projection of actual data. Our results
agree with the estimates of global climate warming issued by the United
Nations' Intergovernmental Panel on Climate Change (IPCC) and are fully
consistent with the conclusion of the IPCC's scientific panel that human
activity is a significant driving force behind global warming."
Pollack's study has been funded by the
National Science Foundation and the Czech-USA Cooperative Science Program. |
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Just to make things more confusing,
new computer modeling, also from the University of Michigan, suggests that
global warming might not be a product of human activity. Ironically,
argues Joyce Penner, professor of atmospheric, oceanic and space sciences,
carbon and sulfur emissions can have the reverse effect, serving to cool
down the planet.
Penner, an expert in computer simulations
of climate change, recently found that, whereas greenhouse gases have led
to a warming of 2.5 Watts per square meter (W/m2), aerosols like soot particles
and sulfuric acid reflect nearly twice as much energy under certain conditions.
"This effect clouds our understanding
of climate change over the last 100 years, but still cannot protect us from
the larger increases in greenhouse gases expected in the future," Penner
said. "If further research serves to uphold these initial findings,
the warming we've seen over the last 100 years may simply be due to natural
variability," she said. Penner is presented the work at last fall's
meeting of the American Geophysical Union in San Francisco.
When floating freely in air, carbon
aerosols from fossil fuel emissions add between .16 and .20 W/m2 to the
atmosphere, and thus heat it. But, according to the latest simulations by
Penner and her colleagues at the Lawrence Livermore National Laboratory,
in Livermore, Calif., and the Centre des Faibles Radioactivites, in Gif
sur Yvette, France, carbon aerosols trapped in clouds may cool the earth's
atmosphere by as much as -4.4 W/m2 or a net decrease of roughly 0.7 to 2.1
degrees Celsius, provided nothing changed in the future.
Penner said that the models are uncertain
because they rely on poorly known estimates for natural sources of aerosols,
so the actual number for negative forcing could be as low as -2.4 W/m2.
Even so, she said, the results are startling. "I had not expected to
get such a large negative forcing from carbon aerosols in clouds. If these
results hold up, we are going to have to do a lot more work to understand
how climate might change in the future."  |
