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By Phillip Gentry Daily University Science
News
Temperature
data from scientific buoys scattered across the Pacific Ocean
are raising doubts about the validity of one of the most important
tools used by scientists to track global climate change.
The
"lock step" link between sea water temperatures and
air temperatures may be less rigid than presently thought, according
to data analyzed by scientists at the University of Alabama in
Huntsville (UAH) and the Hadley Center of the United Kingdom's
Meteorological Office.
Results of
their research are reported in the Jan. 1, 2001, edition of the
scientific journal, Geophysical Research Letters.
The supposed
link between sea and air temperatures let climate scientists use
sea surface temperatures as a "proxy" for air temperature
data over large ocean areas for which air temperature data are
not available, said Dr. John Christy, a professor of atmospheric
science and director of UAH's Earth System Science Center.
"The
global surface temperature datasets -- the data that people commonly
use to track Earth's climate -- are a mixture of near-surface
air temperatures over land and sea water temperatures over the
oceans," Christy said.
Taking the
sea surface data out of the global climate record would have a
significant impact on climate tracking and forecasts. When scientists
take sea surface temperatures out of the global temperature record
for the past 20-plus years and replace them with air temperature
data gathered by ships and buoys, the global warming trend at
Earth's surface drops by about one-third -- from 0.19 to about
0.13 degrees Celsius per decade.
Using high-precision
temperature data gathered by 19 buoys moored throughout the tropical
Pacific Ocean and monitored by NOAA's Pacific Marine Environmental
Laboratory in Seattle, Christy, his British colleagues and a Danish
scientist compared long-term (8- to 20-year) trends for temperatures
recorded one meter below the sea surface and three meters above
it.
"For
each buoy in the Eastern Pacific, the air temperatures measured
at the three meter height showed less of a warming trend than
did the same buoy's water temperatures at one meter depth,"
Christy said. "These are from thermometers separated vertically
by only four meters and monitored at the same time. And the Eastern
Pacific plays an important role in global temperature variations,
through the El Niño heating and La Niña cooling
events."
In the Western
Pacific, it was a "murky picture," Christy said, with
little correlation between water and air temperature changes.
Buoy-by-buoy, seasonal temperature variations in the sea water
explained less than 40 percent of air temperature changes.
That means
if seawater temperatures in the Western Pacific go up from one
season to the next, the air just above the sea surface doesn't
necessarily follow.
By comparison,
water temperatures explained more than 90 percent of the air temperature
fluctuations in the Eastern Pacific.
Over the tropical
Eastern Pacific Ocean, buoy data shows a near-surface seawater
warming trend of 0.37 degrees Celsius per decade, while air temperatures
three meters above the surface were warming by only 0.25 degrees
C per decade during the 20-year test period -- a change of 0.12
degrees C per decade in slightly more than 12 feet.
"It's
odd that over the past eight to 20 years, the air just above the
surface isn't warming at the same rate that the sea water is,"
said Christy.
The supposed
link between sea surface temperatures and air temperatures is
an integral part of both the historic surface temperature record
and the computerized models used to predict what Earth's climate
might do in the future.
Because reliable
low-level air temperature data from over the oceans are more scarce
and more difficult to assess than water temperatures, scientists
monitoring Earth's climate have used sea surface temperatures
as a proxy for air temperatures, assuming that the two rise and
fall proportionally.
"We found
that in the short term, they go up and down essentially simultaneously,"
said Christy. "Over the long term, however, we start to see
differences."
More than
20 years of data gathered by microwave sounding units on NOAA's
TIROS-N satellites shows global warming in the atmosphere from
Earth's surface up to approximately five miles to be about 0.045
degrees Celsius per decade, a trend confirmed by data from "radiosonde"
thermometers lifted through the troposphere by helium balloons.
The apparent
disagreement between climate trends at the surface and in the
troposphere has been the subject of an often heated scientific
debate over the validity of the two datasets. The buoy data offered
the UAH/UKMO/Danish research team a rare opportunity to test the
accuracy of the sea-water-for-air-temps proxy using scientifically
calibrated, co-located instruments.
By comparison,
much of the historic sea water temperature record was generated
by military and commercial ships, which recorded the temperature
of sea water as it was taken aboard as an engine coolant. While
calculated into the temperature record as sea "surface"
temperatures, most modern ships draw in cooling water from as
much as ten meters below the surface.
The authors
looked at the tropicswide difference between the sea water temperatures
and upper air temperatures not only from the satellite data but
from balloons and global weather maps. All three records indicated
the tropical air between the surface and five miles actually cooled
at a rate of about 0.05 degrees C per decade, while the sea water
was warming by about 0.13 degrees C per decade.
The tropicswide
near-surface air temperature (from ships and buoys) warmed at
a rate in between the sea water and the upper air -- about 0.06
degrees C per decade. These differences were all statistically
significant.
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