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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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