 arth's atmosphere has gotten about one-tenth
of a degree Celsius warmer in the past 20 years, with most of the warming
concentrated in the Northern Hemisphere, according to data gathered by NOAA
satellites and released by The University of Alabama in Huntsville.
 All of the global warming trend found
in the 20-year temperature dataset, however, can be attributed to the major
El Niño, Pacific Ocean heating event of 1997-1998, says Dr. John
Christy, an associate professor of atmospheric science in UAH's Earth System
Science Laboratory.
"When we look at the first 19
years of satellite data, from 1979 through 1997, there is no global climate
trend at all -- either up or down," Christy said "Due to the powerful
El Niño, 1998 was a singular year.
"Obviously, El Niños
are part of the natural weather cycle and shouldn't be discounted. When
we look at long-term trends, however, we shouldn't assign excess importance
to individual unusual or extreme short-term events, such as this El Niño
or the cooling that followed the eruption of the Pinatubo volcano in 1991.
"You can't draw any conclusions
about long-term global climate trends based on a heat wave here or a cold
snap there.
"For instance, 1998 was easily
the hottest year in the 20-year global temperature record and the 1998 average
temperature in the contiguous 48 states appears to be the warmest since
1896. But the United States didn't have a single state record its statewide
all-time high temperature in 1998. That didn't make 1998 any cooler from
a global perspective.
"Some individual stations in
the United States had record highs, but that happens frequently. There are
more than 7,000 individual stations, so that's to be expected."
Globally, the temperature trend from
January 1979 through December 1998 was warming at the rate of about 0.06
Celsius per decade. That equals a warming trend of just over one-half degree
Celsius (about one degree Fahrenheit) per century.
Scientifically, any trend that small
over a period of time as short as 20 years could be considered statistical
"noise," according to Christy.
While large portions of the Northern
Hemisphere showed a warming trend over the 20-year study period, most of
the Southern oceans showed a long-term cooling. That split saw the Northern
Hemisphere warm by about 0.2 C in 20 years, while the Southern Hemisphere
got neither warmer nor cooler.
Almost all of North America, the
North Atlantic and Europe saw temperatures warm slowly from 1979 through
1998. The region showing the "fastest" warming trend was along
the coast of Canada's Northwest Territories, where temperatures warmed as
much as 1.6 C (almost 2.9 Fahrenheit).
The next fastest warming region included
most of Siberia, Mongolia, Northern China, Korea and Japan, with temperatures
rising as much as 1.4 C (more than 2.5 Fahrenheit) in 20 years.
At the same time, 20-year cooling
trends were found over Labrador, Antarctica, most of the Southern oceans,
and a region stretching eastward from Central Africa across the Middle East,
the Indian subcontinent and Southeast Asia.
While most of the noteworthy warming
in the past 20 years was over the more industrial and more heavily populated
Northern Hemisphere, Christy said it is unlikely that this regional warming
can be attributed to man-made air pollution or an enhanced "greenhouse
effect."
Burning coal, petroleum products,
natural gas and wood returns carbon dioxide to the atmosphere. Carbon dioxide
is a greenhouse gas which traps heat rising from Earth's surface. In recent
years, climate scientists have theorized that the additional carbon dioxide
might create an enhanced greenhouse effect, which would cause global warming.
While carbon dioxide levels in the
atmosphere have risen in recent years due to burning fossil fuels, the extra
CO2 gas has generally spread almost evenly around the globe instead of concentrating
in the Northern Hemisphere.
What does stay in the atmosphere
over the Northern Hemisphere is sulfate pollution caused by burning fossil
fuels, Christy said. Sulfate particles, which generally stay in the atmosphere
less than two weeks, reflect sunlight back into space. This should cause
a cooling effect that would be strongest near and down wind from the site
of the pollution.
As part of an ongoing NASA/UAH joint
research project, Christy and Dr. Roy Spencer, a space scientist in the
Earth Science Laboratory of the Global Hydrology and Climate Center in NASA's
Marshall Space Flight Center, use data gathered by microwave sounding units
on NOAA's TIROS-N satellites to get accurate temperature readings for almost
all regions of the Earth. This includes remote ocean, desert and jungle
regions for which reliable temperature data are not otherwise available.
The satellite-based instruments measure
the temperature of the atmosphere from the surface to an altitude of about
eight kilometers above sea level. Once the monthly temperature data is collected
and processed, it is placed in a "public" computer file for immediate
access by atmospheric scientists in the United States and abroad.
Neither Spencer nor Christy receives
any research support or funding from oil, coal or industrial companies or
organizations, or from any private or special interest groups.  | 
