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Satellites Shed Light on a Warmer World
While winter may be approaching, researchers using data from satellites and
weather stations around the world have found the air temperature near the Earth's
surface has warmed on average by 1 degree F (0.6 degree C) globally over the
last century, and they cite human influence as at least a partial cause.
Dr. James Hansen of NASA's Goddard Institute for Space Studies, New York,
and Marc Imhoff of NASA's Goddard Space Flight Center, Greenbelt, MD, along
with several other researchers analyzed records for 7,200 global weather stations
and used satellite observations of nighttime lights around the planet to identify
stations with minimal local human influence. Their findings appeared in a recent
issue of the Journal of Geophysical Research-Atmospheres. "Warming around the
world has been widespread, but it is not present everywhere," Hansen said. Warming
in the past 50 years has been rapid in Alaska and Siberia, but Greenland has
become cooler. The lower 48 United States have become warmer recently, but only
enough to make the temperature comparable to what it was in the 1930s.
Hansen and Imhoff are making a special effort to minimize any distortion of
the record caused by urban heat-island effects as they research global warming.
It is recognized that recorded temperatures at many weather stations are warmer
than they should be because of human developments around the station. Hansen
and Imhoff used satellite images of nighttime lights to identify stations where
urbanization was most likely to contaminate the weather records.
Urban heat-island effects are created when cities grow and asphalt roads, tar
roofs and other features are substituted for areas where plants would otherwise
grow. Threes provide shade and cool the air through evaporation. The hard dark
surfaces like pavements store heat during the day, which is released at night,
keeping the city hotter for longer periods of time.
U.S. Defense Meteorological Satellites measure the brightness of nighttime
lights all over the Earth's surface. Hansen and Imhoff used the night-light
brightness to classify the location of each weather station as urban, near-urban
or rural. "We find larger warming at urban stations on average," said Hansen,
"so we use the rural stations to adjust the urban records, thus obtaining a
better measure of the true climate change."
Evidence of a slight, local human influence is found even in small towns and
it is probably impossible to totally eliminate in the global analyses. Although
Hansen and Imhoff have not yet applied satellite data in most of the world,
they adjusted the long-term trend of urban stations to be consistent with the
nearest rural stations. They estimate that remaining urban influence on the
global record is not more than about 0.18 degree F (0.1 degree C).
Hansen and his colleagues classified the global climate into three time segments
between 1900 and 2000. Each segment revealed a small swing in the Earth's global
temperature over a period of time.
From 1900 to 1940, the data showed the world warmed. "that warming may be
in part a response to released greenhouse gases and in part natural climate
variability," Imhoff said.
Between 1940 and 1965, the globe cooled by about 0.18 degree F (a change of
0.1 degree C), which some scientists attribute to the increased aerosols (fine
particles in the air) during this time. Aerosol forcing can lead to more cloud
cover and block incoming radiation. Aerosol increases are related to the rate
of growth of fossil fuel use, which peaked in this period. Hansen noted fluctuations
in ocean heat-transport may also contribute to such climate swings over decades.
The third period, from 1965 to 2000, showed a large and widespread warming
around the world. During this time warming intensified in the El Nino region
of the (eastern) Pacific Ocean, and the Indian, Atlantic and Arctic Oceans also
warmed.
This research was conducted as part of NASA's Earth Sciences Enterprise, a
long-term research effort dedicated to understanding how natural and human-induced
changes affect our global environment. More information and images are available
at www.gsfc.nasa.gov/topstory/20011027heatisland.html
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