David E. Steitz
Headquarters, Washington              November 5, 2001
(Phone: 202/358-1730)

Lynn Chandler 
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-2806)

RELEASE: 01-212

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. 
Trees 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: 
http://www.gsfc.nasa.gov/topstory/20011027heatisland.html
                          -end-