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

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

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

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: