Satellites shed light on a warmer world
Posted: November 11, 2001

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.

Satellite observations reveal this night-lights map of the world. Photo: NASA
"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."

North America
Night-lights view of North America. Photo: NASA
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.

Night-lights view of Europe. Photo: NASA
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.