Night Lights Around the World
November 22, 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. NASA researchers James Hansen and Marc Imhoff analyzed records for 7,200 global weather stations and used satellite observations of night-lights to identify stations with minimal local human influence. U.S. Defense Meteorological Satellites measure the brightness of nighttime lights all over the Earth's surface as seen in the images above. The brightest areas in these satellite images clearly depict populated areas, which are the areas most likely to exhibit urban heat island effects.
Urban heat island effects are created when cities grow and asphalt roads and tar roofs and other features are substituted for areas where plants would otherwise grow. Because dark surfaces like pavements store heat during the day, which is released at night, they keep cities hotter for longer periods of time. The researchers used the night light brightness to classify the location of each weather station as urban, near-urban or rural. They found more warming generally occurred at urban stations, and used neighboring rural stations to adjust the long-term temperature trend at urban stations, thus obtaining a better measure of the true climate change.