Although most scientists are convinced that global warming is very real, a few still harbor doubts. But a new report, based on an analysis of infrared long-wave radiation data from two different space missions, may change their minds. "These unique satellite spectrometer data collected 27 years apart show for the first time that real spectral differences have been observed, and that they can be attributed to changes in greenhouse gases over a long time period," says John Harries, a professor at Imperial College in London and lead author of the study published today in Nature.
As the sun's radiation hits the earth's surface, it is reemitted as infrared radiation. This radiation is then partly trapped by the so-called greenhouse gases: carbon dioxide (CO2), methane (CH4) and chlorofluorocarbons (CFCs)as well as water vapor. Satellites can measure changes in the infrared radiation spectrum, allowing scientists to detect changes in the earth's natural greenhouse effect and to deduce which greenhouse gas concentrations have changed.
The researchers looked at the infrared spectrum of long-wave radiation from a region over the Pacific Ocean, as well as from the entire globe. The data came from two different spacecraftthe NASA's Nimbus 4 spacecraft, which surveyed the planet with an Infrared Interferometric Spectrometer (IRIS) between April 1970 and January 1971, and the Japanese ADEO satellite, which utilized the Interferometric Monitor of Greenhouse Gases (IMG) instrument, starting in 1996. To ensure that the data were reliable and comparable, the team looked only at readings from the same three-month period of the year (April to June) and adjusted them to eliminate the effects of cloud cover. The findings indicated long-term changes in atmospheric CH4, CO2, ozone (O3) and CFC 11 and 12 concentrations and, consequently, a significant increase in the earth's greenhouse effect.