Comparisons of data obtained from satellites that orbited Earth more than 25 years apart provide rare, direct evidence that the planet’s greenhouse effect increased significantly during the closing decades of the 20th century. These new findings support abundant indirect indications that greenhouse gases, such as carbon dioxide and methane, are warming Earth’s atmosphere.
Spectrometers showed that more of the infrared radiation emitted by Earth and leaving the atmosphere was blocked by heat-absorbing greenhouse gases in 1997 than in 1970. This indicates that the planet’s natural cooling mechanism has lost some of its effect, says Helen E. Brindley, an atmospheric physicist at Imperial College in London. She and her colleagues report their findings in the March 15 Nature.
The British researchers used two sets of satellite data: One was collected in 1970 by instruments flown on NASA’s Nimbus 4, and the other was obtained in 1997 from Japan’s Advanced Earth Observing Satellite (ADEOS).
To make valid comparisons between the two data sets, the researchers included only observations that had been collected over the same area of the central Pacific Ocean on cloudless days between April and June of the two years. The researchers also intentionally blurred the ADEOS data to match the resolution of the Nimbus 4 numbers.
“This is the first time that satellite measurements have been available to make this type of comparison,” comments Barbara E. Carlson, a physical scientist at NASA’s Goddard Institute for Space Studies in New York City.
The team’s analysis indicates that most of the increased heat-trapping effects of the atmosphere seen in 1997 occurred at wavelengths of radiation that correspond to absorption by carbon dioxide, methane, and other greenhouse gases, such as ozone and the chlorofluorocarbons CFC-11 and CFC-12, says Brindley.
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The satellite-measured decreases in radiation transmitted into space match those predicted in atmospheric models that incorporate known changes since 1970, are both in Earth’s temperature and in the atmospheric concentrations of greenhouse gases, she adds.
The sea-surface temperatures in the tropical Pacific were higher than normal in 1997, which is the hallmark of the El Nio phenomenon. These higher temperatures boosted the amount of infrared radiation emitted by Earth, Brindley notes. But an increase in greenhouse gases since 1970 intercepted a larger share of that radiation, trapping it in the atmosphere. Analyses of data collected over larger portions of the globe were consistent with findings from the tropical Pacific.
Normal seasonal variations in atmospheric concentrations of greenhouse gases, such as natural summertime increases in stratospheric ozone, are too small to account for many of the differences between the two sets of satellite data, Brindley notes.
Neither of the instruments that collected the spectra used in her analysis was specifically designed to monitor climate, says Richard M. Goody, an atmospheric physicist at Harvard University.
Despite the poor resolution of the Nimbus 4 spectrometer, compared with the modern one that flew on ADEOS, the researchers were able to extract a lot of information about atmospheric changes from the two satellites’ data, Goody notes. “This shows there’s a great future in flying simple, well-calibrated spectrometers to monitor climate,” he adds.