The global warming debate suddenly has grown hotter this week because of a controversial new study that questions the strongest evidence greenhouse skeptics use to deny that there has been any substantial recent worldwide increase in temperature.
Many scientists believe, based on ground-level temperature measurements made since the 19th century, that the Earth’s surface has warmed about 1 degree Fahrenheit during the past 100 years, presumably because of a 30 percent increase in atmospheric carbon dioxide and other “greenhouse gases” produced by modern civilization. Similar data indicate that the planet has heated up at a rate of about one-quarter of a degree per decade since 1979, and that many of the hottest years on record have occurred in the 1980s and 1990s.
But direct measurements of air temperatures a few miles above ground, made by National Oceanic and Atmospheric Administration satellites over the same 18 years, indicate just the opposite: a slight cooling trend of about one-tenth of a degree per decade in the lower atmosphere.
This baffling discrepancy “has been widely cited by skeptics as evidence against global warming,” James W. Hurrell and Kevin E. Trenberth write in Thursday’s issue of the journal Nature. The two scientists from the National Center for Atmospheric Research in Boulder, Colo., report that they re-examined the satellite record and concluded that it contains “spurious trends” caused by “noise” in the measurements, errors introduced as new instruments and new satellites replaced their predecessors over time, and inconsistencies in the satellite orbits.
According to their recalculation, the satellite data actually show a very slight warming trend - one twenty-fifth of a degree Fahrenheit per decade since 1979. The exact amount may not be as important as the impact the finding could have on worldwide debate, Hurrell said Tuesday.
“A lot of people use that (satellite) downward trend somehow as proof that global warming is not occurring,” he said. The result is “a bunch of hype” suggesting that “the surface record cannot be considered reliable at all.”
John R. Christy of the University of Alabama in Huntsville, one of the two chief scientists responsible for constructing the disputed records, said Tuesday that “we have every confidence in the satellite data. It boils down to the question of how do you take the temperature of the atmosphere.”
“We use direct measurements by satellites, and that data set is validated against direct measurement by balloons” at various locations worldwide, Christy said.