Global warning: why is the planet feverish?
But if researchers have any hope of predicting the climate, they need to learn how to separate the natural from the artificial effects that set the planet's thermostat. Two teams now report new insights into Earth's indigenous climate.
Michael E. Schlesinger and Navin Ramankutty of the University of Illinois at Urbana-Champaign detected hints of a natural pattern by studying the record of global average temperatures from 1858 to 1992. Earth has warmed by 0.5 degrees C during this period, but researchers have long puzzled over the uneven rate of change. Temperatures rose quickly in the early third of this century, then leveled off from 1940 to 1975 before starting to climb steeply in the 1980s.
Climate experts suspect that rising concentrations of carbon dioxide and other greenhouse gases have caused some of the global warming, but they cannot explain what stalled that trend for 35 years. Schlesinger and Ramankutty may have an explanation for the unsteady rise. The two used a sophisticated statistical technique to analyze the global record after first subtracting two major human factors: greenhouse warming and the cooling from sulfur pollution.
The remaining temperature changes display a natural oscillation with a period of roughly 70 years. When combined with the steady warming from greenhouse gases, this oscillation caused global temperatures to level off in mid-century, The researchers report their findings in the Feb. 24 NATURE.
Some scientists had previously suggested that the decade-to-decade variations within the temperature record stem from random fluctuations within the climate -- the equivalent of white noise. Others blame the climate wiggles on the sun, suggesting that solar energy might wax and wane by a substantial amount over decades. But Schlesinger and Ramankutty's findings back a different explanation.
If solar or random fluctuations caused the temperature swings, they would affect both hemispheres. But the 70-year-long temperature oscillation appears only in the Northern Hemisphere, particularly in the North Atlantic.
This finding ties in with recent discoveries in oceanography, which suggest that a major current in the North Atlantic plays an important climatic role. By speeding and slowing, this current alters the amount of heat shuttled from the tropics to the Arctic. Schlesinger and Ramankutty suggest that such oceanic changes powered the 70-year temperature oscillation.
The new findings have intrigued other researchers; however, they caution against inferring too much from a temperature record only 140 years long. "It's difficult to be very definitive about oscillations when you're talking about one that is half as long as the period of record," says James Hansen of the NASA Goddard Institute for Space Studies in New York City,
Whereas Schlesinger and Ramankutty focused on natural climatic variations, another recent study investigated whether greenhouse gases caused the recent temperature rise. Ronald J. Stouffer and colleagues from the National Oceanic and Atmospheric Administration in Princeton, N.J., used a sophisticated computer model to simulate a 1,000-year span of climate history, The model did not include the effects of greenhouse gas pollution. Although warming and cooling trends appeared in the simulation, none lasted as long as the observed temperature rise, the researchers report in the Feb. 17 NATURE. The results therefore suggest that the current warming stems from human influences or some other factor not in the model.
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|Title Annotation:||studies indicate fluctuations in temperature record and current warming trend due to human rather than natural influences|
|Article Type:||Brief Article|
|Date:||Feb 26, 1994|
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