Global warming during the fourth quarter of the 20th century may already have hit the rate expected for the 21st century, suggests a new analysis of records going back to 1880. Sharply rising temperatures at the end of the 1990s indicate that the fever pace may be even higher, although other scientists doubt that the blip is a trend.
A series of 16 record-warm months in a row during 1997 and 1998 (SN: 1/2/99, p. 6) prompted Thomas R. Karl and his colleagues at the National Climatic Data Center in Asheville, N.C., to take a new look at global warming patterns. The string of record highs may mark a “change point” in the rate of global warming, they conclude in the March 1 Geophysical Research Letters.
The researchers found that over the past 25 years, Earth has been warming at a rate of 2ºC per century, rather than the 1.5ºC rate previously measured (SN: 9/4/99, p. 150). The new pace matches warming rates predicted for the 21st century by the United Nation’s Intergovernmental Panel on Climate Change.
With average temperature increasing by 2ºC per century, there would be only a 1 in 20 chance of 16 record-setting months in a row, says Karl. “Sometimes unusual events really do happen [by chance],” he acknowledges. More likely, the pace of global warming has increased to 3ºC per century, he says.
Karl adds that he will be more confident of his prediction if temperatures continue to rise at the faster pace through the next few years. Data from 1999 seem to fit his proposal. Despite a La Ni±a cooling trend in the Pacific, last year was the fifth-warmest year on record, with temperatures outside the tropics that were just as high as those in 1998, Karl says.
Climatologist Alan Robock of Rutgers University in New Brunswick, N.J., finds the analysis unconvincing. “From a very short period of time, you can’t extrapolate the rate of change of a very slow process,” he says. “You have to average over 20- or 30-year periods, not 2-year periods.”
The researchers’ statistical analysis ignores the physics of climate change, Robock says. It’s unlikely that the climate system is reacting to steadily increasing greenhouse gases in a different way, as a sudden change in warming rate would require. Data over a longer time are likely to show a constant rate, he says.
“It’s quite an interesting little statistical analysis,” says Tom Wigley, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo. He adds, however, that Karl’s team hasn’t given the Pacific warming known as El Ni±o enough credit. The particularly powerful El Ni±o of 1997 and 1998 could account for the bump in temperature data but not reflect a new rate of warming. In an El Ni±o year, Wigley says, “you don’t need very much to tip the balance from routine to record.”