WASHINGTON — Has Earth's fever broken?
Official government measurements show that the world's temperature has cooled a bit since reaching its most recent peak in 1998.
That's given global warming skeptics new ammunition to attack the prevailing theory of climate change. The skeptics argue that the current stretch of slightly cooler temperatures means that costly measures to limit carbon dioxide emissions are ill-founded and unnecessary.
Proposals to combat global warming are "crazy" and will "destroy more than a million good American jobs and increase the average family's annual energy bill by at least $1,500 a year," the Heartland Institute, a conservative research organization based in Chicago, declared in full-page newspaper ads earlier this summer. "High levels of carbon dioxide actually benefit wildlife and human health," the ads asserted.
Many scientists agree, however, that hotter times are ahead. A decade of level or slightly lower temperatures is only a temporary dip to be expected as a result of natural, short-term variations in the enormously complex climate system, they say.
"The preponderance of evidence is that global warming will resume," Nicholas Bond, a meteorologist at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, said in an e-mail.
"Natural variability can account for the slowing of the global mean temperature rise we have seen," said Jeff Knight, a climate expert at the Hadley Centre for Climate Prediction and Research in Exeter, England.
According to data from the National Space Science and Technology Center in Huntsville, Ala., the global high temperature in 1998 was 0.76 degrees Celsius (1.37 degrees Fahrenheit) above the average for the previous 20 years.
So far this year, the high has been 0.42 degrees Celsius (0.76 degrees Fahrenheit), above the 20-year average, clearly cooler than before.
However, scientists say the skeptics' argument is misleading.
"It's entirely possible to have a period as long as a decade or two of cooling superimposed on the long-term warming trend," said David Easterling, chief of scientific services at NOAA's National Climatic Data Center in Asheville, N.C.
"These short term fluctuations are statistically insignificant (and) entirely due to natural internal variability," Easterling said in an essay published in the journal Geophysical Research Letters in April. "It's easy to 'cherry pick' a period to reinforce a point of view."
Climate experts say the 1998 record was partly caused by El Nino, a periodic warming of tropical Pacific Ocean waters that affects the climate worldwide.
"The temperature peak in 1998 to a large extent can be attributed to the very strong El Nino event of 1997-98," Bond said. "Temperatures for the globe as a whole tend to be higher during El Nino, and particularly events as intense as that one."
El Nino is returning this summer after a four-year absence and is expected to hang around until late next year.
"If El Nino continues to strengthen as projected, expect more (high temperature) records to fall," said Thomas Karl, who's the director of the National Climatic Data Center in Asheville.
"At least half of the years after 2009 will be warmer than 1998, the warmest year currently on record," predicted Jeff Knight, a climate variability expert at the Hadley Centre in England.
John Christy, the director of the Earth System Science Center at the University of Alabama in Huntsville, who often sides with the skeptics, agreed that the recent cooling won't last.
"The atmosphere is just now feeling the bump in tropical Pacific temperatures related to El Nino," Christy said in an e-mail. As a result, July experienced "the largest one-month jump in our 31-year record of global satellite temperatures. We should see a warmer 2009-2010 due to El Nino."
Christy added, however: "Our ignorance of the climate system is still enormous, and our policy makers need to know that . . . We really don't know much about what causes multi-year changes like this."
In addition to newspaper ads, the Heartland Institute sponsors conferences, books, papers, videos and Web sites arguing its case against the global warming threat.
The skeptics include scientists such as Richard Lindzen, a meteorologist at the Massachusetts Institute of Technology, who thinks that climate science is too uncertain to justify drastic measures to control CO2. He calls the case for action against global warming "silly" and "grotesque."
Others go further. For example, Don Easterbrook, a geologist at Western Washington University in Bellingham, thinks the world is in a 30-year cooling phase.
"The most recent global warming that began in 1977 is over, and the Earth has entered a new phase of global cooling," Easterbrook said in a talk to the American Geophysical Union's annual meeting in San Francisco in December.
Government scientists strongly disagree. "Claims that global warming is not occurring . . . ignore this natural variability and are misleading," said NOAA's Easterling.
In reality, global warming "never ceased," said Karl, the climate data center director.
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