Image: Ice Fjord of Ilulissat, Greenland
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Icebergs float in a bay off Ammassalik Island, Greenland.
By AP Science Writer
updated 10/26/2009 4:47:35 PM ET 2009-10-26T20:47:35

An analysis of global temperatures by independent statisticians shows the Earth is still warming and not cooling as some global warming skeptics are claiming.

The analysis was conducted at the request of The Associated Press to investigate the legitimacy of talk of a cooling trend that has been spreading on the Internet, fueled by some news reports, a new book and temperatures that have been cooler in a few recent years.

In short, it is not true, according to the statisticians who contributed to the AP analysis.

The statisticians, reviewing two sets of temperature data, found no trend of falling temperatures over time.

2005 hottest year recorded
U.S. government data show the decade that ends in December will be the warmest in 130 years of record-keeping, and 2005 was the hottest year recorded.

The case that the Earth might be cooling partly stems from recent weather. Last year was cooler than previous years. It has been a while since the superhot years of 1998 and 2005. So is this a longer climate trend or just weather's normal ups and downs?

In a blind test, the AP gave temperature data to four independent statisticians and asked them to look for trends, without telling them what the numbers represented. The experts found no true temperature declines over time.

"If you look at the data and sort of cherry-pick a microtrend within a bigger trend, that technique is particularly suspect," said John Grego, a professor of statistics at the University of South Carolina.

Yet the idea that things are cooling has been repeated in opinion columns, a BBC news story posted on the Drudge Report and in a new book by the authors of the best-seller "Freakonomics." Last week, a poll by the Pew Research Center found that only 57 percent of Americans now believe there is strong scientific evidence for global warming, down from 77 percent in 2006.

Global warming skeptics base their claims on an unusually hot year in 1998. Since then, they say, temperatures have dropped — thus, a cooling trend. But it is not that simple.

Temps rising once more
Since 1998, temperatures have dipped, soared, fallen again and are now rising once more. Records kept by the British meteorological office and satellite data used by climate skeptics still show 1998 as the hottest year. However, data from the National Oceanic and Atmospheric Administration and NASA show 2005 has topped 1998. Published peer-reviewed scientific research generally cites temperatures measured by ground sensors, which are from NOAA, NASA and the British, more than the satellite data.

The recent Internet chatter about cooling led NOAA's climate data center to re-examine its temperature data. It found no cooling trend.

"The last 10 years are the warmest 10-year period of the modern record," said NOAA climate monitoring chief Deke Arndt. "Even if you analyze the trend during that 10 years, the trend is actually positive, which means warming."

The AP sent expert statisticians NOAA's year-to-year ground temperature changes over 130 years and the 30 years of satellite-measured temperatures preferred by skeptics and gathered by scientists at the University of Alabama in Huntsville.

Statisticians who analyzed the data found a distinct decades-long upward trend in the numbers, but could not find a significant drop in the past 10 years in either data set. The ups and downs during the last decade repeat random variability in data as far back as 1880.

Saying there's a downward trend since 1998 is not scientifically legitimate, said David Peterson, a retired Duke University statistics professor and one of those analyzing the numbers.

Identifying a downward trend is a case of "people coming at the data with preconceived notions," said Peterson, author of the book "Why Did They Do That? An Introduction to Forensic Decision Analysis."

Satellite data tends to be cooler
One prominent skeptic said that to find the cooling trend, the 30 years of satellite temperatures must be used. The satellite data tends to be cooler than the ground data. Key to that is making sure that 1998 is part of the trend, he added.

What happened within the past 10 years or so is what counts, not the overall average, contends Don Easterbrook, a Western Washington University geology professor and global warming skeptic.

"I don't argue with you that the 10-year average for the past 10 years is higher than the previous 10 years," said Easterbrook, who has self-published some of his research. "We started the cooling trend after 1998. You're going to get a different line depending on which year you choose.

"Should not the actual temperature be higher now than it was in 1998?" Easterbrook asked. "We can play the numbers games."

That's the problem, some of the statisticians said.

Grego produced three charts to show how choosing a starting date can alter perceptions. Using the skeptics' satellite data beginning in 1998, there is a "mild downward trend," he said. But doing that is "deceptive."

Conflicting data analyses
The trend disappears if the analysis is begun in 1997. And it trends upward if you begin in 1999, he said.

Apart from the conflicting data analyses is the eyebrow-raising new book title from Steven D. Levitt and Stephen J. Dubner, "Super Freakonomics: Global Cooling, Patriotic Prostitutes and Why Suicide Bombers Should Buy Life Insurance."

A line in the book says: "Then there's this little-discussed fact about global warming: While the drumbeat of doom has grown louder over the past several years, the average global temperature during that time has in fact decreased."

That led to a sharp rebuke from the Union of Concerned Scientists, which said the book mischaracterizes climate science with "distorted statistics."

Levitt, a University of Chicago economist, said he does not believe there is a cooling trend. He said the line was just an attempt to note the irony of a cool couple of years at a time of intense discussion of global warming. Levitt said he did not do any statistical analysis of temperatures but "eyeballed" the numbers and noticed 2005 was hotter than the last couple of years. Levitt said the "cooling" reference in the book title refers more to ideas about trying to cool the Earth artificially.

Moving averages over 10 years important
Statisticians say that in sizing up climate change, it's important to look at moving averages of about 10 years. They compare the average of 1999-2008 to the average of 2000-2009. In all data sets, 10-year moving averages have been higher in the last five years than in any previous years.

"To talk about global cooling at the end of the hottest decade the planet has experienced in many thousands of years is ridiculous," said Ken Caldeira, a climate scientist at the Carnegie Institution at Stanford University.

Ben Santer, a climate scientist at the Department of Energy's Lawrence Livermore National Lab, called it "a concerted strategy to obfuscate and generate confusion in the minds of the public and policy-makers" ahead of international climate talks in December in Copenhagen.

President Barack Obama weighed in on the topic Friday at the Massechusetts Institute of Technology. He said some opponents "make cynical claims that contradict the overwhelming scientific evidence when it comes to climate change, claims whose only purpose is to defeat or delay the change that we know is necessary."

Early this year, climate scientists in two peer-reviewed publications statistically analyzed recent years' temperatures against claims of cooling and found them invalid.

Not all skeptical scientists make the flat-out cooling argument.

"It pretty much depends on when you start," wrote John Christy, the Alabama atmospheric scientist who collects the satellite data that skeptics use. He said in an e-mail that looking back 31 years, temperatures have gone up nearly three-quarters of a degree Fahrenheit (four-tenths of a degree Celsius). The last dozen years have been flat, and temperatures over the last eight years have declined a bit, he wrote.

Oceans influence short-term weather
Oceans, which take longer to heat up and longer to cool, greatly influence short-term weather, causing temperatures to rise and fall temporarily on top of the overall steady warming trend, scientists say. The biggest example of that is El Nino.

El Nino, a temporary warming of part of the Pacific Ocean, usually spikes global temperatures, scientists say. The two recent warm years, both 1998 and 2005, were El Nino years. The flip side of El Nino is La Nina, which lowers temperatures. A La Nina bloomed last year and temperatures slipped a bit, but 2008 was still the ninth hottest in 130 years of NOAA records.

Of the 10 hottest years recorded by NOAA, eight have occurred since 2000, and after this year it will be nine because this year is on track to be the sixth-warmest on record.

The current El Nino is forecast to get stronger, which probably will pushing global temperatures even higher next year, scientists say. NASA climate scientist Gavin Schmidt predicts 2010 may break a record, so a cooling trend "will be never talked about again."

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Photos: Climate conditions

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  1. This crack in Greenland's Petermann Glacier is expected to some day completely sever, creating a large iceberg that will float away. Parts of Greenland's vast ice cover are speeding up their flow into the sea, adding a bit each year to rising sea levels. (Nick Cobbing / Greenpeace) Back to slideshow navigation
  2. The Arctic's sea ice recedes each summer but experts fear that human emissions of greenhouse gases are impacting what is an otherwise natural process. The 2009 season saw the third largest retreat since satellite records began in 1979. Satellite data was used to map the extent on July 1, at top, and Sept. 7, which was the lowest point of the summer. The two previous years were the lowest on record, and data for the three years points to a worrisome trend, say experts at the National Snow and Ice Data Center. (NASA via Reuters) Back to slideshow navigation
  3. Climate change also impacts ocean ecosystems, from coral reefs to fish populations. These tuna fishermen in Bali, Indonesia, live in an area known as the "Coral Triangle" because of its wealth of coral species. But rising water temperatures, sea levels and water made more acidic by carbon dioxide are threatening to destroy that coral kingdom. (Sonny Tumbelaka / AFP - Getty Images) Back to slideshow navigation
  4. Reefs like this one off Batangas Bay, south of Manila, Philippines, provide life not only to tiny coral but to the fish that live in and feed off the reefs. (Dennis M. Sabangan / EPA) Back to slideshow navigation
  5. A rare and endangered blue whale swims off Long Beach, Calif. In decades past, blue whales were rarely seen along California's coastline but their migration and feeding patterns are changing. In recent years sightings in Southern California have increased dramatically. Scientists suspect that climate change is among the factors having an effect on the food of the blues, the largest animal on Earth. (David McNew / Getty Images) Back to slideshow navigation
  6. The Indian Ocean nation of Maldives built this raised island in the 1990s to try to keep ahead of rising seas. The nation's main island, Male, is even flatter and fears are that the nation could disappear by the end of the century. (Amal Jayasinghe / AFP - Getty Images) Back to slideshow navigation
  7. Attempts to reduce emissions of greenhouse gases caused by burning fossil fuels includes a boom in wind power. This wind farm, the world's largest offshore, was built in 2009 off the west coast of Denmark near Esbjerg. The platforms are anchored to the sea floor. (Bob Strong / Reuters) Back to slideshow navigation
  8. Students plant mangroves at a conservation area in Jakarta, Indonesia. Native to many coastlines, mangroves are a natural buffer against storm surges and even rising seas. (Bagus Indahono / EPA) Back to slideshow navigation
  9. Deforestation is a major source of greenhouse gases in the developing world. This area was being cleared in Indonesia where the forest is being replaced with palm oil farms. (John Novis / Greenpeace) Back to slideshow navigation
  10. Experts warn that global warming will mean more, and more severe, droughts in parts of the world such as southern Brazil, where a dry spell in 2009 left fields like this one unusable. (Vagner Guarezi / AP) Back to slideshow navigation
  11. An endangered red eyed tree frog rests on a leaf at a nature reserve in Heredia, Costa Rica. Tropical frogs have been in decline and experts believe warming temperatures are making it hard for them to adapt quickly. (Kent Gilbert / AP) Back to slideshow navigation
  12. The Kibera slum in Nairobi, Kenya, is a perfect candidate for the kind of climate "megadisasters" that U.N. staff warn of should warming increase the severity of weather extremes from heat waves to floods. (Riccardo Gangale / AP) Back to slideshow navigation
  13. These farmers in western Uganda rely on water from glaciers atop the the Rwenzori Mountains, yet like most glaciers around the world Uganda's are receding rapidly. (Walter Astrada / AFP - Getty Images) Back to slideshow navigation
  14. A squatters' site is destroyed during a raid aimed at protecting Brazii's vast central savannah region. Illegal farming and settlements have destroyed tens of thousands of square miles of forest in recent years, removing trees and other plants that would have eased the climate impact by absorbing carbon dioxide from the atmosphere. (Stringer/brazil / Reuters) Back to slideshow navigation
  15. Beijing is known for the almost constant air pollution that hangs over China's capital. Efforts to curb smog by making cars cleaner has had a secondary effect of reducing greenhouse gas emissions. (Ng Han Guan / AP) Back to slideshow navigation
  16. China has embarked on a clean energy push that includes building nuclear power reactors like this one in Zhejiang Province. While storing nuclear waste is an environmental issue in itself, nuclear power produces no greenhouse gases. (Eugene Hoshiko / AP) Back to slideshow navigation
  17. A worker unloads coal from a freight train in Xiangfan, China. The nation is still dependent on coal-fired power plants, a major source of carbon dioxide emissions. (Stringer/china / REUTERS) Back to slideshow navigation
  18. Within the United States, the Colorado River has been cited as an example of direct impacts from warming temperatures. The river system, which supplies much of California's water, is being stressed by drought and an increasing population in the West. (Matt York / AP) Back to slideshow navigation
  19. President Barack Obama, left, has showcased clean energy with trips like this one to a massive solar power installation at Nellis Air Force Base in Las Vegas, Nev. (Pool / Getty Images) Back to slideshow navigation
  20. The United States still relies on coal for power given a large supply and relatively low costs. Critics counter that carbon dioxide emissions from burning coal are enormous and that the mining process includes practices like removing mountaintops to get to the coal -- which is what happened here at Kayford Mountain, W. Va. (Jeff Gentner / AP) Back to slideshow navigation
  21. Beetles and wildfires are consuming Western forests like this area in Canada in what scientists say is a preview of a warmer future in which dying, burning forests would then only add to the warming. (Darryl Dyck / AP) Back to slideshow navigation
  22. Coal-fired power plants like this one in Poland -- the largest in Europe -- emit soot pollutants as well as carbon dioxide but also help power industry and homes around the world. This infrastructure won't vanish any time soon, so experts are looking for ways to capture and then safely store the carbon dioxide so that their climate impact is reduced. (Peter Andrews / Reuters) Back to slideshow navigation
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