Why has the warming trend slowed?
Climatologists aren't sure. What they do know is that the average air temperatures at the earth's surface have risen only about 0.2 degrees Fahrenheit since 1998 — the hottest year of the 20th century — even as humanity has continued to pour vast quantities of carbon dioxide and other greenhouse gases into the atmosphere. The world pumped roughly 110 billion tons of CO2 into the air between 2000 and 2010 — about a quarter of the total put there by mankind since the start of the Industrial Revolution. According to the prevailing models of man-made climate change, greenhouse gases heat the planet by trapping solar radiation in the atmosphere that might otherwise radiate into space. So the additional emissions over the past decade should have caused average temperatures to continue to climb as steeply as they did in the 1980s and 1990s. Climate-change skeptics say the plateau in warming proves that the climate isn't as sensitive to greenhouse emissions as scientists claim, and that it would therefore be foolish to adopt costly measures to limit the use of fossil fuels that emit CO2. "There is no problem with global warming," said Ian Plimer, an earth sciences professor at Australia's University of Melbourne. "It stopped in 1998."
Do climate scientists agree?
No. They concede that temperatures haven't risen as rapidly as they did in the previous two decades, but say the world is still getting warmer due to man-made emissions. Despite the plateau in average temperatures, climatologists point out, the 2000s were hotter than the 1990s, and nine of the 10 warmest years on record have occurred since 1998. Overall, the world has warmed by 1.4 degrees Fahrenheit, which sounds modest, but is a tremendous amount of heat for the entire surface of the earth, already causing major melting of the polar ice caps and noticeably more extreme weather throughout the world. Still, that doesn't explain what happened to the "missing heat" — the warmth the last decade's greenhouse gas emissions should have trapped in the atmosphere.
Where did the heat go?
It might be in the depths of the ocean. The world's seas absorb more than 90 percent of the extra energy that greenhouse gases trap on earth, yet the ocean is rarely included in global warming estimates, which are typically based on measurements of air temperature. A recent study in the journal Geophysical Research Letters found that deep ocean waters below 2,300 feet have heated up since 2000, even as the temperature of surface seawater remained stable. "Warming over the last decade has been hidden below the ocean surface," said Richard Allan, a climate scientist at Britain's University of Reading. If you take the oceans into account, he says, "global warming has actually not slowed down." If oceans are indeed the reason for the pause, the extra heat would eventually rise to the surface — causing a sudden new warming trend. Some climate scientists, however, think that much of the heat is missing because it never made it into Earth's climate system in the first place.
Why would that happen?
Possibly because the sun hasn't been shining as brightly. Over an average of 11 years, the sun's energy output rises and falls, subtly influencing Earth's climate. The last solar maximum occurred in 2000; since then, a prolonged solar minimum has kept the sun dimmer than usual. Kevin Trenberth of the National Center for Atmospheric Research says that lower solar radiation could account for up to 15 percent of the missing heat. Another theory is that about 30 percent of the missing heat is due to an influx of sunlight-blocking particles into the stratosphere — vast quantities of pollution from coal-burning China and several mid-sized volcanic eruptions, including on Montserrat in the Caribbean and in Papua New Guinea. These particles work in the opposite way to greenhouse gases, reflecting solar radiation away from the planet.
What does the future hold?
Most climatologists are adjusting their predictions to show a slower pace of warming in future decades. But they say the fundamental threat has not changed. A recent study in the journal Nature Geoscience analyzed data from the past decade and calculated that a doubling of carbon dioxide concentrations in the atmosphere above pre-industrial times — which would occur by 2050 under current trends — would raise temperatures by between 1.6 and 3.6 degrees Fahrenheit. That's lower than the estimates made by the United Nation's panel of climate scientists in 2007, which said that a doubling of atmospheric carbon would raise temperatures by as much as 5.4 degrees. But climatologists say an additional increase of even 2 degrees would be catastrophic, causing massive ice melting, rising sea levels, severe coastal flooding, prolonged droughts, and other disruptions. When the natural factors now holding back warming subside, researchers warn, it could be like a dam breaking, "with more rapid warming appearing over the next few years."
The skeptics' favorite scientist
Richard Lindzen is a climate change skeptic with a novel theory. The MIT meteorologist concedes that greenhouse gases cause warming, but he believes Earth will be able to regulate its temperature, like a thermostat, thanks to clouds. Lindzen argues that when surface temperature increases, the moist air that rises from the tropics will rain out more of its moisture, leaving less to form the wispy, high clouds known as cirrus. Just like greenhouse gases, those cirrus clouds trap heat in the atmosphere, so a decrease in them would counteract the increase of greenhouse gases. "If I'm right, we'll have saved money" by not adopting emissions restrictions, says Lindzen, who recently testified before Congress at the request of Republican skeptics. Most climatologists dispute Lindzen's theory, saying his papers have been riddled with erroneous data and unproven assumptions. Lindzen is "feeding upon an audience that wants to hear a certain message," says Christopher S. Bretherton, an atmospheric researcher at the University of Washington. "I don't think it's intellectually honest at all."