(See an interactive feature on hurricanes.)
Experts emphasize that neither Hurricane Katrina nor any other single event can be linked to global warming.
"But," Steig said, "the statistics [show] that such events are more likely now than they used to be and will become more likely in the future."
Some scientists, however, believe that we are in the high-intensity stage of a decades-long natural hurricance cycle, which they say is primarily responsible for any uptick in storm activity.
Still others aren't even sure hurricanes are gaining strength.
"I've got real concerns about whether this is a real change or whether it's an artifact of the data," Christopher Landsea told National Geographic News in a story published in September ("Hurricanes Are Getting Stronger, Study Says"). Landsea is a researcher with the National Hurricane Center in Miami, Florida.
He noted that scientists now generally use satellite data to gauge hurricane stregth. This technique has greatly improved over the past 30 years, so earlier measurements may depict older hurricanes as weaker than they actually were, he said.
Claim: Heat waves will be more frequent and more intense as temperatures rise.
"There's no question about this," the University of Washington's Steig said. "If the average is going up, the extremes have to go up as well."
2005 was the hottest year on Earth since the late 19th century, when scientists began collecting temperature data. The past decade featured five of the warmest years ever recorded, with the second hottest year being 1998.
Claim: Deaths from global warming will double in just 25 years to 300,000 people a year.
"The exact numbers are, at best, an extrapolation from [a heat wave that] was experienced in Europe in 2003," Steig said.
"However, there is no question that that heat wave was a major event and statistically very unlikely to have happened unless the statistics are changing.
"Since it did happen, the statistics are changingthat is, the globe really is warming up."
Claim: More than a million species worldwide could be driven to extinction in just half a century as a result of global warming.
Steig is "skeptical that climate change itself will cause this [extinction] so much as direct human impacts such as land-clearing." But he noted that he hadn't read the latest studies, some of which do make such a claim.
For example, a study published in Nature in 2004 predicted that climate change could drive more than a million species towards extinction by 2050. (Read "By 2050 Warming to Doom Million Species, Study Says.")
"Climate change now represents at least as great a threat to the number of species surviving on Earth as habitat destruction and modification," said the lead author of that study, Chris Thomas, a conservation biologist at the University of Leeds in the United Kingdom.
Claim: Global warming will also cause the introduction of new, invasive species.
"I take issue with the invasive-species linkage, because the human influencedirectly, by transporting species aroundI suspect is much more important than climate change," Steig said.
Claim: Global sea levels could rise by more than 20 feet (6 meters) with the loss of shelf ice in Greenland and Antarctica, devastating coastal areas worldwide.
There is little doubt that sea levels would rise by that much if Greenland melted.
But scientists disagree on when it could happen.
A recent Nature study suggested that Greenland's ice sheet will begin to melt if the temperature there rises by 3ºC (5.4ºF) within the next hundred years, which is quite possible, according to leading temperature-change estimates.
"It's uncertain how much warmer Greenland would get, [given] a certain carbon dioxide level, because different climate models give different amounts of warming," said Jonathan Overpeck, director of the Institute for the Study of Planet Earth at the University of Arizona in Tucson.
But many experts agree that even a partial melting would cause a one-meter (three-foot) rise in sea levels, which would entirely submerge low-lying island countries, such as the Indian Ocean's Maldives (see Maldives map).
Claim: The Arctic Ocean could be ice-free in summer by 2050.
Some climate models are more conservative, suggesting that there will be no summer ice in the Arctic by the year 2100.
But new research shows it could take as little as 20 years for the sea ice to disappear.
"Since the advent of remote satellite imaging, we've lost about 20 percent of sea-ice cover," said Mark Serreze, a research scientist at the National Snow and Ice Data Center in Boulder, Colorado.
"We're setting ourselves up for very big losses this year."
"We think of the Arctic as the heat sink to the climate system," Serreze said.
"We're fundamentally changing this heat sink, and we don't know how the rest of the climate system is going to respond."
There is no doubt that as sea ice continues to melt, habitat for animals like polar bears will continue to shrink. (See "Polar Bears Being Considered for U.S. Endangered List.")
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