Adds Steven Chu, the Nobel laureate in physics who heads the Lawrence Berkeley National Laboratory, "Twenty years ago, when we talked about global warming, it was somewhat theoretical; it wasn't real. In the last two, three, four years, we're seeing things far more clearly," making it far more difficult to disregard ominous trends. Whether measured in the rate of shrinkage of glaciers, or in rising air temperatures, the process of global warming "is either bad or really bad, and the public still doesn't understand that. It's no longer a question of certainty," he adds, only whether we're 68 percent or 98 percent certain that global warming is accelerating. Either way, "the vast preponderance of the evidence shows that we're warming rapidly," he says.

"What really has us worried is that after 20 years of observations, everything that is happening tells us we are warming, quite rapidly," Dr. Fung explains. "When you see that four of the last five years are the warmest on record, well, then you know the die is loaded." (Data also now show that the first half of 2006 was the warmest on record, according to the National Climatic Data Center.)

For years now, climate scientists have calculated that rising levels of carbon dioxide (from fossil fuel and forest burning) and methane (mainly from decomposing landfills and escaped natural gas) are trapping more and more of the sun's energy closer to the earth. Meteorologists now have little doubt that the effects of global warming are profound and are radically changing our environment. Rising temperatures in the waters off the Gulf Coast coincided with the unusual intensity of last season's hurricanes, which caused more than $100 billion worth of damage and more than 1,400 deaths. Likewise, there no longer seems to be doubt that as the earth warms, and sunlight peels away the glacial mountains that form the polar ice sheets, less of the sun's energy is reflected away from the earth, and more is absorbed resulting in rising temperatures.

But only in the past few years have climatologists begun to calculate "second order" effects they had not previously considered. What happens, for instance, as the warming of the planet forces whole ecosystems to adapt, as soils dry out because of more rapid evaporation, or as seawater becomes fresher as glaciers melt more quickly? Most scientists had not predicted, for instance, that as polar glaciers began to melt, torrents of water would plunge down through the ice sheets by way of large tunnels and crevasses (called moulins) to the bedrock. As warmer weather increases surface melting, these streams intensify, and water pools beneath the glaciers lubricating the base, causing the ice to slide faster. And this, ultimately, hastens the process by which glaciers shrink.

Nor was it clear until recently that as rising concentrations of greenhouse gases cause the earth's surface to warm and dry out, increasing evaporation and dieback of vegetation release more carbon dioxide into the atmosphere, which in turn makes the earth's terrain, like disappearing ice, less able to reflect sunlight back into space. Such so-called "positive feedback loops" amplify and accelerate previously existing phenomena. In the case of the environment, the response of certain plant and geologic systems to warming is to adapt in ways that actually accelerate further warming. (In the case of the earth's atmosphere, a "negative" feedback loop would actually be a good thing, because such reactions would tend to cool the planet.)

"There is a sinister side to synergy, which I call harmful reinforcement," explains John Harte, professor of environmental science at Berkeley, whose long-term field research at 9,600 feet in the mountains of Colorado has attempted to replicate some of the effects of global warming before they actually take place (see Flower power). "These feedbacks increase the speed of the projected warming, and this is the most critical [research] going on in the whole climate story right now."

Fung also believes that even the most sophisticated climate models now must be adjusted to assess how rising temperatures, thinning soils, and drier terrains will affect the future capacity of the earth to absorb carbon. "This suggests that we'll have snowier winters and hotter summers and that weather events will be more serious." But she cannot say for certain when we'll reach a "tipping point" of no return, when the rise in the amount of greenhouse gases in our atmosphere cannot be reversed.

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