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Global Warming - Dimming the Sun - 05
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Most models do not yet take full account of the impact of global dimming and predict warming between two and five degrees Celsius, by the end of the century. But just as global dimming may have lulled the public and politicians into a false sense of security about climate change, has it misled climate scientists about the real power of the greenhouse effect to change global temperatures?
Peter Cox, a leading climate modeler, has come up with a controversial new analysis based on the observed warming of the last century. If scientists have underestimated the cooling effects of global dimming in the past, he believes, they may also be underestimating the heating effects of global warming in the future.
PETER COX: We've got two competing effects, really, that...we've got the greenhouse effect, which has tended to warm up the climate, but then we've got this other effect, that's much stronger than we thought, which is a cooling effect that comes from particles in the atmosphere. And they're competing with one another.
And we know the climate's moved to a warmer state by about six-tenths of a degree over the last hundred years. So the whole thing's moved this way. If it turns out that the cooling is stronger than we thought, then the warming, also, is a lot stronger than we thought. And that means the climate's more sensitive to carbon dioxide than we originally thought, and it means our models may be under-sensitive to carbon dioxide.
NARRATOR: While today's models foresee a maximum warming of five degrees Celsius by the end of the century, Cox thinks that it is not beyond the realms of possibility that by 2100, temperatures could rise by as much as 10 degrees Celsius, 18 degrees Fahrenheit.
Many plant species could not survive such rapid climate change. In his scenario, trees would die all over the planet; the world's best agricultural land would be struck by drought and soil erosion; famine would not be far behind. And in the far north, there would be a risk of releasing a vast natural store of greenhouse gas bigger than all the oil and coal reserves of the planet.
PETER COX: We will be in danger of destabilizing these things called "methane hydrates," which store a lot of methane at the bottom of the ocean, in a kind of frozen form—ten thousand billion tons of this stuff—and they're known to be destabilized by warming.
NARRATOR: If this were to happen, some or all of the ten thousand billion tons of methane, a greenhouse gas eight times stronger than carbon dioxide, would be released into the atmosphere. When this last happened 50,000,000 years ago, when the Earth was already warmer than it is today, the average temperature rocketed by 13 degrees Fahrenheit, making the Earth 25 degrees hotter than today, and life struggled to survive.
Some scientists consider this model extreme, but all climate models contain important unknowns and ranges of possibility. Our new understanding of global dimming has complicated the task of forecasting the future but has also brought the probability of dangerous climate change much closer.
Today, there's a strong scientific consensus that without urgent action to reduce our burning of coal, oil and gas, we risk creating a world very different from the one which has been so hospitable to humanity.
JAMES HANSEN: I think we have less than a decade to avoid passing what I call "point of no return." I think we have to keep global warming less than one degree Celsius, or we're going to get very bad effects. And the problem is that to achieve...to keep the warming less than one degree Celsius, we have to level off the emissions and get them to decline before the middle of the century, substantially.
Right now, the course that we're on—plus 2 percent per year in greenhouse gas emissions—well, if you continue that, even for 15 years, it's a 35 percent increase. And then there's no way that you could possibly meet this alternative scenario with warming less than one degree Celsius. ( Continued at
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