May 4, 2009 | 22
What is the "right" level of carbon dioxide concentrations in the atmosphere to prevent "dangerous" interference with the climate? In the last two centuries, concentrations have risen to roughly 387 parts per million—and are rising by roughly 2 ppm per year thanks to the more than 30 billion metric tons of CO2 humans put into the atmosphere annually through things like burning fossil fuels and cutting down trees. (That's up from 280 ppm for all of recorded history before the Industrial Revolution.)
Climatologist James Hansen, for one, thinks the answer is 350 ppm. While recent changes are small compared to the massive climate shifts in the Earth's history—as much as 10 degrees Celsius warmer—the reasons for today's shift is different (humans) as is the speed. "Humans are now 10,000 times more powerful than natural geologic changes," Hansen said at a conference this past Saturday organized by students of Columbia University's masters program in climate and society to examine whether (and why) 350 ppm might be the right number. "We're now, unfortunately, in charge of future changes."
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