For thousands of years, the Ayles ice shelf sat off the northern coast of Ellesmere Island, a desolate stretch of glaciers and rock 500 miles (800 kilometers) south of the North Pole. Then, on an August afternoon in 2005, a mass of ice the size of 11,000 football fields suddenly broke free. No humans were nearby to bear witness. But it sent tremors flickering across earthquake monitors 150 miles (250 kilometers) away, and satel- lites recorded the image of the shelf floating out to sea.
Why did this happen? Was the collapse simply a normal process of nature?
Most scientists think not. They believe that climate change was at least partly responsible. There’s compelling evidence to support this. The five warmest years on record are all since 1998. The 1990s was the warmest decade and 2005 was the warmest year in over a century. Over the last few years, hundreds of thousands of square miles of Arctic sea ice have melted under these warmer conditions. If current trends continue, by the end of this century, the North Pole’s ice cap could virtually disappear during the late summer season.
Warmer days and nights in the Arctic would seem like a reasonable explanation for the collapse of the Ayles ice shelf. But this raises the underlying question: Why are temperatures rising? Because, the usual scientific answer goes, the amounts of greenhouse gases from human sources (especially carbon dioxide, methane, and nitrous oxide) are rising. World output of carbon dioxide from just consuming fossil fuels currently exceeds 27 billion metric tons a year, up from 18 billion in 1980, and now equal to some 4 metric tons—about the weight of two Hummers—for every man, woman, and child on earth.1
Again, however, this answer gives rise to a more fundamental ques- tion: Why are these amounts rising?

For thousands of years, the Ayles ice shelf sat off the northern coast of Ellesmere Island, a desolate stretch of glaciers and rock 500 miles (800 kilometers) south of the North Pole. Then, on an August afternoon in 2005, a mass of ice the size of 11,000 football fields suddenly broke free. No humans were nearby to bear witness. But it sent tremors flickering across earthquake monitors 150 miles (250 kilometers) away, and satel- lites recorded the image of the shelf floating out to sea.
Why did this happen? Was the collapse simply a normal process of nature?
Most scientists think not. They believe that climate change was at least partly responsible. There’s compelling evidence to support this. The five warmest years on record are all since 1998. The 1990s was the warmest decade and 2005 was the warmest year in over a century. Over the last few years, hundreds of thousands of square miles of Arctic sea ice have melted under these warmer conditions. If current trends continue, by the end of this century, the North Pole’s ice cap could virtually disappear during the late summer season.
Warmer days and nights in the Arctic would seem like a reasonable explanation for the collapse of the Ayles ice shelf. But this raises the underlying question: Why are temperatures rising? Because, the usual scientific answer goes, the amounts of greenhouse gases from human sources (especially carbon dioxide, methane, and nitrous oxide) are rising. World output of carbon dioxide from just consuming fossil fuels currently exceeds 27 billion metric tons a year, up from 18 billion in 1980, and now equal to some 4 metric tons—about the weight of two Hummers—for every man, woman, and child on earth.1
Again, however, this answer gives rise to a more fundamental ques- tion: Why are these amounts rising?