There isn't much to do at the Hotel Arctic except watch the icebergs flow by. The hotel is located in the town of Ilulissat, on the west coast of Greenland, four degrees north of the Arctic Circle. The icebergs originate some fifty miles away, at the end of a long and fast-moving ice stream known as the Jakobshavn Isbrae. They drift down a fjord and through a wide-mouthed bay, and, if they last long enough, end up in the North Atlantic. (It is likely that the iceberg encountered by the Titanic followed this route.)

To the tourists who visit the Hotel Arctic, the icebergs are a thrilling sight: beautiful and terrible in equal measure. They are a reminder of the immensity of nature and the smallness of man. To the people who spend more time in Ilulissat - native Greenlanders, European tour guides, American scientists -- the icebergs have come to acquire a different significance. Since the late nineteen-nineties, the Jakobshavn Isbrae has doubled its speed. In the process, the height of the ice stream has been dropping by up to fifty feet a year and the calving front has retreated by several miles. What locals now notice about the icebergs is not their power or immensity - though they are still powerful and immense - but a disquieting diminishment.

"You don't get the big icebergs anymore," an Ilulissat town councilman named Jeremias Jensen told me. We were having coffee on a late spring afternoon in the Hotel Artic lobby. Outside, it was foggy and the icebergs seemed to be rising up out of the mist. "It's very strange the last few years; you can see a lot of strange changes."

This is a book about watching the world change?..

It grew out of three articles that I wrote for the The New Yorker magazine, which ran in the spring of 2005, and its goal remains much the same as that of the original series: to convey, as vividly as possible, the reality of global warming. The opening chapters are set near or above the Arctic Circle -- in Deadhorse, Alaska; in the countryside outside of Reykjavik; at Swiss Camp, a research station on the Greenland ice sheet. I went to these particular places for all the usual journalistic reasons -- because someone invited me to tag along on an expedition, because someone let me a hitch a ride on a helicopter, because someone sounded interesting over the telephone. The same is true of the choices that were made in subsequent chapters, whether it was a decision to track butterflies in northern England or to visit floating houses in the Netherlands. Such is the impact of global warming that I could have gone to hundreds if not thousands of other places - from Siberia to the Austrian Alps to the Great Barrier Reef to the South African Fynbos -- to document its effects. These alternate choices would have resulted in an account very different in its details, but not in its conclusions.

Humans aren't the first species to alter the atmosphere; that distinction belongs to early bacteria, which, some two billion years ago, invented photosynthesis. But we are the first species to be in a position to understand what we are doing. Computer models of the earth's climate suggest that a critical threshold is approaching. Crossing over it will be easy, crossing back quite likely impossible. The second part of this book explores the complicated relationship between the science and the politics of global warming, between what we know and what we refuse to know.