The vast forests that cover southern Alaska should be evergreen. But not these days. Hop in a tiny plane - which Alaskans seem to do as often as a New Yorker hails a cab - and you'll see patches of brown stretching for miles into the wilderness. This is the work of the spruce bark beetle, which over the past 15 years has killed more trees in Alaska than any other insect in North America's recorded history.

In the Kenai Peninsula on Alaska's southern coast, some 40 million spruce have perished across an area twice the size of Yellowstone National Park. The beetle's population rocketed thanks to changes in the weather, argues Ed Berg, an ecologist with the Kenai National Wildlife Refuge. "We had a really long run of warm summers," he says.

The beetle boom is one of the more dramatic changes that locals and scientists alike attribute to global warming. But it's just one in a long list. Farther down the coast in Prince William Sound, boats pick their way through increased numbers of icebergs calving off the Columbia Glacier. The glacier has retreated 12 kilometres over the past 20 years, and some say it could collapse completely in another ten. Alaska's notorious mosquitoes, so big they're jokingly referred to as the state bird, have spread north to irritate the few residents who used to escape their attentions. Even the plants are changing. The spongy tundra, usually covered in grass and moss, is slowly being invaded by woody shrubs.

This summer saw the biggest melt yet in Alaska's sea ice, and winter in the interior was unprecedentedly mild - for the Arctic. "I've lived here since 1968, and last winter was the first one that didn't drop below -40 ºC" says Gunter Weller, director of the Centre for Global Change and Arctic System Research at the University of Alaska, Fairbanks.

Small change, big difference...