It's the blue that makes the Arctic so unreal - both in pictures and in real life. The color seems so unlikely in nature, that electric hue of the Internet Explorer icon on your computer desktop.

But that amazing color is real, thanks to the minerals in glacial silts and the wacky way they reflect light. Flying last year over the coastline at Barrow, Alaska - the northernmost city in North America - I was among the passengers glued to the windows as huge chunks of ice melted and seem to ooze puddles of cobalt.

Sea ice, of course, comes and goes with the seasons and with the millennia. In the May issue of National Geographic, McKenzie Funk writes of a time when "pieces of the supercontinent Pangaea were drifting apart, and at times greenhouse gases warmed the world to far hotter than it is today." For a time, he says, the Arctic was almost tropical - partly because "global temperatures were higher globally, but more so because parts of the Arctic have not always been in the Arctic."