Moose in the northern U.S. are dying in what scientists say may be the start of climate shock to the world's boreal forests.

The die-off is most dire in Minnesota, where ecologists say moose could be gone within a decade. But it extends across the southern edge of the animal's global range: Populations are falling as far away as Sweden.

No single cause seems to be responsible. In Minnesota, many moose seem to be dying of parasitic worms called liver flukes; in Wyoming, some researchers are pointing to a worm that blocks the moose's carotid arteries; in New Hampshire, massive tick infections seem to be the culprit. This diversity of reasons makes some experts think they need to dig deeper.