If you've ever seen a flock of migrating monarch butterflies, you're one of the lucky ones. Fifty years from now, your memory might be all that's left of the flapping beauties.

A computer analysis suggests that some populations of monarch butterflies could die out in North America if the weather in Mexico changes.

Monarch butterflies fly great distances to spend their winters in warm places. Butterflies that spend their summers in the western United States and Canada migrate to beaches in California during the winter. Monarchs that live east of the Rockies bask in the Mexican sun all winter long. Some 200 million butterflies make the trip every year.

Karen Oberhauser of the University of Minnesota at Minneapolis St. Paul and a colleague used a computer model to predict the future of January weather in the dozen or so places in Mexico where the butterflies usually go.

The computer analysis predicted that temperatures in the butterflies' Mexican habitats would stay monarch-friendly. Precipitation, on the other hand, would more than triple by 2050. Monarch butterflies have never been known to survive in such wet conditions.

If the climate in Mexico changes as predicted, researchers hope that the butterflies will adapt by finding other places to spend their winters. The research also shows just how challenging and uncertain life can be for animals that migrate long distances every year.—E. Sohn