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GENOA, Nev. — There are certain axioms about the natural world we learn as children. The sea is salty. Plants grow toward light. Bears hibernate in winter.

But as climate change leads to warmer winters, later falls and earlier springs — which can disrupt both food supplies and biological rhythms — American black bears are changing their hibernation routines, scientists say. In some cases, bears are not hibernating at all, staying awake all winter. In others, bears are waking from their slumber too early.

For every one degree Celsius that minimum temperatures increase in winter, bears hibernate for six fewer days, a study found last fall. As global temperatures continue to rise, by the middle of the century black bears may stay awake between 15 and 39 more days per year, the study said.

A February visit to the Pine Nut Mountains of northwestern Nevada, near Lake Tahoe, provided a preview of what could lie ahead. The previous fall, regional temperatures were as much as 4.7 degrees Celsius (8.4 degrees Fahrenheit) warmer than the 20th century average. In January temperatures were 5.4 degrees Celsius warmer.