PORTLAND — For the last 35 years, the snowpack in the West’s mountains has resisted the impacts of global warming. But that could soon change, according to a new study out of Oregon State University. The study, published in the journal Geophysical Research Letters, found that although climate change should have caused a steep decrease in snowpack, naturally occurring, decades-long weather variations shielded the Cascades, Sierras and Rockies from some of the effects.