Snowpack declines in Rockies unusual compared to past

The reduction in snowpack in the Rocky Mountains over the last 30 years is unusual compared to the past few centuries, according to a new study that includes University of Arizona researchers. Previous studies attribute the decline to unusual springtime warming, more precipitation falling now as rain rather than snow and earlier snowmelt. The warming and snowpack decline are projected to worsen through the 21st century, foreshadowing a strain on water supplies.

The researchers evaluated the recent declines using snowpack reconstructions from 66 tree-ring chronologies, looking back 500 to more than 1,000 years. The network of sites was chosen strategically to characterize the range of natural snowpack variability over the long term, and from north to south in the Rocky Mountains.

With a few exceptions (the mid-14th and early 15th centuries), the snowpack reconstructions show that the northern Rocky Mountains experience large snowpacks when the southern Rockies experience meager ones, and vice versa. Since the 1980s, however, there were simultaneous declines along the entire length of the Rocky Mountains, and unusually severe declines in the north.

This study supports research by others estimating that between 30-60 percent of the declines in the late 20th century are likely due to greenhouse gas emissions. The remaining part of the trend can be attributed to natural decadal variability in the ocean and atmosphere, which is making springtime temperatures that much warmer.