For western North America over the next three decades, 10 to 30 percent of years will have spring snowmelt runoff amounts below those in the worst year seen between 1976 and 2005, Diffenbaugh said.
By the end of the 21st century, he said, the proportion of years with runoff totals beneath the lowest point for roughly the last 30 years could reach more than 80 percent.
Changes come even with lower warming
What was most surprising, Diffenbaugh said, is that the accelerated melting of the snowpack would occur even if the world were able to limit warming to the target of a 2-degree-Celsius increase agreed upon in international climate negotiations in Copenhagen, Denmark.
At that temperature, in western North America, between 10 and 30 percent of years would see that spring snow amount lower than the driest period over the last 30 years, Diffenbaugh said.
"To see very large increases in extremely low snow years within the occurrence of that [Copenhagen] target suggests that there could be substantial impacts from climate change even if that global warming target is achieved," Diffenbaugh said. "The lowest [snow year] in the recent past occurring about a third of the time in the near future, that's a substantial change in the occurrence of extremely low snow conditions."
At a climb of 4 degrees Celsius, the occurrence of low snow years is much more frequent "at 30, even up to 90 percent of the years have spring snow total that's lower than the lowest of the late 20th century," he said.
The study also looked at the amount of snowpack runoff during the warmer season, compared to what has been seen in the past. It forecasts an increase in number of years that have "extremely high melt" earlier than the past, Diffenbaugh said.
Stanford studied the amount of snow in March, considered the end of the snow season. Lower amounts in that month could come as a result of a decrease in the amount of snow, he said, because precipitation will come as rain instead of snow. There could also be less precipitation overall because of warming, or because what falls as snow melts earlier, so that by spring, there is less snow on the ground.
While there is uncertainty about the amount of precipitation in the future, he said, "even if precipitation increases, the increase in temperature is great enough to cause earlier melt to occur," Diffenbaugh said.