After noting that certain regions appear to be responding to climate change differently than others, Amato Evan, an associate professor of climate science at the Scripps Institution of Oceanography, decided to try to tease out the cause.
He looked at snowmelt, which seemed to be an obvious casualty of a heating climate. But his research shows that snow in certain areas of the western United States is likely to stay put.
He and his team used data on snow depth and air temperature that had been collected for the last 40 years to predict how a warmer climate would change the date of disappearance, or the day when all the snow has melted. What they found seemed counterintuitive at first blush: places with large seasonal temperature swings, like interior mountain regions such as the Rocky Mountains, were more likely to have resilient snow, whereas places with a similar temperature year-round, like coastal mountains such as the Cascades on the west coast, were more likely to see a significantly earlier melt off. But Evan realized that it has to do with proportionality.
"If you have, for example, one degree of warming in a place where the differences between the summertime high temperatures and the wintertime low temperatures isn't very large, like near a coast, one degree warming is really, really big relative to the swings in temperature throughout the year," said Evan. "Whereas out in a place like the Rockies, one degree of warming is a lot smaller relative to how big those swings are in temperature throughout the year."
Global temperatures are predicted to rise year-round, which means that there will be fewer days overall that are below freezing. In an area where there are large swings in seasonal temperature, like the Rockies, one degree of warming won't push many days over the freezing mark. But areas that are relatively the same temperature year-round tend to already have several days just below freezing. Warming would cause those days to rise over freezing.
"We have to think about how much time in a given year is the temperature below zero degrees Celsius and how much time of the year is the temperature above zero degrees Celsius," said Evan. "So instead of thinking about the absolute temperatures, at least when we're talking about snow, we want to think about time. How much time are we above and below zero degrees?"
Evan's data shows that in places that have very small temperature fluctuations between seasons, mountain snowpack is predicted to melt up to a month earlier than it does now, which could cause cascading effects later in the year.
"Now we're really increasing the amount of time for example, that our ecosystems are vulnerable to fire, along with that warming. And really with that changing the amount of time that the temperature is below zero, we also, for example, increase the presence of invasive species, and specifically, I'm thinking about one called the bark beetle," said Evan. "The thing that really kills off the bark beetle are long winters. So as our winters start shrinking and getting shorter, more of these pests can survive throughout the winter and then wreak havoc on the forest during a longer summer."
Snowpack also stores fresh water in the mountains that is then slowly released as temperatures rise. It's integral for keeping reservoirs for drinking water and irrigation full all summer. If that snow regularly melts earlier in the year, reservoirs could start to struggle to keep up with demand.
Evan said they didn't include changes in precipitation in this study as it is often volatile and too hard to predict long-term.
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