Science

Science in Mind

How global warming can worsen snowfalls

The Blizzard of 2015  left giant snow mounds at the ends of many streets in South Boston.
Suzanne Kreiter/Globe staff
The Blizzard of 2015 left giant snow mounds at the ends of many streets in South Boston.

When a historic blizzard dumps a record-breaking amount of snow on the region, it’s only a matter of time before someone ventures a wry joke about climate change. Maybe there’s an upside to a warmer world, after all? Less shoveling.

But the halfhearted punchline doesn’t hold up to scientific scrutiny, according to recent research from a Massachusetts Institute of Technology researcher. In fact, a warming world could mean less overall snow in a given year, but no reprieve from extreme snow events, at least in places like Boston.

To science, not all snowstorms are the same: average snowfall will probably decrease in most places as the climate warms, but the most aggravating, traffic-snarling, work-stopping, back-straining extreme storms like the one that just buried Boston could actually get bigger.

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“Most studies have been about how much snow falls in a season or in a year and call that average snowfall. But of course, in terms of disruption to society or economic disruption, we’re also interested in heavy snowfalls,” said Paul O’Gorman, an associate professor of atmospheric science at MIT who published his findings in Nature. “In some regions, fairly cold regions, you could have a decrease in the average snowfall in a year, but actually an intensification of the snowfall extremes.”

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O’Gorman published his findings last August, back when snow was not on everyone’s mind. He is currently in Australia, where the weather is sunshine and showers instead of snow, but took the time to answer a few questions by e-mail about his counterintuitive finding.

Can you explain how a warming climate might affect snowfall?

A. There are two competing effects as the climate warms: the increasing temperature causes a changeover from snow to rain, but it also increases the amount of water vapor in the atmosphere. For a particular place and time of year, which effect wins out depends on the temperature to begin with.

Most people would think that a warmer climate would mean less snow, more modest blizzards. Why would big storms become more intense?

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A. For relatively mild regions, we would expect heavy snowfall to become increasingly rare as the climate warms. But in colder regions, heavy snowfalls can become more frequent because of increases in the amount of water vapor in the atmosphere or, in some cases, because of changes in the circulation of the atmosphere, such as a shift in position of the storm track.

How did you do this analysis?

I used observations of how snowfall and rainfall vary with temperature to derive a relationship between changes in temperature and heavy snowfall. I also analyzed the output from climate models that are run on supercomputers by laboratories in the US and around the world.

What causes these intense, extreme snowfalls?

A. Extremely heavy snowfall requires a combination of an intense storm and the right temperatures. If it’s too warm, then most of the precipitation falls as rain. If it’s too cold, then there isn’t enough water vapor to give a big snowfall. All else being equal, there is a sweet spot in a fairly narrow temperature range around 24 degrees Fahrenheit.

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How should these trends change average and extreme snowfall in New England and in Massachusetts, specifically?

A. Temperatures in Boston in midwinter are already close to the optimal range for heavy snowfall events, and so I wouldn’t expect major changes in the intensity of snowfall extremes based on the temperature change alone. But changes in the circulation, such as a shift in the position of the storm track, can be very important for a specific location. There are better historical statistics for precipitation than for snowfall, and heavy precipitation events have been increasing in frequency in the Northeast at an unusually fast rate when compared to other parts of the US.

In your paper, you say it might be possible to use snowfall extremes to detect a signal of climate change. Can you explain how that would work?

A It’s not very easy to detect a signal of climate change in snowfall extremes. There is too much variability from year to year, which makes it difficult to discern the long-term trend. We will probably be able to see changes in the fraction of precipitation that falls as snow first, and this is more directly linked to changes in temperature.

Carolyn Y. Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.