With much of New Hampshire covered in a white blanket, this moment in January feels like winter to many.
The rainstorms, flooding, 50-degree temperatures, and bare ground earlier in the season? Not so much.
But that kind of weather is becoming a new kind of normal with climate change. A new study from Dartmouth, focused on snowpack in the last 40 years across the Northern Hemisphere, shows human-caused warming is having a big impact on winter. In New England, the researchers found, many river basins are losing 10% or more of their snowpack per decade.
Justin Mankin, one of the researchers, says New Hampshire’s weird weather in December and January is “emblematic” of the trends that are shrinking the region’s snow. Rain on snow, snow melting days after it falls, and bare ground are all par for the course in our climate-changing world, he said.
Winter, Mankin said, is starting to look like “vacillations between being briefly snow covered and a prolonged stick season.”
Mankin, along with PhD student Alex Gottlieb, set out to study whether climate change has been affecting snowpack in a way that can clearly be attributed to human influence.
That has historically been a tough question for scientists to answer. For one, it’s hard to measure how much snow accumulates on the ground – there are many different ways of measuring that, and they don’t all agree with each other over time, the researchers said.
Climate change is also having a different impact on snow, depending on the average winter temperature of a place.
In places that are very cold, a few degrees of warming won’t have much of an impact, Gottlieb said. Most of the precipitation will still fall as snow instead of rain, and there won’t be big warm spells to melt the snow that is on the ground.
But in places where the average winter temperature is above about 17 degrees, the sensitivity to temperature shifts is different.
The likelihood that a storm will bring rain instead of snow, or that a warm period will thaw snowpack that has accumulated on the ground increases exponentially when a place’s average temperature is closer to the freezing point, Gottlieb said.
And that’s what’s happening in the Northeast.
“What this work makes clear is that in places like New England, we're still going to get these snowy storms, we're still going to get these snowy winters, but they're just going to be kind of increasingly anomalous blips on this overall downward trajectory,” Gottlieb said.
The researchers focused on how river basins are losing snow, studying in particular how the snowpack looked in March – generally the peak of snow accumulation. In about half of the river basins they studied across the Northern Hemisphere, they detected clear trends for snowpack, and in 40% of those they were able to attribute the changes to global warming caused by humans.
With less snowpack, the researchers said, there could be less meltwater running down rivers and streams in the spring, when people are starting to demand more water. That’s particularly an issue in places like the American West, which has a dry season.
For New England, which gets pretty much the same amount of precipitation every month, no matter the season, how losing snowpack could affect water conditions is an ongoing question, the researchers said.
What’s clear is that as humans continue burning fossil fuels and heat up the atmosphere, this area of the world will lose snow more quickly.
“New England basins are particularly vulnerable to even modest amounts of global warming going forward. And the possibility of moving to a regime of near-permanent snow loss, or something that looks quite like that, strikes me as very likely,” Mankin said.
