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Winters are warming faster than summers. These US cities could lose weeks of freezing days by 2050.

The dire consequences of having fewer days below freezing, explained.

White winters are overall beginning to turn mushy gray across the northern United States. And the longer we wait to get serious about limiting climate change, a White Christmas could become a thing of the past for many cities later this century.

As part of our Weather 2050 project, we examined how average winter low temperatures are projected to shift in the 1,000 largest US cities by 2050 if we do nothing to reduce greenhouse gas emissions.

In our analysis, we found that in 67 cities, the average winter low temperature could cross a critical threshold by 2050: the freezing point of water.

For cities like Philadelphia, New York, and Washington, DC, that have historically snowy winters, this shift in the average winter low means that snow and sleet could become rarer.

Around the country, warmer winters could mean the closure of skating rinks, more pollen, and more ticks carrying Lyme disease, since temperatures won’t be dropping below freezing as often to kill them off. Critical water resources out west that depend on snow will suffer large declines.

“I would argue that our winters are getting sick, and the reason why is global warming,” Amato Evan, an associate professor at the Scripps Institution of Oceanography, said at the American Geophysical Union meeting in December.

One way to take winter’s pulse to look at how winter low temperatures are shifting. Here’s a map of the 67 cities we found where the average winter low could shift from below freezing to above freezing by 2050.

Average winter low temperatures, of course, obscure important details about how climate change is affecting winters, like how the cold is distributed throughout the season.

So we also calculated how the number of days with below-freezing temperatures could change by 2050 in the same 67 cities.

Take a look at some cities in the northeastern United States:

You can see that by 2050, these cities could lose a month or more of days with temperatures below freezing.

And as we move further south, cities could experience even larger declines in the number of days with sub-freezing temperatures during winter:

In fact, the majority of the 67 cities that will see their average winter lows rise above 32°F are in southern states like North Carolina, South Carolina, Tennessee, and Georgia. The winters here aren’t especially cold to begin with, so even a slight amount of warming can push temperatures above the melting point of ice.

We also see cities lose freezing days further west:

While these cities aren’t known for ice and snow, they do currently have freezing winters that are projected to become less so.

And the sharpest projected declines in freezing days are actually in the Pacific Northwest:

Here, we see cities that face more than two months-worth of days with temperatures below 32°F, with almost none by 2050.

The loss of freezing days will harm ecosystems, human health, and water resources

In general, scientists expect that winters will warm faster than summers across the US.

This will have several major consequences that are pretty worrying. For starters, many species are adapted to cold weather and snow, so having fewer or no days below 32°F is sure to impact them. We’re already starting to see those effects.

Animals like the snowshoe hare, found in the boreal forests of Alaska, undergo a seasonal molt from brown in the summer to white in the winter to camouflage with their environment. But less snow in the winter is making them more visible to predators.

Changes in winter temperatures are also starting to affect our health and our water resources.

“Some pests that are [typically] killed off by cold winters are able to thrive and survive,” said Daniel Cayan, a scientist at the Scripps Institution of Oceanography at the University of California San Diego.

Bark beetles, for example, have devastated forests across the United States as temperatures have risen. In California, bark beetles combined with years of drought have helped kill off 129 million trees across the state. And these dead trees pose a huge wildfire risk: They were one reason the deadly Camp Fire, which torched the town of Paradise, California, in November, got so big so quickly.

Critters that make humans sick are also benefiting from warmer winters. “In regions where Lyme disease already exists, milder winters result in fewer disease-carrying ticks dying during winter,” according to the Centers for Disease Control and Prevention. “This can increase the overall tick population, which increases the risk of contracting Lyme disease in those areas.”

Cold winters have served as a key check on mosquito populations that carry viruses like dengue and Zika. Freezing temperatures can kill off mosquito larvae, reducing their numbers in the spring. But as winters get milder, we’re seeing the range of species like Aedes aegypti and Culex quinquefasciatus expanding further north and in greater numbers.

Many plants also rely on temperature signals. Seeds detect temperatures to see when it’s appropriate to germinate. Adult plants measure heat and cold to help determine whether to actively grow or go dormant. Some flowering plants are already starting to emerge earlier in the season in part due to warmer winters. This is creating a mismatch between when a plant flowers and when insects like bees are around to pollinate them.

For other plants, like pollen-spewing ragweed, a warmer winter is effectively a shorter winter. That means ragweed and other plants start producing pollen earlier in the season, which will lead to more misery for allergy sufferers.

Another important consequence of winter temperatures rising above freezing is less snowfall. In the western US, the mountain snowpack slowly discharges water as it melts in the spring and summer. The slow release is critical to waterways like the Colorado River to ensure a steady flow.

But the US is now in the midst of a long-term decline in its snowpack, which has already fueled floods in the spring and droughts in the summer. The timing of when the snow melts is also critical. In the Sierra Nevada, earlier snowmelt leads to more wildfires as it leads to a bumper crop of fast-growing vegetation that then dries out in the summer. In some biomes, a dusting of snow can help prevent the ground from freezing, allowing root systems to survive the winter.

The loss of snow and ice is already a major problem for winter recreation. Researchers have found that climate change will shrink the number of days for downhill skiing, cross-country skiing, and snowmobiling across 247 winter recreation areas. For some resorts, the length of skiing season will likely be cut in half by 2050, and across the country, warming winters will cost the industry more than $2 billion. For more on this, check out Climate Central’s new research brief, ON THIN ICE: How Climate Change is Shaping Winter Recreation.

This all goes to show that we cannot ignore the dramatic changes unmitigated climate change could bring to our winter months.

How we did our analysis

To calculate projections for average winter low temperatures in the 1,000 most-populated cities in the United States, we averaged daily minimum temperatures in winter months for a baseline period over 30 years (1986 to 2015). Then we looked at how these cities would warm by 2050, again averaging over 30 years (2036 to 2065). Since we’re only looking at the continental United States, this analysis does not include Alaska and Hawaii.

These projections are based on a suite of climate models aggregated in the Localized Constructed Analogs data set developed by the Scripps Institution of Oceanography at the University of California San Diego.

We’re using a standard set of assumptions in climate models known as RCP 8.5. In general, it’s considered a high-end, some would say pessimistic, projection of warming: It assumes that the world will continue on the same course of carbon dioxide emissions with limited improvements in technology or efficiency.

It’s useful because it tracks closely with where we are now and serves as an upper boundary for what we can anticipate. RCP 8.5 also doesn’t result in a vast difference in climate change estimates compared to other scenarios when looking at the middle of the century. The largest variations under RCP 8.5 emerge around 2100.

The biggest unknown is what the world will actually do to address climate change. Nations recently agreed at the latest round of United Nation climate talks in Poland to a set of rules to govern how they implement their pledges to cut emissions under the Paris climate agreement.

But current pledges are not enough to limit warming to 2°Celsius this century above preindustrial levels, the target under the Paris accord. That means countries will still have to do more to fight climate change, drastically slashing fossil fuel use, electrifying their economies, and sucking CO2 out of the air. They’ll also have to adapt to the warming that’s already happening. We can’t escape its effects, even during the coldest times of year.

Special thanks to David Pierce, of the Climate Research Division at the Scripps Institution of Oceanography, for guiding the data analysis.

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