Climate change won't keep Baltimore warm in the winter

By Joe Catapano and Abigail Bentz | CAPITAL NEWS SERVICE
(Top photo by Julianna Larsen, Capital News Service)

COLLEGE PARK, Maryland — Climate change will drive increases in global temperatures and summer heat waves. But that doesn’t mean cold snaps in cities like Baltimore will disappear.

And, perhaps paradoxically, climate change could mean an increase in extremely cold weather in the Northeast during the winter. That’s because of how climate change will affect the polar vortex, a phenomenon that pushes Arctic air into the United States.

In Baltimore, number of freezing days holds steady

The number of days that dipped to 32 degrees or below in Baltimore has stayed relatively constant over the past 80 years. The 1960s peaked at more than 1,000 such days, but that’s happened just once in the last eight decades.

Source: BWI Airport Sensor Data (1940-2018)

“There is this generalized warming of the planet that would decrease extreme winter weather,” said Judah Cohen, the director of seasonal forecasting at Atmospheric and Environmental Research, a Massachusetts-based research company.

“But also, on the other hand, there are more frequent disruptions of the polar vortex which would increase [it], so it's kind of a tug of war between the two.”

When the temperature in the Inner Harbor fell to 9 degrees on Jan. 31, it marked the third year with a single-digit day in the last six years in the city.

Even as global temperatures have risen over the last century, the number of subfreezing days in Baltimore has stayed relatively constant. But the coldest days — those that dipped below 20 degrees — are occurring at a less frequent rate.

Larger declines in extreme cold days in Baltimore

While the number of subfreezing days hasn’t fluctuated dramatically from decade to decade, the number of extremely cold days has. Compared with the 1980s, there were about one-third as many days in the 1990s and 2000s where the temperature dipped to 10 degrees or lower. But the number of very cold days has trended upward in the last decade.

Source: BWI Airport Sensor Data (1940-2018)

One factor contributing to the continued persistence of cold days: climate change’s effect on the polar vortex. The polar vortex is a mass of air located above the North and South poles that contains trapped cold air. Under normal circumstances, it remains at the top and bottom of the world.

When the stratosphere — the layer of the atmosphere that lies above the one we live in — warms, the polar vortex weakens and can break into multiple sections. This allows the cold air to escape the poles and bring extreme cold temperatures to other parts of the world, such as the continental United States. This happened in January and brought frigid temperatures to the Midwest and, to a lesser extent, the Northeast.

The polar vortex explained

The polar vortex is a climatological phenomenon that can push extremely cold Arctic air into the United States.

Credit: National Oceanic and Atmospheric Administration

The impact from these disruptions could last from a few days to eight weeks, according to Cohen, a wide range that makes it difficult to predict how long future cold snaps will be.

“You may get a cold snap like we had with that polar vortex event in late January in the upper Midwest, where they had that historic cold then it moderated quickly, so that's more typical,” Cohen said. “We think of it as more episodic, but there are cases where it's been continuous when the pattern gets locked in and it doesn't change."

NASA instrument shows movement of the polar vortex

In January 2019, the polar vortex dropped deep into the continental United States, causing record-breaking temperatures in the Midwest and frigid weather across the Northeast. NASA's Atmospheric Infrared Sounder (AIRS) instrument captured the temperature fluctuation in vivid detail.

NASA's Atmospheric Infrared Sounder (AIRS) instrument captures a polar vortex
Credit: NASA

In Baltimore, there is no indication that extended cold snaps are becoming more common. The occurrence of three-day-long cold snaps during which the temperature fell below freezing on all of those days are just as sporadic as previous years.

The most extreme cold snaps — extended periods with temperatures far below normal — in the northeastern United States are the result of polar vortex disruptions, Cohen said. Climate change will make those disruptions more frequent, he said.

Extended cold snaps stay constant in Baltimore

In Baltimore, the number of cold snaps hasn't changed significantly over the years. In the last eight decades, three-day cold snaps peaked in the 1990s while five-day cold snaps peaked in the 1960s. Capital News Service examined two types of cold snaps: three consecutive days and five consecutive days where the daily minimum temperature dipped to 32 degrees or below each day.

Source: BWI Airport Sensor Data (1940-2018)

This winter saw a variety of weird weather across the U.S. Snowfall hit Maui, Hawaii, at its lowest elevation ever. Meanwhile, Seattle recorded record-breaking cold temperatures and Las Vegas had snowfall for the first time in more than a decade. It’s hard to say whether those events were also driven by climate change or happened at random, Cohen said.

“How do you determine if that's an indication of a changing climate and not just the weather event?” said Tim Canty, a professor of oceanic and atmospheric science at the University of Maryland. “That's the trick and that's a question among climate communities.”

What experts do know is that sporadic extreme winter weather will continue, including in the city of Baltimore. Cold snaps have regularly occurred in the northeastern U.S. over the last 30 years. The pattern is not expected to change, according to Cohen.

Temperatures recorded in Baltimore Harbor dipped to 9 degrees on Jan. 31, 2019. It marked the lowest temperature collected by that sensor in four years. The last time temperatures recorded by the BWI sensor dipped below zero came in 1996, but dangerous, frigid temperatures persist annually throughout the city amid climate change.

Extreme cold weather events have always happened, are still happening and show no signs of completely disappearing, despite a decrease in extremely cold days.

“And the difficulties with this and trying to get people to care about this is … we had this massive cold snap this year,” Canty said. “But what about if next year it's 70 degrees all winter long and everyone's happy about that?”



Cold weather has a profound effect on public health, worsening certain chronic conditions and disproportionately affecting some groups. Read the story.


At the heart of the issues of climate change and public health are residents trying to stay well in the cold. Read the story.

Safety Net

What are government agencies and institutions in Baltimore and Maryland doing to help people affected by the cold? Read the story.

Stay tuned. During the summer, Capital News Service, NPR and Wide Angle Youth Media will be reporting on the public health effects of climate change during hot weather. Follow the Climate and Health Project blog or sign up for our e-newsletter for details.