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The amount of cold air above the Northern Hemisphere this winter is near a record low, an unambiguous signal of the planet’s warming climate, according to a new analysis of 76 years of temperature data from about a mile above the ground.

The depleted cold-air supply means blasts of Arctic air generally lack the vigor of the past, while incursions of unusually mild weather — <a href="https://www.washingtonpost.com/weather/2024/02/26/record-warm-weather-midwest-greatlakes-south-northeast/" target=_blank>such as the one swelling over the central United States now</a> — can be more frequent and intense.

The cold-air supply in the Northern Hemisphere is being evaluated using temperature data from about 5,000 feet high in the atmosphere. For about a decade, Jonathan Martin, a professor of meteorology at the University of Wisconsin, has analyzed the size of the cold pool at this level — or the area of the hemisphere covered by temperatures at or below 23 degrees (minus-5 Celsius). This winter’s cold pool will finish the winter as the second-smallest on record, Martin said.

Nine of the 10 smallest winter cold pools since 1948-1949 have occurred in the 2000s, continuing a trend of shrinking winter cold pools that Martin identified in a 2015 <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00496.1" target=_blank>study</a> published in the Journal of Climate.

“The climate continues to stay at the same pace of warming that it’s been at for the last 20 years,” Martin said. “The hemisphere is showing us in the wintertime, unequivocally, that the globe is warming up.”

This winter’s shrinking supply of cold air in the atmosphere has coincided with what is probably to be one of the warmest winters on record. Many locations in the United States are on track for a record-warm winter as <a href="https://www.washingtonpost.com/weather/2024/02/26/record-warm-weather-midwest-greatlakes-south-northeast/" target=_blank>temperatures soar to near and above 30 degrees warmer than normal in the season’s final days</a>. On Monday, Dallas hit a record high of 93 degrees, while Minneapolis reached 65 — 32 degrees above average. Chicago could reach the upper 70s on Tuesday, a February record.

Globally, <a href="https://twitter.com/extremetemps/status/1762455302933188969" target=_blank>more than 200 countries have seen record warmth this week</a>, according to weather historian Maximiliano Herrera. January was the eighth consecutive month to register as that month’s warmest on record, while 2023 was <a href="https://www.washingtonpost.com/climate-environment/2024/01/09/record-hot-year-2023-global-temperatures/" target=_blank>Earth’s warmest year on record</a> for both the land and oceans.

Some who are skeptical that the climate is warming claim that surface temperatures are rising because of urbanization, changes in sensor location and other factors they say taint the data. While scientists are aware of such influences, and adjust surface data accordingly using methods that have been verified by peer-reviewed research, Martin says the validity of upper-air data is even more difficult to refute.

“These measurements are completely uncontaminated by any of that urbanization,” Martin said. “So if you get a signal [higher up in the atmosphere,] you can be pretty sure that that’s a really robust signal, and it’s got none of those limitations [of] surface-based measurements.”

The year-to-year fluctuations in the size of the cold pool also appear to be unrelated to El Niño, according to Martin. El Niño is a naturally occurring, periodic warming of the ocean surface in the central and eastern Pacific, which scientists say has contributed to extreme surface warmth during the past year.

Jonathan Martin, University of Wisconsin

Martin calculates the size of the Northern Hemisphere cold pool for each day of winter compared with the average for that day going back to the winter of 1948-1949, when weather balloons started to consistently collect upper-air temperature data. From that, he ranks each winter season (Dec. 1 to Feb. 28) by its average cold-pool size.

<a href="https://www.washingtonpost.com/news/capital-weather-gang/wp/2015/02/25/even-as-the-eastern-u-s-freezes-theres-less-cold-air-in-winter-than-ever-before/" target=_blank>Only the 2014-2015 winter saw a smaller cold pool</a> than the current winter. Still, the cold pool reached a record small size on four days this winter which Martin said was “pretty unbelievable.”

Even when the cold pool contracts, it doesn’t mean mild weather will occur everywhere in the Northern Hemisphere, Martin stressed. When the cold pool shrank to a record low in 2014-2015, the United States experienced an extended period of bitter cold.

“So it really points out how regional our own impressions of the winter can be, and how different that can be from the hemisphere’s perspective,” Martin said.

The cold pool decreased to what Martin defines as a warm extreme (two standard deviations smaller than average) on 15 days this winter, while reaching a cold extreme (two standard deviations larger than average) on no days. The last cold extreme occurred in February 1994, making this the 29th straight winter with no cold extremes.

The dominance of upper-air warm extremes is consistent with <a href="https://www.washingtonpost.com/weather/2024/01/17/climate-change-cold-extremes-arctic/" target=_blank>heat records outpacing cold records</a>. In the United States, for example, there have been nearly 8,000 warm-weather records set this winter, compared to around 2,300 for cold weather.

<i>Jason Samenow contributed to this report.</i>

Dan Stillman is a meteorologist and editor for the Capital Weather Gang. He earned an M.S. in Meteorology from Texas A&M University, and a B.S. in Atmospheric, Oceanic and Space Sciences from the University of Michigan.

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