Residents of the eastern United States are enduring one of the most painfully cold periods in modern times. Since January, Syracuse, N.Y., has never had more days below zero. Bangor, Maine is witnessing its coldest month ever recorded. On Tuesday, Washington Dulles Airport experienced its most bitter morning measured so late in the season, plummeting to minus-4.

Yet, in what may seem like a paradox, the amount of wintertime cold air circulating around the Northern Hemisphere is shrinking to record low levels. This winter (2014-2015) is on track to see the most depleted cold air supply ever measured.

“We are still on pace to break the all-time record — no question about it,” says Jonathan Martin, a professor of meteorology at the University of Wisconsin-Madison. “Despite the brutal cold in the eastern U.S., the whole hemisphere is warmer this winter than it has ever been in history.”

Using an analysis of atmospheric temperature data from the National Center for Atmospheric Research, Martin has been tracking the size of the Northern Hemisphere cold pool during winter (December-February) over time. Specifically, he has examined the total area of the hemisphere covered by temperatures 23 degrees (-5 degrees Celsius) or lower at an altitude of about 5,000 feet, for the period 1948-1949 to present.

In a study accepted for publication in the Journal of Climate, Martin found that four of the five smallest Northern Hemisphere cold pools on record — averaged over the winter — have occurred since 2004.

“Only 12 of the 43 winter seasons before 1990-1991 had below average seasonally averaged areas whereas 20 of 24 winter seasons have had below average seasonally averaged areas since,” the study says.

The study only incorporates results through last winter but reported last year’s “desperately cold” conditions in the eastern U.S. coincided with the most diminutive Northern Hemisphere cold pool on record up to that point in time.

Taking into account Martin’s analysis of the current winter, the size of the Northern Hemisphere cold pool has reached records low levels in back-to-back years.

One may wonder how the cold air supply is so compromised after the relentless blasts of frigid air in the eastern U.S. the past two winters.

“You just need to look around and see how big the globe is,” Martin says. “The thing this simple analysis makes clear is that there is such an obvious difference between regional weather and global climate. There’s a better way to measure global change than backyard thermometers.”

Martin points out that while the U.S. has shivered, Alaska and northern Europe, in particular, have been much warmer than normal. And he is convinced the hemispheric warming signal reflects growing greenhouse gas concentrations in the atmosphere from the burning of fossil fuels.

“The only way to have systematically smaller pools of cold air is to have greater retention of infrared energy [from greenhouse gases],” Martin says. “The planet can’t cool the way it used to.”

The long-term shrinking of the Northern Hemisphere cold pool identified in Martin’s work mirrors the decline in Arctic sea ice and rise in global surface temperatures, adding another independent line of evidence for climate warming.

While some warming naysayers have attempted to discredit surface temperatures datasets because of adjustments made for quality control (the methods for which have been published in the peer reviewed journals and gained widespread acceptance), Martin says his results from upper air data are difficult to refute.

“Skeptics have jumped all over the surface data, but you can’t really do that with temperatures about a mile above sea level [analyzed from atmospheric data],” Martin says. “They make a pristine signal.”