You are reading Part 2 of 9 in this series.What are Quick Facts?
Periodic bouts of extreme cold do not invalidate or weaken the overwhelming scientific evidence that human activities are warming the Earth to dangerous levels. Some evidence even suggests that human-induced global warming may be a driving force behind some winter cold snaps, and that Arctic warming in particular may ultimately increase the frequency and persistence of U.S. winter cold snaps.
There is a clear scientific consensus that human activities—primarily the burning of fossil fuels—are causing the Earth to warm, and that the Arctic is warming two to three times as fast as the globe as a whole.1https://www.ipcc.ch/srocc/ View Source2 Miller, G.H. et al., (2013) Unprecedented recent summer warmth in Arctic Canada, Geophysical Research Letters, 22 Oct. 2013 View Source3https://www.noaa.gov/news/2019-was-2nd-hottest-year-on-record-for-earth-say-noaa-nasa View Source
This overarching warming trend has been punctuated periodically by episodes of unusually cold winter weather (“cold snaps”) in northern temperate regions, including the United States and Eurasia.4Kug, J., et al., (2015), Two Distinct influences of Arctic warming on cold winters over North America and East Asia View Source
The vast majority of these cold snaps are not setting new records for coldness. In fact, almost all the temperature records that have been broken in recent decades have involved record-breaking heat.5Meehl, G.A. et al., (2009), Relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S., Geophysical Research Letters, 1 December 2009 View Source Nonetheless, these cold snaps bring temperatures that are significantly colder than average for the time of year, and they can have serious effects on human health, transportation systems, and energy consumption.
Scientists do not know for certain which factors contribute most to periodic cold snaps. Some science suggests that changes in the tropics could be an important factor.6Lee, S.H., (2019), Wintertime North American Weather Regimes and the Arctic Stratospheric Polar Vortex, Geophysical research Letters View Source Other research has suggested that climate-change-related Arctic warming may be key.7Pedersen, R.A. et al., (2016), The Impact of Regional Arctic Sea Ice Loss on Atmospheric Circulation and the NAO, Journal of Climate View Source8Tang, Q., et al., (2013), Cold winter extremes in northern continents linked to Arctic sea ice loss, Environmental Research Letters, 8:1 View Source9 Wallace, J.M., et al., (2014), Global Warming and Winter Weather, Science 343:6172, pp. 729-730 View Source
Two lines of evidence support the possibility of an Arctic warming connection: direct observations and computer simulations.
Scientists have observed that unusually warm temperatures in the northern polar region can destabilize the jet stream—a high-altitude current of wind that circles the polar region and generally carries weather systems from west to east.
A destabilized jet stream tends to “wobble” northward and southward as it moves eastward, allowing pools of cold, polar air to penetrate far southward in some regions of the globe (and at the same time bringing warmer air north).10Francis, J. A., et al. (2017), Amplified Arctic warming and mid-latitude weather: new perspectives on emerging connections. WIREs Climate Change View Source
A wobbly jet stream in some parts of the globe can also weaken the polar vortex, a persistent swirl of cold air in the northern polar stratosphere. That in turn can amplify jet stream wobbles, adding to the southward flow of cold air into temperate regions.11Cohen, J., et al. (2020), Divergent consensuses on Arctic amplification influence on mid-latitude severe winter weather. Nature Climate Change, 10, 20-29 View Source
In general, when the jet stream meanders, it also travels east more slowly, which can make these cold plunges and (and concomitant northward warm surges into polar regions) more persistent.12Francis, J.A., et al., (2018). North American weather regimes are becoming more persistent: Is Arctic amplification a factor? Geophysical Research Letters View Source
When scientists collect meteorological data at relatively high spatial resolution (“high resolution” in this context means pockets of air about a cubic kilometer in volume, or about 400 times the volume of the great pyramid of Giza) and combine this information with the latest understanding of how such volumes of atmosphere interact with each other, the resulting computational simulations predict that as the Arctic warms, we can expect an increase in the frequency and persistence of cold spells in northern temperate regions for some years ahead, albeit fewer broken cold records as global warming continues.13Sung, M-K et al., (2016), Arctic-North Pacific coupled impacts on the late autumn cold in North America, Environmental Research Letters View Source
Pitfalls to Avoid
Avoid amplifying the misconception that cold snaps somehow undermine the clear evidence of human-caused global warming—or that any single weather event is representative of what is happening with the climate. In the United States, long-term records show that heat waves are increasingly outnumbering cold snaps5Meehl, G.A. et al., (2009), Relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S., Geophysical Research Letters, 1 December 2009 View Source and the same is true globally. Between January and early November 2019, for example, 20 nations or territories set all-time heat records and recorded 107 instances of a hottest all-time temperatures for a given month. During the same period there was not a single all-time national cold record set for any month. 2019 was the second warmest in recorded history and 2019 ocean temperatures were warmest ever recorded.
Also, keep in mind that the severity of a cold snap may not be its most disruptive attribute. Even if temperatures are not record-breaking, a long-duration cold spell may have a greater impact than a short, intense one.