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. 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 be increasing the frequency and persistence of U.S. winter cold snaps.
Facts for Any Story
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 has warmed two to four times faster than the rest of the world over the past four decades,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 Source3 Cohen, Judah et al., (2021) Linking Arctic variability and change with extreme winter weather in the United States, Science, Vol 373, Issue 6559 View Source4 Rantanen, Mika et al., (2022) The Arctic has warmed nearly four times faster than the globe since 1979, Communications Earth & Environment, 168 (2022) View Source5Cohen, J., et al. (2020), Divergent consensuses on Arctic amplification influence on mid-latitude severe winter weather. Nature Climate Change, 10, 20-29 View Source6Screen, James A. and Ian Simmonds, (2010), The central role of diminishing sea ice in recent Arctic temperature amplification, Nature 464, pages1334–1337 (2010) View Source a phenomenon known as Arctic amplification.
This overarching warming trend has been punctuated periodically by episodes of unusually cold winter weather (“cold snaps”) in northern temperate regions, including North America and Eurasia.7Kug, Jong-Seong, et al., (2015), Two distinct influences of Arctic warming on cold winters over North America and East Asia, Nature Geoscience, 8, pages759–762 (2015) View Source
Most cold snaps are not setting new records for coldness. In fact, the vast majority of temperature records broken in recent decades have involved record-breaking heat.8Meehl, Gerald A. et al., (2016), US daily temperature records past, present, and future, PNAS, 113 (49) View Source9van Oldenborgh, Geert Jan. et al., (2019), Cold waves are getting milder in the northern midlatitudes, Environmental Research Letters, 14 114004 View Source Nonetheless, some severe cold snaps bring temperatures significantly colder than average for the time of year, and they can have serious impacts on human health, transportation systems, infrastructure, and energy consumption.
Global warming is leading to later freeze-up of the Great Lakes and higher lake water temperatures. This increases lake-effect snowfall, which occurs when frigid air blows across relatively warm, ice-free water. Global warming is thus paradoxically leading to heavier snowfalls in regions bordering the Great Lakes, such as western New York State.10Wuebbles, Donald, (2019), An Assessment of the Impacts of
Climate Change on the Great Lakes, Environmental Law and Policy Center, 70 pp. View Source
Global warming also has increased the amount of water vapor in the atmosphere via additional evaporation, which may be augmenting snowfall in sufficiently cold regions.
Mounting evidence from observations and computer simulations suggests that Arctic warming is linked to U.S. cold snaps.3 Cohen, Judah et al., (2021) Linking Arctic variability and change with extreme winter weather in the United States, Science, Vol 373, Issue 6559 View Source11Pedersen, R.A. et al., (2016), The Impact of Regional Arctic Sea Ice Loss on Atmospheric Circulation and the NAO, Journal of Climate View Source12Tang, Q., et al., (2013), Cold winter extremes in northern continents linked to Arctic sea ice loss, Environmental Research Letters, 8:1 View Source13 Wallace, J.M., et al., (2014), Global Warming and Winter Weather, Science 343:6172, pp. 729-730 View Source
Disproportionate Arctic warming has reduced the temperature difference between the Arctic and temperate latitudes, which is thought to favor a “wavier” jet stream—the high-altitude current of wind that circles the Northern Hemisphere and that creates and steers weather systems from west to east.14 Moon, Woosok, et al., (2022), Wavier jet streams driven by zonally asymmetric surface thermal forcing, PNAS 119 (38) e2200890119 View Source A wavier jet stream tends to swing northward and southward as it moves eastward, allowing pools of cold, polar air to penetrate as far south as Texas and elsewhere in the U.S. South while also sending tropical air far northward, sometimes even creating winter “heat waves.”15Francis, J. A., et al. (2017), Amplified Arctic warming and mid-latitude weather: new perspectives on emerging connections. WIREs Climate Change View Source
In general, when the jet stream has large meanders, its temperature waves also travel east more slowly, which can make these cold plunges (and concomitant northward warm surges into polar regions) more persistent.16Francis, J.A., et al., (2018). North American weather regimes are becoming more persistent: Is Arctic amplification a factor? Geophysical Research Letters View Source
In addition, uneven Arctic sea ice melt and related heating in the Arctic have increased disruptions of the polar vortex, a persistent, very-high altitude swirl of cold air in the northern polar stratosphere, causing it to stretch and elongate, like a rubber band, or even split into separate swirls. This can cause the jet stream to meander and exacerbate the southward flow of cold air into temperate regions including the United States, and the flow of warm air into far northern regions.1https://www.ipcc.ch/srocc/ View Source5Cohen, J., et al. (2020), Divergent consensuses on Arctic amplification influence on mid-latitude severe winter weather. Nature Climate Change, 10, 20-29 View Source17Lee, S.H., (2019), Wintertime North American Weather Regimes and the Arctic Stratospheric Polar Vortex, Geophysical research Letters View Source
Strong Arctic warming and the associated loss of sea ice combined with La Nina conditions (cool ocean temperatures in the Tropical Pacific Ocean) were found to be factors in the February 2021 extreme cold snap that caused huge societal disruption in Texas and the southern Great Plains.18Zhang, Ruonan, (2022), Arctic and Pacific Ocean Conditions Were Favorable for Cold Extremes over Eurasia and North America during Winter 2020/21, Bulletin of the American Meteorological Society, Volume 103: Issue 10 View Source
Computer simulations that complement high-resolution meteorological observations predict that as the Arctic continues to warm, we can expect an increase in the frequency and persistence of cold snaps in northern temperate regions including the United States for some years ahead, albeit with weaker intensity.19Sung, 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 observations show that high-temperature records are being broken more often than cold records8Meehl, Gerald A. et al., (2016), US daily temperature records past, present, and future, PNAS, 113 (49) View Source and the same is true globally.
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. And when cold penetrates farther southward than normal, communities unfamiliar and unprepared for abnormal cold can be severely disrupted.
The jet stream exists all year and resides in the atmosphere (about 5-7 miles high, at altitudes where jets fly). It is responsible for creating and steering most of the weather systems experienced in temperate latitudes, including North America and Eurasia. The polar vortex, by contrast, exists only during winter and resides in the stratosphere (about 30 miles over the North Pole); its relationship to weather is less well defined and understood than the jet stream’s. Because the polar vortex typically stays close to the North Pole (and it is the winds associated with the polar vortex that expand southward across the United States), it is best to avoid saying “the polar vortex is coming to X location.”