Putting a freeze on climate doubt

The polar vortex that afflicted 50 million people in the Midwest and Northeast with an extreme chill last week may have struck many (including President Trump) as entirely at odds with the concept of global warming.

But cold spells like this are not unexpected by climatologists.  In fact, more extreme weather – more often hotter, but sometimes colder – is among the major risks projected as global temperatures rise. Indeed, just a week later, we saw temperatures rise to 20-plus degrees above the seasonal average in parts of the eastern United States, with Washington, D.C., experiencing a record high on Feb. 5.

Here’s why extreme cold is consistent with global warming.

To begin with, the world is hotter than it was 100 years ago (or 50 years ago, or 20 years ago). Globally, January was more than 0.4 degrees C (0.7 degrees F) warmer than the 1981-2010 average and the fourth warmest January on record. Within the United States, while the Midwest saw record lows, the Pacific Northwest saw record daily highs. These warmer averages all followed 2018, the fourth hottest year on record (behind 2015, 2016 and 2017) and every year since 1977 has been warmer than the 20th century average.

Record High and Low Temperatures

More Record Highs: This figure shows the percentage of daily temperature records set at weather stations across the contiguous 48 states by decade. Record highs (blue) are compared with record lows (green).

Second, the role of climate change in winter weather is complex and far from conclusive, but a growing body of evidence has linked colder North American winters to a warming arctic (especially northern North America and the northeastern United States). This particular cold patch is attributed to a “sudden warming in the Arctic stratosphere” that allowed the cold air spinning above the Arctic to bow into North America and Europe. Additionally, heavy winter snow storms have been linked to warm arctic weather in some studies. The four nor’easters that the East Coast experienced in 2018 (remember the bomb cyclone?) were caused in part by shifting jet stream patterns and La Niña, but higher sea temperatures causing greater precipitation may have contributed to the storms’ energy and snowfall.

Finally, not all weather is caused by climate change. Studies show the clear fingerprints of human-caused climate change in many extreme weather events, but a 2018 analysis found that about 29% of the events studied between 2004 and 2018 showed no clear evidence of human influence (which indicates that over 70% of the weather events studied were made more extreme by climate change). So chalking up every weather event – be it a heat wave, blizzard, or the recent frigid temperatures – to climate change (or the lack thereof as the case may be) is a conflation of the day’s weather and a region’s climate.

Even as scientists work to better understand the extent to which climate change impacts particular extreme storms or changing weather patterns, we also have to consider how we talk about the effects. Better universal understanding is an important tool in building support for the action needed to protect our communities and infrastructure from growing climate risks while contributing to strong, sustainable economic growth.