What role did climate change play in this winter’s US freezes, heat, and drought?

CCL’s Science Policy Network explains how human-caused global warming is causing rapid changes in the Arctic, which in turn is altering patterns in the atmosphere. 

What role did climate change play in this winter’s US freezes, heat, and drought?

By Dana Nuccitelli and Doug Sinton, CCL Science Policy Network Team

There is growing scientific evidence suggesting that human-caused global warming is causing rapid changes in the Arctic, which in turn is altering the atmosphere, causing wavy patterns to form more frequently in the jet stream. On the West Coast, this can cause persistent high-pressure systems to form in the Pacific, exacerbating droughts by blocking storm systems. It can also allow frigid Arctic air to spill into the USA, creating especially cold winter weather. In sum, these freezes, heat, and droughts are made more likely by rising global temperatures, and as they rise further, such extremes may well become more common.

Abnormal winter weather

This winter, the eastern USA was hit by frigid cold weather, although at the same time, the western states (and most of the rest of the world) were relatively toasty:

Surface temperatures, Arctic, North America, NASA

North American surface temperatures for Dec. 26, 2017 – Jan. 2, 2018, from NASA Earth Observatory

This prompted a presidential tweet suggesting, “Perhaps we could use a little bit of that good old Global Warming.” It’s a natural reaction, when in the midst of frigid weather, to wonder how such cold conditions can strike in a world that’s being heated by global warming. However, scientific research has suggested that, counterintuitively, climate change appears to be playing a role in making these cold winter weather events happen more often in some regions.

Connecting global warming, the Arctic, and the wobbly jet stream

The jet stream plays a key role here. Jet streams are bands of fast-moving air currents about five to seven miles above Earth’s surface. The polar jet stream influences weather in North America, and in turn is influenced by changes in the Arctic due to human-caused global warming.

The Arctic is the fastest-warming part of the planet, in large part because sea ice is disappearing so rapidly. White ice is reflective, but dark oceans aren’t. When sea ice sitting on top of the ocean melts, the Arctic surface becomes less reflective, absorbing more sunlight, which in turn melts more ice in what’s known as a “positive feedback.” Because the colder Arctic is warming faster than the warmer area to its south (e.g. North America), the temperature difference between the regions is shrinking.  

That temperature difference is a big part of what normally keeps the jet stream strong and moving in a straight west-east line. When it shrinks, that allows more and larger wobbles (waves) to form in the weakened jet stream. As a result, weather patterns can get stuck as those waves slow down in the weaker jet, and the weather systems can become quasi-stationary or “blocked.” Picture a river with a slower current having more and larger meanders than a faster flowing one. (2)

Consequences: Drier West Coast, cold weather in the East

These jet stream waves (3) tend to cause different types of extreme weather in different parts of the country. On the West Coast, persistent high-pressure ridges tend to form in the Pacific Ocean, creating warm, dry weather in California as storms pass to its north. One such high pressure system, coined “the Ridiculously Resilient Ridge,” contributed to California’s 2012–2016 drought, which a 2014 study concluded was the state’s most intense in over 1200 years. A 2016 study published in “Science” found that these persistent high-pressure ridges are forming more often off the West Coast in a hotter world.

In the Midwest and on the East Coast, a southward dip in the jet stream tends to cold Arctic air to spill into those states. Rutgers climate scientist Jennifer Francis described this scenario:

“In response to the strengthened western ridge of atmospheric pressure, the winds of the jet stream usually also form a deeper, stronger trough downstream. Deep troughs act like an open refrigerator door, allowing frigid Arctic air to plunge southward, bringing misery to areas ill-prepared to handle it. Snowstorms in Texas, ice storms in Georgia and chilly snowbirds in Florida can all be blamed on the Terribly Tenacious Trough of December 2017 and January 2018.”

Jet stream trough Arctic air North America

While this is a topic of active research and significant uncertainty, a 2018 study published in “Nature Communications” found additional evidence that the jet stream has become increasingly wavy over the past 50 years, to a degree unprecedented in the past 290 years, which “can generate more frequent mid-latitude blocking patterns and facilitate persistent periods of extreme weather.”

How to accurately communicate climate-extreme weather connections

Communicating these sorts of climate-weather connections can be tricky. It’s often tempting to begin by hedging that “we can’t say that any given weather event was caused by global warming.” However, that approach emphasizes what we don’t know instead of what we do know.

A National Academy of Sciences’ Committee on Extreme Weather Events and Climate Change Attribution report tackled this question:

“A reoccurring theme of this report is the importance of the framing of any attribution question. Although climate scientists are frequently asked ‘Was a given observed weather event caused by climate change?’ we believe this is a poorly formed (or ill-posed) question that rarely has a scientifically satisfactory answer. The report discusses appropriate ways to frame attribution questions as well as the interplay between meteorological and human-made factors in the realization of extreme events.”

When Barry Bonds was setting home run records during the steroid era, people used to ask if his latest homer was due to steroid use. The same dynamic occurs after each extreme event and global warming. It’s more useful to assign a higher probability to extreme weather events because of global warming just as it was more instructive to look at Barry Bonds’ overall home run record than any single home run.

As climate scientist Kevin Trenberth noted in a 2012 paper, all weather events now happen on a planet with a climate that we’ve made hotter:

“The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question. All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.”

That is the proper framing when discussing these types of extreme weather events. In this case, there is growing scientific evidence suggesting that human-caused global warming is causing rapid changes in the Arctic, which in turn is altering the atmosphere, causing wavy patterns to form more frequently in the jet stream. On the West Coast, this can cause persistent high-pressure systems to form in the Pacific, exacerbating droughts by blocking storm systems.  It can also allow frigid Arctic air to spill into the USA, creating especially cold winter weather.

Footnotes:

  1. Volunteers should carefully consider what audience this information may be most appropriate for. For example, if presented to a “layperson,” ensure it is done in a manner that is easy to understand.
  2. Rutgers climate scientist Jennifer Francis explains these changes to the jet stream and impacts on weather systems here.
  3. A video illustrating the jet stream waves is available here.

Dana Nuccitelli is an Environmental Scientist and writes about climate change for The Guardian and Skeptical Science. Doug Sinton is a meteorology professor at San Jose State University (retired).  

The Science Policy Network is a team of CCL leaders and supporters with a diverse background in the field of climate science. These network contributors write regular guest posts, offering thorough insight into topics that fall within their expertise. This post and other resources are available in the form of white papers on CCL Community

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