Connections Between Climate Change and Extreme Weather

More than 98 inches of snow has fallen in Boston this season, while workers have spent about 170,000 hours plowing the streets and distributed more than 76,000 tons of salt on roadways. At the same time, much of the American West, Rocky Mountains, and Northern and Central Plains have experienced warmer-than-average temperatures. California, in the grip of an epic drought, had its fourth-driest January ever recorded with just 15 percent of average precipitation.

So what is going on with this extreme weather, and what does it have to do with global climate change?

Due to recent analytical advancements, climate scientists are now able to more accurately determine how climate change impacts the odds of an individual extreme event occurring.

More research is planned in coming years to examine links between extreme weather and climate events and climate change, and global research already tells us a lot about the trends, including these three counterintuitive connections between climate change and extreme events:

1. Record cold temperatures can still occur in a warming world.

During 2014, cities in regions like the Midwest and Northeast endured record cold months. Zoom out to the state level, however, and three states (California, Nevada and Arizona) saw record warm annual temperatures in 2014, while none experienced record cold annual temperatures. The national average temperature was warmer than normal, and at the global scale, last year was more than just above average—2014 was the warmest year ever recorded.

While portions of the eastern United States have faced historic snow totals and frigid temperatures this year, the opposite can be said for much of the western U.S.

A growing body of research suggests that a contributing factor of this drastic east-west temperature contrast could be the accelerated warming taking place in the Arctic, which can have a weakening effect on the polar jet stream, a west-to-east river of wind in the atmosphere where cold Arctic air meets milder subtropical air in the mid-latitudes (e.g., the U.S.) of the Northern Hemisphere. A weaker polar jet stream creates more favorable conditions for a “wavy” north-south oriented path around the Arctic, which can increase the frequency of phases where Arctic air seeps south into regions like the eastern United States while warmer air protrudes north in the western half of the country.

2. A warming planet can make some regions much snowier.

The warmer the air is, the more water vapor it can hold. This additional moisture can bring more intense rain or snowfall. In addition, when sea surface temperatures are warmer than average (as they are currently in the Northeast Atlantic Ocean), the atmosphere becomes fueled with more moisture and energy.

Precipitation amounts vary by region, but in the United States, all regions except Hawaii have experienced an increase in very heavy precipitation events since the late 1950s. However, snowfall has decreased in most parts of the country since recordkeeping began, in large part because more winter precipitation has come from rain instead of snow. Thus, temperatures play a significant role.

Gradual warming is reducing the number of very cold days in many regions, but when temperatures are cold enough, the increased moisture can cause heavier snowfall events. In fact, the heaviest snowstorms occur when temperatures are just below freezing as opposed to when they are much colder (when available moisture is reduced)—conditions becoming more likely in mid-winter in a warmer world.

3. Climate change can contribute to a double whammy of drought and extreme precipitation in the same location.

As mentioned above, a warmer atmosphere can hold more water, fueling more intense rain and snow events. But at the other end of the spectrum, the warming climate can amplify conditions conducive to drought—like heatwaves, evapotranspiration and reduced soil moisture. The combination of these two extremes in one location can increase disasters like flooding and landslides, and recent history suggests parts of the United States may already be grappling with these double-whammy impacts.

Since 2010, regions like the Midwest have been impacted by numerous extreme drought and flooding events that have each exceeded $1 billion in losses. Right now, California is in the midst of a drought that researchers have found to be its worst in at least 1,200 years. And while not found to have been caused by climate change, scientists have determined the drought has been driven by record warm temperatures and reduced precipitation. These prolonged warm and dry conditions were then met with an incredible deluge of rainfall in some areas of the state last December (San Francisco received more rain in a matter of days than it did all of 2013) causing flooding and mudslides, washing out roads and damaging homes.

A Need for Action

A growing body of evidence shows strong connections between climate change and extreme events, and impacts once thought of as a distant future threat are already occurring and widespread.

As decision-makers scramble to keep trains and buses running among feet of snow, make plans to save infrastructure from flooding, and revise planning to contend with drought, it’s time they pause to acknowledge the role that human-induced climate change is playing in our changing weather—and commit to ambitious policy changes that reverse these trends.