Last updated December 4, 2018

Atmospheric River Change

Recent observations reveal a rising trend in land-falling atmospheric rivers on the West Coast consistent with climate warming. Looking forward, climate models project that the intensity and duration of atmospheric river storms increase in a warming climate, with the most intense of these storms becoming more frequent.

Toplines connecting changes in atmospheric river storms to global warming

  • A warmer atmosphere drives more extreme precipitation across all storm types, which in turn increases the the risk of flooding
  • Atmospheric river events have been identified as the primary cause of flooding in the western United States
  • Research shows a rising trend, associated with global warming, in atmospheric river events on North America’s West Coast[1]
  • Atmospheric river storms are projected to increase in intensity and duration in California in a warming climate, with the most intense atmospheric river storms becoming more frequent
  • Overall conditions in California become more polarized in a warming climate, shifting toward both more drought and more flood, often alternating
  • The fingerprint of global warming has already been found in one recent atmospheric river storm that broke meteorological and hydrological records in the British-Irish Isles[2]

What are atmospheric river storms?

Atmospheric rivers are wide paths of moisture in the atmosphere composed of condensed water vapor. They bring extreme precipitation to land areas they pass over. They occur globally but are especially significant on the West Coast of the United States, where they create 30 percent to 50 percent of annual precipitation and are linked to water supply and problems such as flooding and mudslides.

How does global warming affect these storms?

Warming is linked to increases in precipitation extremes partly because of the physical effect of warming on atmospheric conditions. Warmer air has the capacity to hold more moisture (for each 1°C of warming, saturated air contains 7 percent more water vapor).[3] Therefore, a given volume of warmer air has the capacity to drop greater amounts of rain and snowfall than the same volume of cooler air.[4] Warmer air also provides greater latent energy to drive storms.[4]

Climate change increases the risk of extreme precipitation and flooding

Extreme precipitation is increasing worldwide as the warming atmosphere is holding and dumping more water when it rains. Across the United States, observational data shows an increase in the intensity and frequency of extreme precipitation events.[5] 

Recent work has revealed a critical role for atmospheric rivers (ARs) in driving extreme precipitation events.[6][7][8][9] These storms drive “high‐impact hydrologic events” (HIHEs) including floods, flash floods, and debris flows.[10] In the western United States, atmospheric river events have been identified as the primary cause of flooding.[11]

In California, ARs deliver up to one-half of the state's entire annual precipitation over the course of only 10 to 15 days.[12] Between 1996 and 2007, all seven declared floods on California's Russian River were linked to atmospheric rivers.[8] A longer term analysis showed that of 39 declared floods on the Russian River since 1948, 87 percent were caused by atmospheric rivers.[13] A July 2015 study found that climate change may increase horizontal water vapor transport by up to 40 percent in the North Pacific, due mainly to increases in air moisture.[14]

Land-falling ARs are on the rise in the western US

An August 2017 study spanning seven-decades of data revealed a rising trend in land-falling atmospheric rivers consistent with a long-term warming of the North Pacific, which sends more water vapor to North America.[1]

High-resolution climate models report that atmospheric storms linger and become more intense in a warming climate. In addition, the models project that these storms will land more frequently in Southern California.