El Niño: Unaffected by climate change in the 21st century but its impacts may be more severe

While climate change will not modify the extent or frequency of El Niño variability in the next 100 years, the environmental consequences of such events may become more extreme, according to a new collaborative study between scientists at the Cooperative Institute for Research in Environmental Sciences (CIRES) and the National Center for Atmospheric Research (NCAR).

"Based on the latest state-of-the-art model, it does not appear that the warm water/cold water anomaly in the Pacific—known as El Niño and La Niña—is changing," said study coauthor Baylor Fox-Kemper, a CIRES Fellow and assistant professor at the Department of Atmospheric and Oceanic Sciences at the University of Colorado Boulder. "But due to a warmer and moister atmosphere the impacts of El Niño are changing even though El Niño itself doesn't change."

El Niño events—anomalous warming of the surface water of the eastern and central Pacific Ocean, occurring every four to 12 years—typically coincide with atmospheric changes like reduced trade winds and a displaced jet stream, which can cause unusual weather patterns such as flooding or droughts. These weather patterns can have dire consequences: "Tens of billions of dollars are associated with big El Niño events or La Niña events," Fox-Kemper said.

Advance knowledge of variations in El Niño behavior would allow a community to better prepare, for example, by altering planting seasons or water usage, Fox-Kemper said. "We would like to ask questions such as whether the flooding that occurred in Australia will happen more or less often over the next 100 years," he said. "If one of the impacts of climate change is a changing El Niño, we would like to know as soon as possible so we could start planning."

To determine whether El Niño may become stronger or more frequent in a warming climate, and whether the impacts of El Niño may change, the researchers used the most recent and advanced climate model available from NCAR—the latest version of the Community Climate System Model that scientists are using for the experiments that will inform the next Intergovernmental Panel on Climate Change (IPCC). They compared the model reproductions of variability in ocean surface temperature in the 20th century with model simulations that extended into the 21st century, and found that the changes were not significant. The study was published online in September in the Journal of Climate.

The impacts of El Niño, however, were affected by a warming climate, Fox-Kemper said. "What we see is that certain atmospheric patterns, such as the blocking high pressure south of Alaska typical of La Niña winters, strengthen in the model in the future climate as compared to the 20th century," he said. "So, the cooling of North America expected in a La Niña winter would be stronger in future climates."

Moreover, Fox-Kemper cautions that although El Niño does not appear to be changing in the short-term, it may change in the future under the influence of current climatic warming. Because the oceans heat up slowly and ocean currents move slowly there is a subsequent time lag before the tropics warm up. This means scientists will have to wait longer to see if there are any changes to El Niño with a changing climate, he said. "Even 100 years from now, ocean warming is still working its way through the system," he said. "This study isn't saying there isn't going to be a change to El Niño—it is just that the adjustment process is still happening."

The National Aeronautics and Space Administration, National Science Foundation, the US Department of Energy and the National Energy Research Scientific Computing Center provided funding for the study "Will there be a significant change to El Niño in the 21st century?" Lead author of the study is CIRES scientist Samantha Stevenson and coauthors on the study include NCAR researchers Markus Jochum, Richard Neale, Clara Deser and Gerald Meehl.

Baylor Fox-Kemper, CIRES, 303-492-0532, bfk@colorado.edu
Jane Palmer, CIRES, 303-492-6289, jane.palmer@colorado.edu