california 2100
October 22, 2015

More frequent, extreme floods and droughts forecasted for California in 2100

The date is 2100 in California and the “Terrible Twins” El Nino and La Nina, the Spanish names for boy and girl, are more unruly than ever. Their behavior is still unpredictable, but the end result is a doubling in the number of drought and flooding for the poor state of California.

A new study published in Nature Communications shows that the combined effects of El Nino and La Nina will cause more frequent extreme events. A better understanding of what gives rise to El Nino and La Nina cycles, collectively known as the El Nino-Southern Oscillation (ENSO), might help California to be better prepared for potential threats in the coming century.

ENSO is the danger

“Wet and dry years in California are linked to El Nino and La Nina. That relationship is getting stronger,” said atmospheric scientist Jin-Ho Yoon of the Department of Energy’s Pacific Northwest National Laboratory (PNNL). “Our study shows that ENSO will be exhibiting increasing control over California weather."

As California is coping with one of the most severe droughts in its history, climate scientists still don't know if a warmer world will make droughts worse, more frequent, or perhaps even improve the situation.

However, some research predicts future rain will be more in the form of light drizzles or heavy deluges rather than steady moderate rainfall. Yoon and colleagues from PNNL and Utah State University in Logan, Utah, wondered if droughts might follow a similar pattern.

So the researchers simulated two periods of time, 1920 to 2005 using historical measurements, and 2006 to 2080 using conditions in which little is done to reduce greenhouse gas emissions. This future scenario is in effect an examination of the most extreme case.

Not looking so hot for California

Two tactics were used to check how well the simulations worked, and the all-important reproducibility. In one tactic, they used a compilation of 38 different models, while in the other, they re-ran a single model 30 times. The more similar the results, the more sure the researchers would be of the model’s predictions.

If emissions continue to increase, the model predicts that California seasons will experience more excessively wet and excessively dry events, meaning droughts could double and floods could triple in frequency between the early 20th century and late 21st century.

“By 2100, we see more – and more extreme – events. Flooding and droughts will be more severe than they are currently,” said Yoon.

Yoon suspected the El Nino-Southern Oscillation was causing the predicted increases. Every two to seven years, El Nino comes in and warms up the tropical Pacific Ocean a few degrees, increasing winter rain and snowpack in California. On a similar schedule, La Nina cools things off. Both disrupt regular weather in many regions around the globe.

Yoon and colleagues ran a climate model with and without El Nino to explore its connection to California precipitation. In both simulations, they ramped up the concentration of carbon dioxide by 1 percent every year for 150 years. In just one of the runs, they removed El Nino's cyclical contribution by programming the sea surface temperatures to reflect only steady warming.

With El Nino and La Nina out of the model, the frequency of extreme precipitation in California stayed constant for the century and a half of the simulated period. When ENSO was factored in, California experienced wide swings in rainfall by the end of the period.

Even though the researchers expect rain and snowfall to increase as the climate warms, the way in which the precipitation affects California could be highly variable.

In a statement from the Pacific Northwest National Laboratory, the El Nino-Southern Oscillation is still a bit of a mystery. Scientists can only forecast El Nino and La Nina years by studying sea surface temperatures and other weather hints, and studies that investigate what controls them could help scientists predict unruly weather in the future.

-----

Feature Image: On the left, La Nina cools off the ocean surface (greens and blues) in the winter of 1988. On the right, El Nino warms up it up (oranges and reds) in the winter of 1997. Credit: Jin-Ho Yoon/PNNL