A warming climate coupled with more intense El Niño and La Niña events could cause twice as many droughts and three times as many floods in California by 2080, according to a study published Wednesday in the journal Nature Communications.
The findings come while California suffers its most severe drought in recorded history, a four-year disaster that has caused an estimated $2.2 billion in economic loss from 2013-14 alone. At the same time, heavy rainfall––which triggered mudslides last week in Southern California––is anticipated through the winter from a strong El Niño event predicted by many climate forecasters.
The findings provide a more detailed understanding of how the region's climate will respond to global warming in the coming decades. Prior studies predicted a slight increase in rainfall for California over the 21st century. These studies, however, looked at mean rainfall over periods of a decade or more. They failed to take into account increasing variability from extreme El Niño and La Niña events, changes in surface water temperatures in the Pacific Ocean that affect rainfall patterns across the globe.
"Mean is one thing, but the changes, the extremes are really another thing that we need to pay attention to," said Jin-Ho Yoon, an earth systems scientist at Pacific Northwest National Laboratories and the study's lead author.
If, for example, the coming winter brings intense flooding, the combined drought and intense rainfall might average out to moderate rainfall in each of the last few years, but this creates a deceiving picture of reality.
Yoon and colleagues based their findings on historic observations and climate models for precipitation focusing on unusually dry and wet events from 1920 to 2080. Simulations for future decades were based on a worst-case scenario where greenhouse gas emissions continue unabated.
Extreme and rapid changes in precipitation like the current drought and the intense rain predicted for coming months will pose new challenges—with increasing frequency—according to the study.
One challenge is an increased potential for mudslides, Yoon said. During periods of drought, wildfires occur more frequently, killing vegetation that would otherwise help stabilize hillsides. When heavy rains follow prolonged drought, the chance of mudslides increases significantly.
"We are going to see more of this switching from one side to the other," Yoon said. "We need rain, but it may come down as torrential rain."
The study draws increasing attention to changing weather patterns as climate change increases the intensity of droughts and floods worldwide.
Ethiopia, for example, is suffering through its worst drought in more than a decade, a situation made more severe by El Niño. By next year, 15 million Ethiopians may face starvation if they don't receive food assistance, according to recent reports.
The increased intensity and frequency of droughts and flood, however, is not due solely to El Niño and La Niña events.
"As the climate warms, the changes in precipitation are complex," Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, said in an email.
Moisture in the atmosphere increases along with air temperature, resulting in more intense precipitation events and longer dry spells in between, Trenberth said. "Heavy rains deplete the atmospheric moisture and it takes time to recharge," he said. "The result is more widespread and intense droughts."
How El Niño and La Niña, the cyclical warming and cooling of the tropical eastern Pacific Ocean––also known as the El Niño Southern Oscillation (ENSO)––will behave in a changing climate remains a topic of debate.
"This has always been a real big question in the community––'what happens to ENSO over the 21st century?' and models are typically all over the place," said Matthew Newman, a climate scientist with the National Oceanic and Atmospheric Administration. "Not all models have that increase in ENSO variance."
Climate forecasters are now better able to predict El Niño events for the coming year but still struggle to ascertain just how strong near-term events will be.
"Now we are starting to ask what is the ENSO variability going to do in the next several decades based on [human activity], that gets to be even harder," Newman said.
Whatever the root cause, more extreme flood and drought events are already occurring, said Michael Hayes, director of the National Drought Mitigation Center in Lincoln, Neb.
"Sometimes the major crises that occur are not from individual events, but from events that compound one another," Hayes said. As an example, he cited the flooding of the Missouri river basin and the upper Mississippi river basin in 2011, followed by a drought in the same region in 2012.
"You had folks that were dealing with flood issues and folks dealing with drought issues," Hayes said. "The fact that they had to deal with them back to back put a strain on resources."
Policy makers need to start taking into account the increasing possibility of such events happening in close succession, Hayes said.
"The future is going to be different from the past and we need to be incorporating some of these future [changes] into our decision-making."