A new study warns that global trends are moving California toward more frequent extreme rain events.

By Bob Berwyn

The Geostationary Operational Environmental Satellite captures an image of the famous Pineapple Express on December 19th, 2010. (Photo: NASA Goddard Space Flight Center)

After a five-year drought, parts of California are now facing a deluge of rain, mudslides, and car-crushing snowfall. An atmospheric river — also known as the Pineapple Express — is shunting moist Pacific air from near Hawaii to the slopes of the Sierra Nevada. While the surge of moisture is mostly welcome to boost snowpack and reservoir storage, California will likely experience similar extreme events much more frequently if global greenhouse gas emissions aren’t cut to near zero — and soon.

Right now, California sees about six extreme atmospheric river rain storms per winter season, but if greenhouse gases keep increasing, global temperatures will rise by 4 degrees Celsius by 2100. That means the number of extreme rain events will increase to nine per winter, a team of Massachusetts Institute of Technology researchers concluded in a study published this week in the Journal of Climate.

“This is actionable information,” says Adam Schlosser, senior research scientist in MIT’s Joint Program on the Science and Policy of Global Change; Schlosser explains that previous studies had suggested there were equal odds of atmospheric rivers increasing or decreasing. That made it hard for resource planners to choose an adaptation path. “In our method, all the results point toward an increasing trend,” Schlosser says.

As a benchmark, the researchers used a December 11th, 2014, storm that dropped three inches of rain in the San Francisco area in just one hour. It was a classic “Pineapple Express” that resulted in mudslides, floods, and power outages across the state, and was acknowledged as one of the state’s most extreme precipitation events in recent history.

With their new model, the scientists identified large-scale patterns in atmospheric data that helps predict the frequency of such local, extreme rainfall events with greater confidence than other previous techniques.

Schlosser explains the modeling with a simple analogy: “Think about the weather report you see every night. What we do is, we don’t focus on where the television meteorologist shows where it’s going to rain and snow. We look at the high pressure areas and the lows, the fronts, and study how they match when extreme precipitation events happen.”

A new climate regime swinging between extreme drought and extreme rainfall will require new approaches in planning.

Existing climate models do well at predicting large-scale atmospheric shifts. By analyzing the connection between those shifts and atmospheric rivers, the study is able to show “how frequently these events are occurring now, and how they will change locally, like in New England, or the West Coast,” Schlosser says.

The findings are important in the context of other climate change projections, including higher odds of extended droughts and heat waves in California, because the impacts of extreme rainfall include a heightened risk of massive mud slides in areas scarred by wildfires. A new climate regime swinging between extreme drought and extreme rainfall will require new approaches in planning.

“It’s a really bad combination of two extremes. The drought dries, and, in some sense, cooks up the ground. It becomes more susceptible to heavy rain. You’re putting together a meta-event that could be quite destructive,” he says.

“We already applied this technology for heatwaves and other extreme events,” says MIT researcher Xiang Gao, who led the study. Work is under way to use the modeling to identify the vulnerabilities of electric grid infrastructure to global warming.

European scientists are also interested in the study because it will help them examine similar climate change impacts, says Monica Ionita-Scholz of theAlfred Wegener Institute for Polar and Marine Research. This week, coastal northern Germany was hit by the strongest storm surge in 10 years.*

“In Europe, there is definitely an increase in extreme rain events. The more you increase temperature, the more water you’ll have in the atmosphere. In some cases there will be less average precipitation, but when you have it, it will be very extreme,” Ionita-Scholz says. “In terms of social impacts and economic costs, it will be quite dramatic.”

“That is a very interesting study and somewhat similar to what we are planning to do with heavy rainfall events in Germany,” PIK climate scientist Jascha Lehmann says. “In our paper we show that, in Europe, the number of record-breaking rainfall events has increased by 31 percent over the time period 1981–2010 compared to a climate with no long-term warming. On the global scale we find a surplus of 12 percent more record-breaking rainfall.” Another study shows strong increases in short-lived heavy rainfall over Germany in response to temperature increases, he adds.

The MIT research is also of interest in the United Kingdom, where atmospheric river rainstorms caused $1.6 billion in damages in 2012. A study published in Environmental Research Letters in 2013 concluded that atmospheric river events will strengthen and could double in number by the end of the century if global warming continues unchecked, “bringing a greater risk of higher rainfall totals and therefore larger winter floods in Britain.”

For California, the findings may be a mixed bag, Schlosser says. “The occurrences of those storms are make-or-break for the water year and for storage, but there can be too much of a good thing.”

*Update —January 6, 2017: This post has been updated to reflect Monica Ionita-Scholz’s professional affiliation with the Wegener Institute.