Has Desertification Begun In The American Southwest?

A warning buoy sits on a dry bed in Lake Mendocino, a large resevoir near Ukiah, California.

PHOTOGRAPH BY RICH PEDRONCELLI, AP

Climate Forecast: More Southwest Droughts

Global warming will drive La Niña to greater extremes, a new study says—and El Niño too.

Warren Cornwall

for National Geographic

PUBLISHED JANUARY 26, 2015

People living around the Pacific Ocean, including in parts of Asia, Australia, and western North and South America, should expect wilder climate swings in the 21st century.

"The results are actually very, very convincing, and terrifying in a way because we know the impact can be dramatic," said Wenju Cai, a climate scientist who was the lead author of two recent papers about the research, the second of which was released Monday.Extreme versions of El Niño and La Niña, the sibling Pacific weather patterns that can translate into torrential rains or searing droughts, will likely occur nearly twice as often—approximately once every decade—if greenhouse gases continue increasing on their current trajectory, an international team of scientists has concluded.

If the predictions prove true, it could mean tens of thousands more weather-related deaths and devastating economic damages.

The paper published Monday reveals the La Niña side of the equation and appears in the journal Nature Climate Change.

Using computer models that simulate how increasing greenhouse gases alter ocean and land temperatures and wind patterns, the study finds that particularly intense La Niñas will occur approximately every 13 years in this century, compared with every 23 years in the past one. And those La Niñas will follow more frequently on the heels of a severe El Niño—which according to the earlier study is to be expected every 10 years rather than every 20.

During the most recent extreme La Niña, in the late 1990s, the southwestern United States endured a severe drought, while more than half of Bangladesh was underwater and flooding in China killed thousands and displaced over 200 million people. The preceding El Niño is blamed for weather that did more than $33 billion in damage and claimed 23,000 lives worldwide. (Related: "El Niño/La Niña: Nature's Vicious Cycle.")

Focusing on Extremes

Just five years ago, there was little clarity about how climate change would alter what is known as the El Niño/Southern Oscillation (ENSO), said Cai, a climate scientist based at Australia's Commonwealth Scientific and Industrial Research Organisation.

Computer models offered conflicting answers. Cai and his colleagues brought greater computing power to bear on the problem, and they simplified it by focusing on the most extreme and damaging versions of the climate shifts.

In an El Niño, the trade winds that blow west along the Equator weaken, and warm water that's normally pushed by the winds into the western Pacific comes sloshing back east. The warm, moist air and thunderstorms that collect over that warm pool, typically around Indonesia, follow it east. That takes rain away from parts of Asia and Australia and brings more of it to the Americas.

During La Niña, by contrast, the trade winds intensify, driving warm water and wet weather even farther west toward Asia, and causing cooler, drier conditions from the central Pacific east. The American Southwest gets hit by drought.

What happens to this cycle as greenhouse gases warm the whole planet? Cai and his colleagues used 21 different computer models to find out. The key seems to be that different regions warm at different rates—in such a way as to shift the cycle into overdrive.

Nearly all 21 models showed that in the future, the Asian land surface would warm faster than the surface waters of the central Pacific Ocean. In a La Niña year, when the central Pacific is already cooler than normal, the increased temperature difference could fuel stronger winds blowing toward Asia, and the winds would push warmer surface waters with them.That would cause even more cold water to rise to the surface in the central Pacific, further reinforcing the temperature difference and feeding still stronger winds.

In other words, global warming increases the likelihood that a La Niña will turn nasty.

Likewise it's expected to warm the eastern Pacific near the Americas more than it does the central Pacific. During an El Niño year, that will diminish the temperature difference between the two regions, weakening the trade winds still further—and making the El Niño more extreme.

As a result of global warming, the new study suggests, the ENSO cycle will have a greater tendency to swing between extremes, like a seesaw with heavyweights at each end. As an intense El Niño burns itself out, it helps set the stage for a strong La Niña, said Cai. The study found that 75 percent of the increase in La Niñas comes in years just after extreme El Niños. That means the regions bordering the Pacific are likely to see more cases of climate whiplash, with flood seasons following droughts and vice versa.

Mysteries Persist

Of course, the studies by Cai and his colleagues won't be the last word on the subject. Just how much remains to be learned about the ENSO cycle has been evident in the past year: Predictions of a blockbuster El Niño made last spring have so far failed to materialize.

The Pacific trade winds pose a challenge for climate models, said Antonietta Capotondi, an oceanographer at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colorado. Most models suggest that on average, independent of La Niña and El Niño, the winds should be losing force as the climate warms—but recent observations indicate that the winds are actually strengthening.

Still, most of the 21 models used by Cai and his colleagues agree in forecasting greater swings for the ENSO cycle. That makes the conclusion credible, said Capotondi, who wasn't involved in the new study but wrote an article analyzing it. "The analysis they have done is probably as accurate as one can do," she said.