News•January 3, 2013

Global Warming-El Nino Link Stronger but Still Not Proven

The natural climate cycle known as the El Niño/Southern Oscillation (ENSO) would wreak havoc even if humans weren’t warming the planet. During its El Niño phase, the Americas get floods and torrential rains while Asia suffers drought. When it swings over to the La Niña phase, it’s the opposite. But humans are warming the planet, and a report published Thursday in Science says that both phases of ENSO have become more intense over the past century, suggesting that as warming continues, ENSO may become increasingly volatile.

Centered on the Pacific Ocean, these maps show patterns of sea surface temperature during El Niño and La Niña episodes.
Credit: Steve Albers/NOAA.

Knowing how ENSO reacts to global warming is crucial to understanding what might happen as the planet continues to warm, of course, but so far, climate models have disagreed on what the future of ENSO will look like. Some suggest the cycle will get more intense, some say less, and some project little change. This new study, which looks at changes in the chemical composition of Pacific corals caused by variations in temperature and rainfall, could help sort things out.

But it shouldn’t be taken as definitive proof of a cause-and-effect link between global warming and a more intense ENSO, cautioned lead author Kim Cobb, of the Georgia Institute of Technology, in an interview. “Our paper comes with caveats,” she said. “We see a roughly 20 percent increase in strength in the 20th century, but it could be a coincidence.”

In fact, when Cobb and her colleagues surveyed the relationship between ENSO and global temperatures going back roughly 7,000 years, they saw no clear sign that the swings of El Niño and La Niña went up or down in intensity intense during periods of natural climate change — despite earlier studies that showed otherwise.

Weekly averaged sea surface temperatures (°C) for the past twelve weeks.
Credit: Climate Prediction Center/NOAA.

Instead, they found that ENSO has been “noisy,” meaning it seems to vary in strength somewhat randomly, for no obvious reason at all. “It certainly looks very different from what we thought we knew,” Athanasios Koutavas, of the College of Staten Island, a scientist who recently published one of those earlier studies, said in an email. “This is going to force a healthy re-evaluation of things, and hopefully a push to replicate these results from more sites.”

This doesn’t mean Koutavas and other experts buy into the idea that the volatility of ENSO is unrelated to climate change. “I’m not sure whether I believe it or not,” said Columbia University’s Richard Seager in an interview. And while Koutavas praised the new data Cobb and her colleagues have gathered as “an impressive addition to what we had before,” he added that there are still large gaps to filled in.

“Most of the records are from a localized region in the Line Islands of the central Pacific, with a few more from Papua New Guinea in the west. Although these are strategic locations, they are a drop in the bucket given the size of the Pacific,” Koutavas said.

The new evidence also suffers from large gaps in time: the Science paper’s conclusions are based on just 17 sets of samples, each covering between 20 and 80 years, over the entire 7,000-year period. It’s a spot-check, not a comprehensive record — a point Cobb agrees with wholeheartedly.

The new samples have tripled the amount of data scientists have to work with, which, she said, “is revolutionary. But it’s still fairly sparse. If we can triple the number of samples again, we’ll be able to speak with more confidence. Collecting these kinds of data sets are clearly the way forward.”

Knowing how ENSO reacted to climate warming or cooling in the past — or, conversely, didn’t react — is crucial to making climate models more reliable. But even if global warming doesn’t have much effect on the intensity of ENSO, the precipitation and temperature swings that come with El Niños (which tend to raise global temperatures temporarily) and La Niñas (which tend to keep them in check), will come on top of what humans are doing to the climate.

In 1998, the planet set a temperature record, in part because a strong El Niño gave an extra kick to an already warming planet. This year, the U.S. has experienced its warmest year on record even though there was no El Niño in sight. “Once you superimpose this natural [ENSO] cycle on greenhouse warming,” Cobb said, “things just get worse.