Ecocentric

Weather: What’s Responsible for Devastating Floods? Blame La Nina

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As terrible floods rage simultaneously around the world—inundating half of Australia, killing more than 50 people in the Philippines and hundreds in Brazil—it’s natural to want a scapegoat. Well, you can blame the “little girl”—the weather phenomenon known as La Nina, which many meteorologists are blaming for the unusually heavy rainfall that’s led to these devastating floods.

A little weather background first. La Nina is the extreme phase of a naturally occurring climate cycle called the El Nino/Southern Oscillation, with El Nino periods themselves on the other end of that cycle. The cycle is governed, like so much else on the planet, by the sea—in this case, large-scale changes in the sea-surface temperature in the eastern tropical Pacific. Normally the sea-surface temperatures in that region fall into the 60s to 70s F, with warm pools that can rise above 80 F in the central and western Pacific. In El Nino years, those warm pools expand across much of the tropics, but during La Nina years the opposite occurs, and an upwelling brings cold water to the surface that can lower temperatures by as much as 7 F.

For both El Nino and La Nina, abnormal changes to sea-surface temperatures in turn alter global weather patterns, changing both air temperatures and precipitation. El Nino often leads to drought and unusually hot weather in parts of the world, but La Nina reverses that effect, leading to more clouds and wetter weather in places like Australia and Indonesia. The last time the Australian city of Brisbane flooded was in 1974—the same year as a particularly strong La Nina episode. The flooding in Australia, Sri Lanka and the Philippines all bear the hallmarks of La Nina—though scientists say the heavy rains in Brazil are an aberration, since the southwestern part of the country currently suffering from floods usually experiences below-average rainfall during La Nina.

The bad news for Australia and all the other countries fending off flooding is that the current La Nina is one of the strongest on record, according to NASA—and that it’s not likely to end any time soon. La Nina events can usually last a year or longer, with the entire El Nino/Southern Oscillation cycle lasting three to four years. The official word from Australia’s Bureau of Meteorology is that the current La Nina is just peaking, and that it will persist into the Southern Hemisphere’s autumn over the next two months, which could mean more rain and more flooding.

Worse, some Australian climatologists worry that this La Nina may not be the end:

Hydroclimatologist Associate Professor Stewart Franks from the University of Newcastle believes the phenomena occur in clusters and we could be in for a series of La Ninas that could continue to drench the eastern regions of Australia for years to come.

“That’s actually misleading,” he said about the estimates about the frequency of La Nina and El Nino.

“Our research has shown that they tend to cluster. For instance, in the first half of the 20th century, we had very frequent El Nino events, with very few La Nina. For eastern Australia, that was a time of drought.”

At the same time, however, climatologists say that it’s impossible to blame a specific weather event on the La Nina effect—just as it’s still impossible to say a hurricane or heat wave is definitely cause by global warming. But climate change “loads the dice” for certain kinds of weather, making extreme events more likely on average, so does La Nina change the norm for much of the world. It doesn’t cause a biblical flood like the ones that has ravaged Australia’s Queensland state, but it can make it more likely—just as smoking and high cholesterol levels would make a heart attack more likely.

Of course, as I’ve written before, it’s human action—or inaction—that transforms an extreme weather event into a disaster. This is the worse flooding Australia has seen in over 30 years, but the death toll is still around 20 people. That’s 20 too many, but Australia’s wealth and its experience of floods has helped its government and its people respond to the catastrophe in a way that has kept the human damage at a relative minimum. Compare that to Brazil, where less rain has likely fallen but where hundreds of people have died—in part because the country isn’t as accustomed to floods and hasn’t been prepared. Just as Haiti was unready for a strong earthquake—with horrific results—Brazil was caught off guard by the storms.

Unlike man-made climate change, the La Nina/El Nino cycle is a natural one, and there’s little we can do to change that—although some scientists worry that warmer temperatures cause that cycle to become even more extreme. We can protect ourselves with better monitoring that would allow forecasters to track and predict coming El Ninos and La Ninas with greater precision—it doesn’t help much to know after the fact. The ocean, unfortunately, is many ways still a mystery to us—though a proper global ocean monitoring system would help a lot. We’re victims of the weather—and we’re greenhouse gas emissions, we’re also causing it. The least we can do is get ready.

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