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Understanding extreme weather in an era of climate change

Scientists try to ID climate signals in weather as public draws conclusions.

Understanding extreme weather in an era of climate change

The US has clearly seen some pretty extreme weather events over the last year. These events have caused both billions of dollars in property damage and endless arguments over how much can be attributed to climate change. Even as scientists work on the problem of attribution, the public has often made up its mind on what's to blame.

To try to bring some sanity to the discussion, the meeting of the American Association for the Advancement of Science hosted a session on US weather extremes. Although there were a variety of talks, three presentations nicely captured the challenges: one on the state of the US climate, another on a recent climate event, and a third on trying to convey all of this to the public.

Turning up the heat

The first speaker was Donald Wuebbles of the University of Illinois. He started out by saying that you can view the climate as a bell curve, with extreme heat and cold events occurring where it starts to flatten out to the left and right. In that view, changing the climate could do any of three things. The curve itself could shift, with hot events becoming more common and cold events becoming less frequent. You could also potentially flatten the curve, with the typical climate remaining roughly the same but the instances of extreme events increased. Or, he said, you could do both.

Depending on which aspect of the climate system you're looking at, you may get any one of these options.

For temperatures, both globally and in the US, the curve has shifted: there are far more extreme hot events than there are cold ones, a trend that has been apparent for decades. In the middle of the last century, the US was setting roughly equal numbers of hot and cold records; by the decade of the 2000s, record heat was being recorded twice as often as record cold. In 2011, the ratio was 3:1, and last year it hit a staggering 10:1, suggesting the trend shows no sign of abating.

As far as rain, the US is seeing more overall, but it's mostly falling in the Northeast and Midwest, and in the form of unusually heavy rains. The heaviest one percent of events are up everywhere, but the Northeast is seeing 74 percent more of them compared to the 1950s. The Midwest has seen a 45 percent increase, and that's been enough to make the typical one-in-twenty year event into a one-in-thirteen. The West and Southwest, meanwhile, are seeing more droughts, although the extent of this change depends on the measurement standard used to define drought.

In terms of the most dramatic weather events, hurricane intensity is going up, although the numbers have varied and the probability of their making landfall is influenced by the climate in complex ways. Huebbles said that other dramatic weather events, like tornadoes and hail storms, have a complex relationship to climate, and our understanding of this relationship is limited.

Heubbles also mentioned a variety of events that have been specifically attributed to climate change, including the recent drought in Texas, which analysts have suggested was made anywhere between two and twenty times more likely by climate change. This fact served as a great bridge to the next speaker, John Nielsen-Gammon, a Texas state climatologist.

Dry in Texas

Nielsen-Gammon set the stage for the drought talk by noting that a few tropical storms had passed over Texas in the late summer of 2010, leaving most of the state wetter than normal. But the last storm hit on September 27, and three months later eastern parts of the state entered a drought. They were then hit by the windiest spring on record, and by June of 2011 they had run out of colors to represent the depth of the drought.

With few clouds to reflect sunlight and no water to remove heat through evaporation, the state baked, with the mean temperatures rising by 3°F. "In Texas, we call that an outlier," Nielsen-Gammon joked.

Some areas of the state went over 100 days with temperatures reaching the triple digits. All of which made the wildfires inevitable. At their peak, they covered an area that would match the size of Connecticut and Rhode Island, "and we had to throw in New York City as well." All told, the drought and fires racked up over $7.6 billion in agricultural costs.

"Saying the drought was due to climate change is like saying the airplane crashed due to gravity," Nielsen-Gammon said. It's true, but it doesn't tell you much. So, his team used climate models to try to understand the causes of the drought. One of them was La Niña, the cold phase of the tropical Pacific Ocean, which has historically been associated with reduced rainfall in Texas and neighboring states. On its own, La Niña clearly doesn't produce events like the extreme drought of 2011, so they began testing other factors.

Rather than simply stipulating La Niña conditions, the team put the actual sea surface temperatures into their model. While the event was still unusual, it was no longer an extreme outlier within the set of results produced by the models. Adding in the warming that Texas has experienced since 1995, events like 2011 "become even easier to see."

But it was still clear that La Niña was a key enabler. What does that mean for the future? As Nielsen-Gammon put it, "All the reviews agree: La Niña may get stronger, or maybe not. La Niña may get more frequent, or maybe not." In other words, we don't know how the Pacific will respond to our planet's increasing temperatures, and the ENSO cycle that swings between El Niño and La Niña is complex and variable, making the identification of long-term trends very challenging. Right now, we simply don't have enough data to know.

Managing complexity

If Nielsen-Gammon's job was complicated by the mix of natural variation and climate-driven events, then Andrew Freedman has an even greater challenge: accurately conveying it to the public. He's on the staff of Climate Central, an organization that is funded by everyone from the Nature Conservancy to the Army Corps of Engineers, and he is tasked with getting accurate climate information out through a variety of outlets.

Freedman said that the public can no longer escape paying attention to climate change, because the distinction we used to make between weather and climate change has vanished. In the last year alone, lots of events—the sea ice record, the warmest year in the lower 48, the warmest La Niña year on record, Australia's need for a new color to register the record heat on its temperature maps—all made the news, often as part of the weather reporting.

Even if scientists and reporters are cautious about how they describe the events, people are not. People have a fundamental drive to try to understand the world around them (Freedman quoted climatologist Gavin Schmidt as saying, "people are actively trying to interpret their world"), and in Freedman's words, "people are connecting dots ahead of media and scientists." In some cases, that means they'll see patterns that don't actually exist.

For example, the warming climate has driven sea level rises that will ensure that normal events have more significant consequences, as New York City experienced with Sandy. But on the other side of the globe, the Thai floods that set back the hard drive industry were largely caused by poor water management upstream. And Freedman noted the big blizzard that had just hit Boston the week before the meeting was a classic northeaster, which is not an exceptional event.

To try to get accurate information to the public, Climate Central is working with TV meteorologists, who already reach the public but don't always have the tools to handle the complexities well. And the emphasis is on getting them to ask better questions—instead of "did global warming cause this?", they should focus on questions that are more relevant and can be answered, like "how did it [climate change] influence the odds for or severity of this event?"

This certainly won't guarantee that we'll end up with a public that can easily grasp the complexities of climate. But it could help them to start recognizing that the complexities exist.

Channel Ars Technica