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Scientists analyze recent extreme weather events in relation to climate change

August 17, 2016 at 6:20 PM EDT
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HARI SREENIVASAN: Two major disasters in two different parts of the country have sent tens of thousands of people fleeing from their homes, and caused millions of dollars in damage. Are these just freak events, or are they in some way related to climate change?

William Brangham brings us the latest.

WILLIAM BRANGHAM: It’s called the Blue Cut Fire, and it’s wreaking havoc in Southern California. The massive blaze closed major roadways like part of Interstate-15 that connects Los Angeles and Las Vegas. And last night, officials issued evacuation orders for more than 34,000 homes. That’s some 82,000 people.

WOMAN: I think this is the worst that I have ever seen, you know? And it’s kind of getting used to the idea of being homeless.

WILLIAM BRANGHAM: The fire erupted yesterday in the Cajon Pass, a critical corridor just north of San Bernardino, only 60 miles from Los Angeles.

It quickly expanded to more than 45 square miles. Ten air tankers, 15 helicopters and some 1,300 firefighters were deployed within 24 hours. They faced hot and windy conditions.

MICHAEL WAKOSKI, Southern California Incident Management Team: The fuels are extremely dry and very explosive this time of year. And in my 40 years of fighting fire, I have never seen fire behavior so extreme as it was yesterday.

MARK HARTWIG, Chief, San Bernardino County Fire and Rescue: I was able to get up this morning and get some eyes on it from the air. In a word, it was devastating, a lot of homes lost yesterday. There’ll be a lot of families that come home to nothing.

WILLIAM BRANGHAM: Governor Jerry Brown declared a state of emergency for the Blue Cut area, as he did earlier this week for a major blaze north of San Francisco. That fire has since started to fade. The man suspected of sparking it, as well as 16 smaller fires over the last year, has now been charged with arson.

Seventeen hundred miles across the country, a different kind of disaster is unfolding in Louisiana, where some of the worst floods in history have hit the state. As the water begins to recede in some parts, the numbers are stark. At least 11 people have died, 30,000 people have been rescued, and 40,000 homes damaged.

MAN: We lost everything, God, just about. We got out safely and all of our friends are safe, so that’s the main thing.

WILLIAM BRANGHAM: So far, about 68,000 people have signed up for assistance from the Federal Emergency Management Agency. The American Red Cross says this flooding has triggered its largest disaster operation since Superstorm Sandy in 2012.

The catastrophes in California and Louisiana have again raised the question: Are these events caused in part by global climate change? Both events follow a July that was the planet’s single hottest month since records began in the 19th century, and many computer models have indicated that, as temperatures rise, droughts and extreme weather are likely to follow.

To help us sort out what’s driving these extreme events, we turn to two scientists well-versed in these matters. Barry Keim is a climatologist for the state of Louisiana and a professor at Louisiana State University, and Adam Sobel is a professor of environmental science at Columbia University. He also directs its Initiative on Extreme Weather and Climate.

Thank you both for being here.

Barry, I would like to start with you.

Before we get into the science of all of this, can you just tell us, how are things in Baton Rouge right now?

BARRY KEIM, Louisiana State University: Well, in Baton Rouge, they are improving, but across the broader area, there still are some issues.

Things are slowly starting to settle down or simmer down a little bit. But, right now, across most of my immediate region, people have already started ripping out carpets and gutting houses. So, we’re already starting the cleanup.

WILLIAM BRANGHAM: So, this was obviously a historic amount of rainfall that fell on your state and that flooded you all out. Can you give us a sense of the context of how serious these storms were, and should we be thinking about these in terms of climate change?

BARRY KEIM: This event was — it was a tropical disturbance. It was a very weak — in fact, it — I would classify it practically as an easterly wave.

And the amount of rainfall amounts, as I suggested, were staggering. Just to put this in perspective, a 100-year rain event in this region is roughly 14 inches. A 1,000-year event is about 21 inches.

Well, we have about eight sites, seven or eight sites across southeast Louisiana or south central Louisiana that exceeded the 1,000-year mark. And, in fact, we have one site that had 30 — over 31 inches of rain.

Now, 1,000-year event is 21 inches, and this was 31 inches. So, that ought to give you some context on just how severe this rainstorm was across this area.

WILLIAM BRANGHAM: In that context, do you think of this as something that is — what we would expect to see with climate change, or is that not part of your thinking?

BARRY KEIM: Well, no single event really tells you anything about climate change specifically, and even one year’s worth of events.

And, admittedly, this past year has been very strange. We had the floods in South Carolina. Texas had some serious floods. We even had a major flood earlier this year in Louisiana. In fact, we even had an early one back in this past October which was quite extraordinary.

So, it does seem like there is something unusual going on. But I still wouldn’t quite characterize this as being a sign that the climate has officially changed. We are just in a very unusual period right now. And this could be indicative of climate change, but it’s way, way too early. And we need a lot more data and understanding of the science to be able to say something conclusively like that.

WILLIAM BRANGHAM: Adam Sobel, I would put the same question to you.

In California, we have seen this historic drought. It has dried out the landscape. It is partly why we have seen these explosive fires across the state there. But, again droughts are not unusual for California, the same way floods are not unusual for Louisiana. How do you see these in relation to climate change?

ADAM SOBEL, Columbia University: Well, I think you have to talk about each event separately.

So let’s start with the Louisiana event. I think the California situation is a little bit different and has been going on for a number of years. So, there have actually been a few studies specifically on that event.

But since we’re talking about Louisiana and the floods now, just for some general context, I mean, across the United States, as well as most places in the world where there is adequate data to ask the question, we have seen increases over the last decades, or in some cases the last century, in the extremity of severe rain events.

So, we see heavy rain events becoming heavier and a larger fraction of the rain falling at heavy events. And this is what our climate projections tell us should happen as the climate warms, although to uncertain degrees, but it’s qualitatively consistent. And we expect it on physical grounds, as a warming climate means that there is more water vapor in the atmosphere.

Now, it’s true that each individual event is a result of many factors, and climate change is at most one of them, and usually a small one. So a lot of things have to happen for an extreme event to occur. Otherwise, it wouldn’t be an extreme event. And so climate change can at most push the odds a little bit in one direction.

So, I mean, we now have a new emerging area of science called extreme event attribution, where people actually do studies and have ways of making probabilistic statements. You can’t ever say this event was caused by climate change or this single event is conclusive evidence of climate change.

But you can say that climate change appears to have made it such a percent more probable, or given that it occurred, make it such and such percent more extreme.

Those studies haven’t yet been done on the Louisiana event. My guess is that they will be done pretty soon. They can get done pretty quickly now. And I would cautiously predict that they are going to show some increase in the likelihood of this happening due to climate change, although I wouldn’t want to put a number on it.

So, that is the sort of broad context. I think it is the kind of thing we expect to see more as the future proceeds. And it’s the kind of thing we have been seeing. But I agree that to make stronger statements than that about a specific event, it is certainly possible to overstate the connection. But there is a lot of science behind the statement that there may be some connection there.

WILLIAM BRANGHAM: Barry, back to you.

I’m just curious, do you think that this is what we have just got to become accustomed to, that what was once considered an extreme event is now just the new normal? And, if so, what does that mean for policy-makers?

BARRY KEIM: Yes, well, we’re certainly in a period right now where we have seen quite a bit — quite a few extremes.

And — but we have had some periods in the past where we have had clustering of really big, catastrophic rainfall events in the past as well. But I tell you, the last decade or so really does raise some eyebrows, and very, very suspicious.

And as what this means for policy implications, I mean, this is just so complex, so politically loaded, that, I mean, it’s really tough to navigate through these waters, so to speak. And so, yes, it is really hard to say a whole lot conclusively on what kind of policy should proceed as a result of these recent extremes.

WILLIAM BRANGHAM: Adam, with relation to the fire in California and the many people appointed to the very long drought that has dried out much of California and makes these fires more common and more violent when they actually occur, how do you see climate change in relation to that particular event?

ADAM SOBEL: Right.

So, if we look at the drought in California, there have actually been several studies of it, some of it done by my colleagues here at Columbia. And those studies actually show the complexity of studying events like this, because the answer actually depends how you ask the question.

If you ask about just the lack of rainfall — of course, the cause of a drought is just lack of rainfall — then the studies have tended to come to a conclusion that it’s not — this event is not fundamentally caused by climate change.

That said, once you have a lack of rainfall, the hotter temperatures that we see as a result of human influence on climate cause water to evaporate from the soils more quickly, and so you have overall a dryer land surface and lower reservoirs and all of that.

And that, there, there is a climate change influence. So if you ask about the lack of rainfall, this event appears to be largely natural, is what the studies have shown. But if you ask about the overall dryness of California, there does seem to be some influence of higher temperature, which is a result of climate change.

So, the studies have come to different conclusions, in part because they ask different questions. But I think that is the complicated answer to that question, as far as we know now.

WILLIAM BRANGHAM: All right, Adam Sobel, Barry Keim, thank you both very much for being here.

ADAM SOBEL: Thank you.

BARRY KEIM: My pleasure. Thank you.

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