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What’s Causing Unusually Hot Temperatures in U.S.?

July 2, 2012 at 12:00 AM EST
Lack of water, "the great air conditioner", is causing unusually high temperatures and extreme weather events in the United States, Kevin Trenberth with the National Center for Atmospheric Research tells Judy Woodruff.


JUDY WOODRUFF: We take a closer look now at the connection between these weather events and changes in the environment.

Kevin Trenberth is a senior scientist with the federal government’s National Center for Atmospheric Research.

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Mr. Trenberth, thank you very much for being with us.

Let’s start with these wildfires. Are they worse this year than usual?

KEVIN TRENBERTH, National Center for Atmospheric Research: This year has certainly gotten off to a very bad start. We always have some wildfires in patches. But we don’t expect to see them until much later in the season.

And, of course, this relates to how dry it has been and exceptionally dry in all of the Southwestern parts of the United States.

JUDY WOODRUFF: And the dryness, how unusual is that? How does that compare to a normal, a typical year?

KEVIN TRENBERTH: Well, certainly, right in the area where we are, in the Boulder-Denver area, there’s about a third of the normal annual precipitation up until now that has occurred.

But in the mountains, there’s no snow left. Normally, you don’t see any snow left until late August, if then. And by early June, there was no snow left. Snow is a reflector. When it melts, it provides water. It cools the atmosphere. And water is the great air conditioner.

So the absence of all of the water and the snow in the Southwest has meant all of the heat from the sun is going into raising temperatures. This is the building of the heat wave.

And this has been building throughout this year, from the relatively mild winter to the June temperatures in March this year. And now we get to June, the peak of the — getting to the peak of the summer. And there is no parallel to this. We’re now breaking records.

And the heat then begins to — begins to move eastward. It picks up moisture out of the Gulf. The humidity goes up to enormous amounts because the oceans have a strong memory of the climate change being warmer and moister over the oceans. And that empowers all of the weather systems, such as the ones that came through, the thunderstorm clusters that came through and did all of the damage yesterday.

JUDY WOODRUFF: And when you say — you used the term no parallel. You literally meant that?

KEVIN TRENBERTH: I don’t think there has been anything quite like this before.

Back in the 1930s, the Dust Bowl era was very hot and dry. But, you know, I mean, every year is somewhat exceptional. Last year, of course, there was heat waves and all kinds of wildfires in — centered in Texas, Arizona, New Mexico. And it was extraordinarily hot in Oklahoma.

The previous year, it was in Russia. In 2009, there were exceptional conditions in Southern Australia, in the Melbourne area. And so these areas where the really hot and dry conditions leading to wildfires has — is moving around. We certainly don’t expect them to occur every year, but we do expect more of them.

The odds are changing for these to occur with climate change, with the global warming from the human influences on climate.

JUDY WOODRUFF: Well, that’s what I wanted to ask you about. To a scientist, what does this say is going on?

KEVIN TRENBERTH: Well, I think it’s — you know, you look out the window and you see climate change in action. This is the way it gets manifested. There’s normal weather events. There’s the normal seasons.

If we have June temperatures in March, well, you know, we have experienced them before because we get them in June. If we have a very mild winter, actually, people like that, because the winter isn’t as cold. But we were breaking records then.

Now we’re breaking records, but we’re in the peak of the heat season. And now we’re going outside of the realm of conditions previously experienced. And so that’s when the damage really becomes extreme, and we get all of these wildfires. Houses have been burned, tremendous damage to the environment and, you know, maybe some other consequences to come with regard to things like bugs that have survived the relatively warm winter.

So these are all manifestations of climate change that we expect to see more of as time goes on.

JUDY WOODRUFF: I was just hearing a few minutes ago that the Weather Service reporting more than 2,000 record temperatures matched or broken just in the last week.


Well, of course, the — it’s easy to break an individual record because the weather system just happens at that particular location. With an unchanging climate, you expect that the number of highs and the number of low temperature records are about the same. And that was the case in the 1950s, ’60s and ’70s.

And then by the 2000s, we were breaking high temperature records at a ratio of 2-to-1 over cold temperature records. But this year, we have been breaking high-temperature records at a rate of about 10-to-1. And, I mean, ironically, there are some still — still some cool spots mainly in the Pacific Northwest. And cold temperature records continue to be broken.

So, breaking records is not an indication of climate change. But breaking records at a rate of 10 to-1 vs. the cold records, that’s a clear indication of climate change.

JUDY WOODRUFF: Something for scientists like you and others who study the climate to keep you busy, it sounds like.

Well, Kevin…

KEVIN TRENBERTH: Well, this is a view of the future. So, watch out.

JUDY WOODRUFF: We hear the message. Kevin Trenberth, thank you very much.

KEVIN TRENBERTH: You’re most welcome.