Is U.S. Heat Wave Linked to Greenhouse Gases?

Sweltering temperatures are sweeping across the United States this summer, and many scientists say overall temperatures this year are the hottest on record. Jay Lawrimore of the National Climatic Data Center talks with Madeleine Brand about the link between greenhouse gases and hot weather.

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MADELEINE BRAND, host:

It's been hot, and not just normal end of summer hot. This year so far, has been the hottest on record, and the records go back for more than 100 years.

According to Jay Lawrimore of the National Climatic Data Center, greenhouse gases are partly to blame.

Jay Lawrimore joins me now from Asheville, North Carolina.

And when you say partly to blame, what do you mean by that?

Mr. JAY LAWRIMORE (National Climatic Center): What I mean is that the increase in greenhouse gases that has been observed over the past 150 to 200 years is having an effect on the global climate.

While we don't attribute that to any specific event, such as the heat wave that has been experienced over most of the U.S., what we can say is that the climate is changing, in part, due to the increase in greenhouse gases.

BRAND: And just to clarify, greenhouse gases are emissions caused by cars, by factories, by other…

Mr. LAWRIMORE: Yeah, the burning of fossil fuels.

BRAND: But from what I understand, scientists say that global warming has contributed to an increase in temperature of just one degree.

Mr. LAWRIMORE: Yes, that is correct. Global temperatures have increased about one degree Fahrenheit over the past 100 years. However, we've always experienced extremes. There have always been heat waves, flooding, hurricanes. But when the mean temperature goes up, it's just the distribution of temperatures. And so, with that shift, it increases the likelihood that those extremes that we've experienced in the past will happen more often.

So, take the Dust Bowl years of the 1930s - very warm temperatures. And we've seen very similar temperatures this July. Now, as the mean temperature continues to rise, those type of events will become more frequent.

Instead of occurring once every 50, 75, 100 years, they may occur once every 25 years. Eventually, maybe, once every 10 years.

BRAND: You noted high nighttime temperatures last month. How much hotter are the nights now and then what's contributing to those hot nights?

Mr. LAWRIMORE: Well, during July there were actually 3200 new daily records for the highest minimum temperatures. And one thing that's driving that is increases in moisture in the atmosphere which acts to hold up the nighttime temperatures. And why this is important is, during heat waves, the ability of people to adapt and to withstand the heat wave is made more difficult the warmer the nighttime temperatures are. Many of the deaths occur, occur because night time temperatures are not decreasing.

BRAND: So why is it wetter now?

Mr. LAWRIMORE: Well one reason is that with warming temperatures, there's more - more moisture has popped into the atmosphere from the earth's surface, from land surfaces, ocean surfaces.

BRAND: So basically, because it's getting hotter, there is more evaporation of the world's waters and therefore more moisture in the air.

Mr. LAWRIMORE: Yes, that is a contributing factor.

BRAND: How do we know, though, that this cannot be explained by natural cycles in the climate? That next year might be unseasonably cool in the summer and we won't be talking about global warming as a factor.

Mr. LAWRIMORE: Oh it certainly could be cooler next summer, that's part of the natural variability. But one way the science community studies and understands the influence of greenhouse gasses, is through computer modeling and being able to simulate what is happening with the atmosphere. And what these models show us is that in fact, what we are observing cannot be explained by natural variability alone. Only through the increase in greenhouse gasses have these increases in temperature been able to be explained. So the majority of the science community has come to the conclusion that man has had an influence of the earth's climate.

BRAND: You mention the first seven months of this year were the hottest on record, how hot were they?

Mr. LAWRIMORE: Well the average, if you are interested in numbers was 55.3 degrees Fahrenheit - which was about one half degree Fahrenheit warmer than the previous record, which was January to July, 1934 - which was during the dust bowl era.

BRAND: Jay Lawrimore is chief of the Climate Monitoring Branch of the National Climatic Data Center. That's a government group. Thank you very much Jay Lawrimore.

Mr. LAWRIMORE: Thank you.

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