ROBERT SIEGEL, HOST:
It's turning into an annual conversation. There are record-high temperatures. There seem to be more extreme events than we used to be accustomed to: intense storms, droughts, wildfires. Are these the symptoms of climate change or is it just a very hot summer? Joining us now is Kevin Trenberth, who is a climate scientist. He's at the Climate Analysis Section of the National Center for Atmospheric Research in Boulder, Colorado, used to be head of the section for several years, and he joins us now from Boulder. Welcome to the program.
KEVIN TRENBERTH: Thank you very much, Robert.
SIEGEL: Let's take the fires in Colorado near you. A symptom of climate change?
TRENBERTH: It's one of the consequences of the climate change influence that has exacerbated the drought and the heat waves. Now, many of these fires have been started by lightning, by natural events. Some of them have been started by human activities of other kinds. But, you know, the underlying cause is really the very dry conditions and tender dry vegetation that has been set up by the prevailing climate.
SIEGEL: And record-high temperatures. We can look at them and say there really is a trend toward higher temperatures year in year out?
TRENBERTH: That's very clear, especially in the United States. You know, as time goes on, we always expect to set new records, but there should be an equal number of highs and lows. And in the 1950s and the '60s and '70s, that was the case. But by the time we got to the 2000s, the ratios of highs to lows was about two-to-one and this year so far it's running at about a ratio of ten-to-one. And so clearly this is just not natural variability anymore. There's something more than that that's rearing its head.
SIEGEL: To some people who - and I'm not talking about outlying scientists or polemicists - but some people who have difficulty with the notion of climate change, say, you know we - yes, we had a beastly month of May in Washington, D.C. this year and you could groan about climate change, then came an idyllic June. That there's so much change here that it's hard to measure objectively they would say.
TRENBERTH: Well, there's a tremendous amount of natural variability - the day-to-day stuff we call weather. There is also a natural variability in climate system - the dominant phenomenon there is the El Nino phenomenon. And over the last couple of years, we've been under the other phase, the cold phase, of that called La Nina. And so there is this natural variability. But when the natural variability from both the weather and the climate system are going in the same direction as the global warming from human influences, that's when we really break records. And the breaking of records is a clear symptom of this. And it's apt to have very large impacts, and we've seen that a lot this summer.
SIEGEL: The question of when extreme weather indicates climate change is one that draws upon, obviously, analysis of data, which few of us have either the training for or the access to for that matter. It draws upon our trust or mistrust of scientific authority and it inevitably draws upon our everyday experience and our recollection of what things used to be like. How would you advise people to process that mix of subjective experience and understanding of scientific data?
TRENBERTH: This is one of the difficulties. There are all of these events around the world which the scientists can see and put together as part of a larger scale pattern, which is much more difficult to see from the standpoint of the individual. And so it's this large-scale synthesis and then averaging over a month or season or something like that, which really helps to pull it all together.
SIEGEL: Kevin Trenberth of the climate analysis section of the National Center for Atmospheric Research in Boulder, Colorado. Thanks a lot for talking with us.
TRENBERTH: You're most welcome. Transcript provided by NPR, Copyright NPR.