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From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

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And I'm Melissa Block. Next month, a scientific committee sponsored by the United Nations will put out its latest assessment of climate change. The report is expected to underscore, yet again, that climate change is a serious problem and that human beings are largely responsible.

The IPCC, as the committee is known, represents a consensus view of hundreds of scientists around the world. The group shared the 2007 Nobel Peace Prize with Al Gore.

CORNISH: Yesterday, we heard from a scientist who is skeptical of this majority view. Today, we meet Kevin Trenberth. He's been part of the IPCC since its early days in the 1990s and is an outspoken defender of the scientific consensus on climate change.

Trenberth's own research has sought to explain an issue frequently raised by climate skeptics - why global air temperature hasn't increased over the past 15 years. NPR's Richard Harris visited him recently at his lab in Boulder, Colorado.

RICHARD HARRIS, BYLINE: The National Center for Atmospheric Research isn't quite the ivory tower, more like beige. The towering building sits on a ridge on the edge of the Colorado Front Range with breathtaking views of the mountains in one direction and the Great Plains stretching out below.

KEVIN TRENBERTH: Let me show you in here first.

HARRIS: Kevin Trenberth is the most prominent denizen of the research center, its distinguished senior scientist. You'd never guess he's 68 years old, either by his spry appearance or his scientific productivity.

And while these days he's a staunch advocate for the scientific consensus, his first foray into climate science was a cut across the grain. There was a devastating drought in 1988, which attracted the attention of one of the most renowned climate scientists.

TRENBERTH: Jim Hansen famously went before Congress and declared that the drought was due to global warming, essentially. And I wrote a paper, along with two others, that appeared in Science magazine which basically said that it wasn't.

HARRIS: Instead, Trenberth said the drought had to do with what at the time was an unappreciated part of the climate system: the El Nino warming phenomenon in the Pacific Ocean.

Fast forward 25 years, and Trenberth still sees changes in ocean temperature as key to understanding the ups and downs of global climate. That includes the current plateau in global temperature. Climate skeptics are quick to point out that the Earth's average temperature hasn't risen for 16 years, and to them, that casts doubt on global warming.

But Trenberth says the planet has been heating up during that time, it's just that the heat has been flowing into the oceans, which have a vast capacity to absorb it.

Will the oceans come to our rescue, essentially?

TRENBERTH: That's a good question, and the answer is maybe partly yes, but maybe partly no.

HARRIS: The oceans can, at times, soak up a lot of heat. Some of it goes into the deep oceans, where it can stay for centuries. But heat absorbed closer to the surface can easily flow back into the air. That happened in 1998, which made it one of the hottest years on record.

Trenberth says since then, the ocean has mostly been back in one of its soaking-up phases.

TRENBERTH: They probably can't go on much for much longer than, you know, maybe 20 years, and what happens at the end of these kind of hiatus periods is that suddenly there's a big jump to a whole new level and you never go back down to that previous level again.

HARRIS: So you can think of it like a staircase. Temperature is flat when a natural cool spell cancels out the gradual temperature increase caused by human activity. But when there's a natural warm spell on top of the long-term warming trend, watch out.

TRENBERTH: When the natural variability or when the weather is going in the same direction as global warming, suddenly we're breaking records, we're going outside of the bounds of previous experience. And that is when the real damage occurs.

HARRIS: Consider Hurricane Sandy. Trenberth figures the storm was maybe five or 10 percent more powerful as a result of global warming. And sea level is eight inches higher than it was a century ago. Doesn't seem that dramatic, but he argues that made a huge and costly difference.

TRENBERTH: I reckon that without climate change, we would not have exceeded the thresholds that caused the flooding of the subways in Manhattan and the tunnels from Manhattan to New Jersey and to Brooklyn.

HARRIS: Now it's taken quite a few years for Trenberth and his colleagues to piece together the role of oceans in climate variability. It involved a huge amount of data taken from ocean buoys that take the temperature of the deep sea, along with satellites that measure energy flowing into and out of the atmosphere.

And a few years ago, Trenberth was lamenting to his colleagues in an email that the Earth observing system still didn't give them all the data they needed to fully explain the ups and downs of global temperatures.

TRENBERTH: And I said that it was a travesty that we couldn't account for, essentially, the global warming in some sense.

HARRIS: This email ended up being taken from a British computer and published along with a flood of other private conversations, in an episode dubbed Climategate. Trenberth's comment was singled out by skeptics who claimed scientists like him were covering up the truth about global warming.

TRENBERTH: That email was taken completely out of context and misused in many respects.

HARRIS: Trenberth readily acknowledges that there are still some gaps in understanding the Earth's overall heat balance. But that doesn't undercut the basic observation that carbon dioxide and other gases from human activity are driving up the Earth's temperature in the long run. Indeed, the last decade was the warmest on record, even though temperatures didn't keep climbing during that period.

Over the decades that Kevin Trenberth has been working on climate change, the role of scientists has gradually expanded. Prominent scientists like him are trying to reduce the risk of global disruption by pushing society to act. These are frustrating times.

TRENBERTH: This is very much the role of the politicians, who are supposed to do what's in the interests of everybody as a whole. And I'm not sure many politicians fully understand their role in this.

HARRIS: There's a deep current on Capitol Hill that says it's pointless even to try because China and India seem destined to produce so much carbon dioxide, curtailing U.S. emissions won't do much at all. But wading into this policy debate, Trenberth argues that the United States should and could lead the world toward a less dangerous trend.

TRENBERTH: If you play the right kind of role, then other countries will follow.

HARRIS: You could argue that's simply wishful thinking, or you could argue that China and India would be even less likely to address climate change if the United States wasn't even going to try.

Trenberth has no illusions that we can do anything to stop the climate from changing altogether. After all, nature is changing the climate all the time. But the question now is moderating the speed of that change.

TRENBERTH: Some of the human-induced changes are occurring 100 times faster than they occur in nature. And this is one of the things that I think worries me more than climate change itself. It's actually the rates of change that is most worrying.

HARRIS: Ecosystems are not prepared for this jolt. And neither, he argues, are many human endeavors, built around assumptions about how hot it's going to be, how much it's going to rain on our croplands, and how high the seas will rise. Richard Harris, NPR News.

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