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With the Kyoto Conference continuing in Japan, Margaret Warner and guest take a look at the the science and politics of global warming.

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MARGARET WARNER: This week representatives from more than 150 countries are in Kyoto, Japan, trying to negotiate binding limits in the greenhouse gas emissions that are blamed for global warming. At stake are issues of science, economics, and public policy. Tonight we begin a series of conversations about what we know and don’t know, and what we should do and not do about global warming. First up is Kevin Trenberth, chief of climate analysis at the National Center for Atmospheric Research. He was a lead author of the so-called Intergovernmental Panel on Climate Change Report. This 1995 document, which serves as the scientific foundation for the Kyoto Conference, aimed to produce a consensus on the science of global warming. Welcome. Let’s start with some basic concepts here. Explain what is global warming?

KEVIN TRENBERTH, National Center for Atmospheric Research: Global warming is often in the public view regarded as the global increases in temperature. It’s really much more than that. It’s really the global heating that’s associated with the increases in various gases in the atmosphere, the greenhouse gases, and this heating--some of it goes into raising temperatures, but most of it at the surface actually goes into evaporating moisture and changing the climate.

MARGARET WARNER: You mean, by causing more rain, among other things?

KEVIN TRENBERTH: By causing more rain and other things. It means that in places where there’s no rain occurring, there’s more drying going on, there’s more evaporation, more moisture goes into the atmosphere. The humidity of the atmosphere is increasing in many places, and that means there is more moisture available for rain systems. And when it rains, it rains harder.

MARGARET WARNER: Now explain another term you use, which was greenhouse gases. What are those?

KEVIN TRENBERTH: Greenhouse gases are gases which are transparent to the incoming radiation from the sun but the earth also radiates, and it radiates to space at infrared wave lengths. And the greenhouse gases tend to trap that radiation and they act like a blanket on the planet Earth, therefore providing this warming that we referred to earlier. Carbon dioxide is the main greenhouse gas that is increasing through human activities. Water vapor is the main greenhouse gas that produces our current greenhouse effect.

MARGARET WARNER: That is what you might call a natural one.

KEVIN TRENBERTH: That’s the natural greenhouse effect. 25 percent of the current natural greenhouse effect comes from carbon dioxide. Most of it comes from water vapor at the present time.

MARGARET WARNER: Now, the report you helped author concluded that--and I’ll quote from that--that the balance of evidence suggests that there is a discernible human influence on global climate.

KEVIN TRENBERTH: That’s right.

MARGARET WARNER: Now, let’s start first with the warming of the earth. What’s the scientific evidence that the earth is warming?

KEVIN TRENBERTH: Well, we have measurements all around the world, and the best analysis of that suggests that there’s been a warming of about 1 degree Fahrenheit over the last hundred years. The last ten years have been the warmest ten years on record. The warmest single year on record was 1995, although it looks as though at the surface 1997 could well replace that and become the warmest year on record. A big contributor to that is this current El Nino event, which is going on in the tropical Pacific. Further evidence that this warming is really taking place comes from melting of glaciers all around the world. The only place where glaciers are increasing is in Scandinavia, in Sweden. And that’s primarily because of increases in snowfall that’s been occurring there. Also, going along with the melting of glaciers is a rise in sea level and also the expansion of the ocean. So sea level has risen by about six inches in the last hundred years.

MARGARET WARNER: All right. And then what is the scientific evidence that human activity is partially causing this warming?

KEVIN TRENBERTH: Well, we know that carbon dioxide has increased about 30 percent in the last 200 years. Most of that has occurred in the last 50 years, and, indeed, that carbon dioxide and methane is also increasing, and nitrous oxide and so on, and that these are greenhouse gases. One way we know--look for--

MARGARET WARNER: And these are--I’m sorry--let me interrupt--and these are caused by the burning of fossil fuels mostly.

KEVIN TRENBERTH: Carbon dioxide in particular is caused by the burning of fossil fuels and there’s various other human activities that are contributing to this. And that’s pretty well established as well.

MARGARET WARNER: And are there natural activities that contributed to this?

KEVIN TRENBERTH: Well, there’s a natural cycle of carbon dioxide always through photosynthesis being taken up by plants and in the oceans and so on that’s going on in the background as well. But now we’re adding more carbon dioxide to the atmosphere. Some of that does get immediately taken out of the system and goes into the oceans and into the biosphere and increases the growth of trees and so on. But about half of it gets left behind in the atmosphere and is building up over time. One of the insidious things about this is that carbon dioxide overall has a lifetime of about a hundred to two hundred years, and so the emissions that we put in each year are actually accumulating and building and building. And we can’t make this problem go away rapidly if we want to.

MARGARET WARNER: All right. Now, since we don’t have a skeptic here tonight, let me pose some of the skeptics’ points to you. The first one--one first one is that the earth has gone through all kinds of fluctuations in climate in the millions of years, billions of years, and that this could well be just a natural variation, even if it is warming slightly.

KEVIN TRENBERTH: It’s true. Natural variability has occurred in the past. The thing that scientists are especially concerned about is that the projected rates of change over the next hundred years are much greater than anything that we have seen in nature. And so this is likely to be very disruptive to natural systems, and, in fact, it’s likely to be quite disruptive to human systems, human infrastructure, dams, and buildings, and so on. One of the things which we already have evidence for, for instance, is that what used to be hundred year storms now occur in seventy years or maybe even fifty years, or something like that. And so the risk of flooding is increasing in a number of areas. And so it’s the rate of change I think which is one of the chief concerns.

MARGARET WARNER: That brings up, though, another point they make, which is that these projections which are the really sort of alarmist--give us this sort of dreadful picture--are based mostly on computer modeling that they say just really isn’t reliable enough, given how variable weather is and so on; that the technical expertise just isn’t there.

KEVIN TRENBERTH: Well, computer models are not perfect by any means. They’re tools that we used to try to make projections into the future. We also use computer models to try to know what we should be looking for in the first place with regard to the changes that we’re seeing. We’re beginning to see a number of things that the computer models tell us we should be seeing. That’s what the statement from the IPCC says, that at least some of this warming that has gone on in the last hundred years is probably due to the increases in the greenhouse gases. It doesn’t say that all of it is, and it’s calling a warning to say that because we can’t stop this quickly and, in fact, in order to reverse this trend, we would have to reduce emissions substantially below what we have today, and it seems as though the Kyoto agreements are unlikely to be able to achieve that, then we’re going to have to live with some warming, and the best we can hope for is to slow the process down.

MARGARET WARNER: Now, the skeptics also say that that’s too alarmist of you; that it isn’t so terrible if the earth gets a degree warmer; that, in fact, plant life flourishes, animals do better, that even humans do better with it being just a little warmer.

KEVIN TRENBERTH: The problem is that the climate is not just changing from one state--the one we have now--to another state; it’s going to continue to change; and it’s going to continue to change in the future. And that means that any kind of planning is going to be different--difficult. A farmer is not going to know by his past experience exactly what to plan for in the future. And some of the biggest problems, I think, are not a degree warmer or so, but the changes in the water availability, the fact that droughts are likely to be more severe, the fact that thunderstorms, when they occur, cause flooding more readily. Even in North Dakota, in the Red River Valley, and last April the big snow buildup that occurred last winter produced major flooding, and there’s tremendous economic loss attached to that as all of the dams burst. Earlier in the year in California there were a number of levees that burst in association with flooding. And we see signs of this occurring. We can’t point a finger and say, yes, this is caused by global warming. But global warming is probably contributing. And it’s that difficulty which makes this problem hard to come to grips with.

MARGARET WARNER: All right. Well, thank you very much.

KEVIN TRENBERTH: You’re most welcome.

MARGARET WARNER: Thanks for being with us.

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