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U.N. Panel Says Humans ‘Very Likely’ Causing Global Warming

February 2, 2007 at 6:10 PM EST
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MARGARET WARNER: Today’s announcement in Paris from the U.N.’s Intergovernmental Panel on Climate Change was its fourth such assessment since 1990 and its most urgent warning yet about global warming: what’s already occurred, and what lies ahead.

By unanimous agreement, the 2,500 scientists and government representatives said there’s now at least a 90-percent certainty that mankind is to blame for the warming already being observed.

As to the future, the projections were stark. Among them: in this century, the planet will warm up by between three and nearly eight degrees Fahrenheit; the weather will be hotter everywhere, with some areas becoming dryer, while others see more rain; and sea levels will rise.

Yvo de Boer, head of the U.N. Climate Secretariat, said it’s time for the world to act.

YVO DE BOER, U.N. Climate Secretariat: It’s important that all governments have agreed to the conclusions of the scientists, and therefore these conclusions can no longer be the subject of discussion in the political negotiations, but should be considered as a given, and that’s an important step forward. The signal that we’ve received from the science today is crystal clear.

MARGARET WARNER: The next phase of the group’s report, due this spring, will focus on the impact of global warming and how humans might adapt to it.

For more now on these findings, we turn to Kevin Trenberth, one of the draft contributing authors of the report. He is the director of climate analysis at the National Center for Atmospheric Research in Colorado. And he joins us from Paris.

And Michael Oppenheimer, professor of geosciences and international affairs at Princeton University’s Woodrow Wilson school, he’s also a member of the U.N. climate change panel and a contributor to the report.

Welcome to you both.

Mr. Trenberth, beginning with you, the finding that’s attracted the most attention today is the one saying that there is really 90-percent certainty now that the climate warming that’s already occurred since the middle of the last century is due to human activity. How much warming has there been? And what lead you to that kind of certainty that mankind is at the root of it?

KEVIN TRENBERTH, National Center for Atmospheric Research: Well, there’s two-steps to this. The first one is what has happened, the observations of what has happened. And I thought a very important statement in the report, you know, to quote, is that, “Warming of the climate is unequivocal.”

And then it goes on to qualify that, and say that, you know, it’s not just the global mean temperatures which, you know, the six years since the last report are in the top warmest seven years on record, but also a whole host of other variables, from snow cover and sea ice, rising sea levels, melting glaciers, drought around the world, changes in hurricanes, all of these kinds of things come together to provide really compelling evidence from many different lines of evidence to suggest that, indeed, warming is happening.

In addition, the models have improved substantially so that they can now simulate a lot of what has happened in the models themselves. And the ability to match those things provides a lot more confidence than in what we can say in the future.

And this has led to this statement, which is actually greater than 90-percent certainty. It is very likely that global warming that is happening is due to human activities.

Acceleration of climate change

Michael Oppenheimer
Princeton University
If we don't bring the emissions under control, we can expect potentially very, very much greater changes than what we have already seen.

MARGARET WARNER: Professor Oppenheimer, let me go to you. First of all, if you use the word model, do explain what a model is. But what jumps out at you about this report? You worked on the previous one, as well, as I understand.

MICHAEL OPPENHEIMER, Princeton University's Woodrow Wilson School: Right. What jumps out at me, in addition to what Kevin mentioned, is the fact that both the temperature increase and the rate of sea level rise have accelerated.

And, furthermore, that, unless the emissions that are causing the climate change are brought under control, we can expect more climate change in the future. In fact, we can expect more climate change regardless.

But if we don't bring the emissions under control, we can expect potentially very, very much greater changes than what we have already seen. So to use maybe an unfortunate metaphor, this is just the tip of the iceberg compared to what may be in store for us in the future.

MARGARET WARNER: But, Professor, why couldn't it be just a normal cyclical development?

MICHAEL OPPENHEIMER: Well, as Kevin has said, the matching of what the computer models project for what the pattern of warming and the pattern of climate change should be with what has actually been measured now for, you know, essentially going back over a hundred years, shows convincingly and definitely that the warming is by and large due to human activity and that we will get more substantial warming in the future.

The impact of rising temperatures

Kevin Trenberth
Contributing Author, Climate Report
The extremes of water are going to be much more unmanageable, both the drought extreme and the risk of floods.

MARGARET WARNER: Kevin Trenberth, the report predicted or forecast that now, in this century -- and I converted from Celsius to Fahrenheit, so I hope this is correct -- that the planet is really sure to warm up between 3.2 and 7.8 degrees Fahrenheit. Give us an understanding of, first of all, how dramatic that is and what it might lead to.

KEVIN TRENBERTH: Well, the warming we've had is around about -- let's see. It's 1.5 degrees Fahrenheit since the beginning of the 20th century and about 1 degree Fahrenheit since about 1970. So the rate of warming has increased. That's one of the key aspects of the global temperature increases.

The warming is also greater overland and at high latitudes in the northern hemisphere in the Arctic region, and so this is the thing that affects the polar bears and their livelihood.

And so, with a greater warming, we expect to -- the amplification of the effects we're already seeing. So around the subtropical region, there is a drying that's been going on, increases in droughts, and some increases in rainfall at higher latitudes. And it is rainfall, some of the what used to be snowfall is now occurring as rain.

And so there are large-scale patterns of changes in the rainfall and the rainfall distribution, which has real consequences for human activities and for the environment, because it increases the risk of droughts in the subtropics and other places around the world, because, as temperatures warm up, it creates a drying effect off the atmosphere, on the surface.

And at the same time, in other places where it does rain, it's likely to rain harder. And this relates also to changes in hurricanes. And so it means that the extremes of water are going to be much more unmanageable, both the drought extreme and the risk of floods.

Coastal zones most at risk

Michael Oppenheimer
Princeton University
The last time the poles were as warm as they could get over the next few decades, due to the greenhouse gases, substantial parts of the Greenland ice sheet and perhaps the Antarctic ice sheet had either melted or disintegrated away into the ocean.

MARGARET WARNER: So, Professor Oppenheimer, which areas of the planet where lots of -- where many people live are most at risk?

MICHAEL OPPENHEIMER: Well, I'm most worried about the coastal zones everywhere. One sort of nugget in the report is that the last time the poles were as warm as they could get over the next few decades, due to the greenhouse gases, substantial parts of the Greenland ice sheet and perhaps the Antarctic ice sheet had either melted or disintegrated away into the ocean. And sea level was something like 15 feet higher than today.

Now, that isn't going to happen overnight. It's a relatively slow process. But we may be essentially remaking the face of the Earth by putting a lot more water into the ocean, reconfiguring the coastal zone, drowning areas like river deltas, where tens of millions of people live in some countries, like the Netherlands, Bangladesh, the Louisiana delta in this country.

And to my mind, this is the most pervasive and most threatening consequence of global warming. It will be very expensive. And once it gets under way, it's essentially impossible to stop.

MARGARET WARNER: Why is that?

MICHAEL OPPENHEIMER: There's a long inertia in the system. Ice does not respond immediately. The warming could push us to the point, with only a relatively few degrees of global warming, where we would start to lose, eventually, large portions of the ice sheets.

So policymakers ought to take heed of that and start to think about reining in the warming before we cross thresholds that we're not even sure of the location of.

The control societies still have

Kevin Trenberth
Contributing Author, Climate Report
The policy decisions that are made today, with regards to emissions and what we put into the atmosphere, are very important. But, indeed, you also need to recognize that we're going to have to live with some climate change.

MARGARET WARNER: So, Kevin Trenberth, go to that point about what really can be done. Your report was saying it's inevitable, is it not, that some warming is going to occur no matter what we did. I mean, if tomorrow we stopped emissions, all emissions, which obviously isn't going to happen, the planet would still continue warming?

KEVIN TRENBERTH: This is correct. And this relates to the fact that the carbon dioxide that we've already put into the atmosphere has a long lifetime. And so, even if we don't put anymore in, what we've already got hangs around there for many decades, maybe more than a century. And so that's one component.

And then the other component is the fact that the oceans are still responding to the climate change that we've already had, the changes in the heat that's flowing through the climate system. And so that has an adjustment time of order of 20 years.

And this means that these kinds of things are going to occur no matter what we do in the future. It's mainly after about 30 years or so that what we do now has its main impact, and it can be very important.

And so further out, the policy decisions that are made today, with regards to emissions and what we put into the atmosphere, are very important. But, indeed, you also need to recognize that we're going to have to live with some climate change. And that relates then very much to a proper assessment of the impacts and how you adapt to those changes that we're expecting to happen in the future.

MARGARET WARNER: Very briefly, Professor Oppenheimer, so how much control does mankind have at this point over this process?

MICHAEL OPPENHEIMER: The bottom line is that the future, by and large, still rests in our hands, that we can avert the most threatening part of the warming, but we will have to start action now. We can't wait around. And it's encouraging that California, the Northeastern states, the European countries have all introduced caps on emissions and then intend to bring them down.

Really it's an inside-the-beltway problem at this point. Washington has to take some leadership.

KEVIN TRENBERTH: Well, it's also a global problem.

MARGARET WARNER: All right. Kevin Trenberth and Michael Oppenheimer, thank you both.

MICHAEL OPPENHEIMER: Thank you.

KEVIN TRENBERTH: Thank you.