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

The Intergovernmental Panel on Climate Change issued a report Friday saying temperatures and sea levels will continue to rise for centuries even if greenhouse gas emissions stabilize. An expert and a report co-author discuss the findings.

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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.