Report: Humans 'Very Likely' Cause of Climate Change An international team of climate scientists prepares to release its global warming report. The report is expected to assert that it's "very likely" humans have caused warming leading to stronger hurricanes and melting glaciers.
NPR logo

Report: Humans 'Very Likely' Cause of Climate Change

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript
Report: Humans 'Very Likely' Cause of Climate Change

Report: Humans 'Very Likely' Cause of Climate Change

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript



After a week of back-to-back meetings to finalize their findings, scientists from around the world released a much-anticipated report on climate change this morning. The scientists belonged to the Intergovernmental Panel on Climate Change and the report is considered the gold standard on the state of scientific knowledge on climate change, and what they say is sure to answer critics of global-warming theory.

They say in this report that they are about 90 percent certain that human activity - in particular, our burning of fossil fuels - is the driving factor in the Earth's warming over the past 50 years. The report also says that for the foreseeable future the course has been set. Even aggressive changes we make now to our fossil-fuel consumption won't prevent future warming, but it could slow the pace of change.

So we'll start this hour by talking with one of the report's authors about warming planets and what these warmer temperatures will mean for our planet. If you'd like to talk about the new climate-change report, give us a call. Our number 1-800-989-8255, 1-800-989-TALK. And as always, you can surf over to our Web site at

Kevin Trenberth is a senior scientist and head of the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colorado. That's NCAR. He's also the lead author on one of the chapter's of the new IPCC report, and he joins us now by phone from Paris, where he's been in meetings all week. Good evening, and welcome back to the program, Dr. Trenberth.

Dr. KEVIN TRENBERTH (Senior Scientist, Climate Analysis Section, National Center for Atmospheric Research): I guess it's good afternoon to you, Ira, but here it is indeed evening.

FLATOW: How difficult has it been to arrive at this report?

(Soundbite of laughter)

Dr. TRENBERTH: You know, the rules of the United Nations for this is to have a unanimous consensus, and there are many delegates who I might say are a little long-winded and there are a number of delegations who have particular political agendas. You know, the way this is supposed to work is that we have worked hard on producing the report over the last three years.

There's - it'll be, I don't know, close to a 900-page report, something like that, and then there is this policy for summary - a policymaker summary that we've been producing over the last few days, where the policymakers - the way I think of it is that they help create the language that we use to communicate the report, while the experts determine what goes into the report.

And on Wednesday night - it's hard to remember which day it is now - we got into a mode where we were substantially behind in terms of the amount of text we had to deal with versus the time available, and so we had very brief breaks. There was no coffee breaks, worked through the dinner hour and closed at 10:40 in the evening. Then last night - I'm sorry, 12:20 was when we closed on…

FLATOW: We won't hold you to any of these things in your state of drowsiness.

(Soundbite of laughter)

FLATOW: Tell us about what's the difference between this report and the last report and how much you've increased your confidence.

Dr. TRENBERTH: Well, there's certainly been a lot of literature that's been published, and that's the main basis for the IPCC assessment. The - I'm sorry, it's a little disconcerting. I've got an echo in my ear, here. I wonder if someone can turn that off.

FLATOW: We'll try to deal with that.

Dr. TRENBERTH: And what has happened is that there is a lot more observations. One of the highlight statements in the report from my standpoint is that the warming of climate is unequivocal, and that really comes from multiple lines of evidence and extensive evidence within each line.

And since the last report, which was published in 2001, we've had six new years of data, and Mother Nature keeps telling us that the planet is warming. Those six years are among the seven warmest on record, for instance. And the multiple lines of evidence referred to not just changes over land but also the sea temperatures, the so-called satellite temperatures, which is the temperatures above the surface of the Earth.

The ocean temperatures are increasing. That gives rise to expansion of the ocean and rising sea level. Sea ice is melting, especially in the Arctic. The glaciers are melting. That contributes also to sea-level rise, and so we can then measure the sea-level rise - and that's gone up about an inch and a half in the last 12 years. The snow extent has decreased in the Northern Hemisphere.

And then there are some things that we can really add that are new in many ways. The water vapor has increased in the atmosphere. That contributes to heavy rainfall events and the risk of flooding.

The pattern of rains has changed around the world. There's more drought and drying in general in the subtropics and more rains and less snow at high latitudes, and increases in heat waves, decreases in frost, and also even, you know, intense hurricane activity is increasing.

So these multiple lines of evidence are all support for this statement that the warming of climate is unequivocal, and then the report goes on to say it is very likely to do human activities, as you mentioned before, which is greater than 90 percent.

FLATOW: What about the sea-level rise. Have we noticed that change, too?


FLATOW: Dr. Trenberth?

Dr. TRENBERTH: Yes, I lost you there for - briefly.

FLATOW: It's OK. A lot of people claim to want to do that.

(Soundbite of laughter)

FLATOW: What about the sea-level rise. Has that been documented, too?

Dr. TRENBERTH: Well it has, and there was a certain amount of blogs and various other publicity about sea-level rise in various places before the report. It turns out that the numbers in this report are actually very similar to the previous report, overall. However, they are reported in a somewhat different way and there's an increased confidence in the report in at least part of that.

But there are parts of sea-level rise that we also recognize are very uncertain. And in the last five years, for instance, there's been noticeable melting of the Greenland ice sheet and increased flows of glaciers, as several ice shelves around Greenland - and this is also true in the West Antarctic region - indicating that glacier dynamics comes into play.

And so these great big ice sheets are not just like a big ice block which sort of just gradually melt. Instead, there's a lot of active features that come into play where they crack, the crevices form, water flows down in them, lubricates the base of the glaciers. They become dynamic, and so the collapse of the ice sheet can occur much faster than we thought about five years ago.

FLATOW: The ice will just slide into the ocean.

Dr. TRENBERTH: Some of that…

FLATOW: Instead of just waiting for it to melt, yeah.

Dr. TRENBERTH: Yes. And so this is not really taken into account in the report, but it's clearly identified as an uncertainty.

FLATOW: There are critics who say that you didn't go far enough in this report, that you really were coddling the issue of it in your conclusions.

Dr. TRENBERTH: Well, I think it's much more carefully discussed, and part of it was presentational in I think some of those critical comments. And I hope the critics are much more satisfied now. The problem is that a lot of this is speculation and is something that needs to be pinned down in the future, but there's very little basis to go on. There's not enough understanding and there's no modeling of these aspects and how to treat it. So it needs to be identified as an uncertainty.

FLATOW: What about mitigation? You seem to be saying that at least in the short term, meaning the next 100 years, we're set on a course and it really can't be reversed, no matter what kind of greenhouse-gas mitigation we take.

Dr. TRENBERTH: Well, it says that for the next 20 years because of the long lifetime of the greenhouse gases - in particular carbon dioxide - in the atmosphere. What we already have right now guarantees, you know, what's going to happen in the next 20 years to a very large extent, along with the infrastructure that we have. We're not going to shut down a whole lot of power plants in the near term, and so that contributes as well.

And then the oceans respond very slowly. The uptake of heat and when they reach equilibrium, you know, that adds about a 20-year timeframe to the response function. But what it also does say is that the different what we call emissions scenarios, the different decisions that we make about how much we put into the atmosphere does make a difference after about 30 years. And so the rates of change can be affected by policy decisions that are made and, you know, whether we have things like carbon taxes and other options that politicians consider for perhaps reducing emissions.

FLATOW: Speaking of policy decisions, next week you're going to present these results to both the House and the Senate. What do you hope that they will take away from this?

Dr. TRENBERTH: Well, certainly we hope that our results are policy relevant. The purpose of the IPCC is to inform decision makers and help them to make decisions. But the debate as to exactly what should be done is one that becomes very much part of the political arena, but it's also a part of public awareness-raising. And in order to provide the decision-makers the support, they need in order to make some of these critical decisions. You know, a key thing is that this is truly a global problem and it's important for the U.S. I think to be much more engaged than they have been on the international scene. Because from the U.S. standpoint, even though at the moment the U.S. is the biggest emitter of carbon dioxide, China is catching up and we have a global atmosphere.

So what China does to the atmosphere affects us just as much as what we do affects the Chinese. It's a global problem, and so we are going to have to recognize that it's happening. We're going to have to adapt to it, but we also need to work to slow it down.

FLATOW: And you think those policy decisions need to happen very quickly.

Dr. TRENBERTH: Well, you know, we don't have to solve this tomorrow or even next week, but we do have to give it attention. And one of the problems is that it's a long-term sustained problem, so it has to be given continued attention. And this is sometimes incompatible with the relatively short lifetime of people who are in office, where they're looking at two-year or six-year timeframes, or something like that.

FLATOW: Well I want to thank you or taking time to be with us. And are you looking forward to your hearing next week?

Dr. TRENBERTH: Well, I haven't given it much thought at the moment. It's been so consuming with this.


Dr. TRENBERTH: And I'll be flying back to the United States tomorrow, and then to Washington later in the week. So it's been a busy time.

FLATOW: Are you hopeful that people will finally change their minds - I mean the skeptics will change their mind now that this meeting - this report is out.

Dr. TRENBERTH: You know, the whole - you know, I think many of the so-called skeptics certainly agree that the planet is warming. They may argue a little bit about what it is due to or just how much is due to human influences, but we state very clearly in this report that the solar influence - for instance, the sun - is a very small component of what has gone on, and we've got good measurements of that in the last 25 to - last 35 years. So there's not much basis for them there. But then the argument is, well, you know, maybe this is good for us. And so that relates to the second IPCC report, which will be coming out in April that deals with the vulnerability and the impacts and the (unintelligible) adapting to it.

FLATOW: We'll invite you back then. Thank you, Kevin Trenberth, for taking time to be with us.

Dr. TRENBERTH: Thank you.

FLATOW: He is lead author in one of the lead chapters of the new IPCC report. We're going to take a break and come back and talk more about wind power as a possible mitigating factor, so stay with us.

I'm Ira Flatow. This is TALK OF THE NATION: SCIENCE FRIDAY from NPR News.

Copyright © 2007 NPR. All rights reserved. Visit our website terms of use and permissions pages at for further information.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.