There is broad scientific agreement global warming is happening and that humans are at least partially to blame. But there are some important scientific skeptics. Last month, in the New York Times, one of the most important of them did a public about-face. Richard Muller, a physicist at the University of California at Berkeley, now says it’s real and humans are almost entirely to blame. Here’s his conversation with Fareed Zakaria from the latest episode of GPS.

You say in that piece that all scientists should be skeptics. And I think you’re right. I remember Niels Bohr once said that every statement should be taken by a scientist as a provisional hypothesis that has to be tested. So what made you start doubting your original skepticism? What evidence convinced you that something real was happening here?

The issues were so large that about two-and-a-half years ago, my daughter and I began a major scientific research effort in which we recruited a dozen of some of the top scientists in the world, including Saul Perlmutter, who won the Nobel Prize last year – well after he joined our team. So we felt there were questions that were valid, questions about data reliability, about data adjustment, about the choice of the stations which had been used. These demanded attention and I couldn’t get the answers. The only way to do it is to do the study ourselves.

So, after a great deal of work, largely done by Robert Roady, who I can’t compliment enough for his superb work in data analysis, which we all carefully participated in...I came to the conclusion that, yes, global warming was real. Then, over the last three to six months, Roady was able to extend the record back to 1753. We now had a really long record, beginning before the American Revolution, when Benjamin Franklin and Thomas Jefferson were the ones taking the data. With this long record, we could look for the signatures of the various possible causes. We’re able to rule out solar variability, able to rule out volcanoes. They had an effect, but it was short-lived. When we tried fitting it to see whether it looked like carbon dioxide, it was right on. It was a shock to me at how well that carbon dioxide curve fit our new temperature data set.

So when you look at the historical data now, is it fair to characterize the situation thus, that ever since the Industrial Revolution, human beings have been pumping more carbon dioxide into the atmosphere and that that increase in CO2 has been having the effect that we call global warming?

That is my viewpoint on this. You can’t prove it. It’s always possible that something random is happening that just happens to match the carbon dioxide data. But [the data] leads me to conclude that essentially all of this warming over the last 250, 260 years, has been caused by greenhouse gases emitted by humans.

Now, do you also worry about the potential effects that this will have on life on Earth? Because a lot of people talk about global warming and then they say, if it gets to a certain point, the higher end of the U.N. estimates, you could have very significant coastal flooding. You could have unintended consequences, the sort of follow-on effects that could be even more damaging to life on Earth.

Well, I am deeply worried about it. [With] the coastal flooding, the U.N. estimate is something like between two and three feet. That’s not huge. But I am concerned. I think rising temperatures soon will be in a realm that’s higher than we, Homo sapiens, have ever experienced. I don’t personally believe that’s good for our civilization. I think we really do need to do something about it.

When you look at the issue of what to do about it, there are people who say, look, the only thing we can do is what’s called adaptation. We should rotate crops. We should build dikes. We should do those kinds of things. And then there are people who say, no, the problem is so serious, you have to actually get at the root cause and slow down the emission of CO2.

I believe in the latter. [W]e’re a very adaptable species. But adaptation is always disruptive and it hurts…The biggest thing we have to do, we have to recognize that the reason that carbon dioxide is shooting up is not because of the United States. Ours has actually been going down over the last few years, as we switch from coal to natural gas. Natural gas emits only one third the carbon dioxide that coal does. If we are going to do something about this, there are two things we have to do. One is energy conservation and efficiency. That’s really important. A huge amount we can do there. Number two is we’ve got to switch the world – China, India and particularly the developing world – away from coal and onto natural gas. Now, that’s a solution that a lot of my environmentalist friends don’t like, because they’ve decided they have to oppose hydraulic fracturing, known as fracking. But, in fact, that is one of the two biggest things we could do – energy conservation and a switch to natural gas from coal.

Some of your funding came from the Koch brothers, who are famously anti-global warming or believe that it isn’t happening. How did that play out? Were they disappointed by the results of your research? Have they asked for their money back?

I actually find it amusing how many people think they know what the Koch brothers are thinking. It’s a caricature…I did speak with them, and they made it clear to me, from the very beginning, that they recognized that there were serious issues raised about prior estimates of global warming, everything from urban heat islands to data selection bias to other things. And they knew that I wanted to look into that, that our team would do a good, unbiased job. And all they were asking for was scientific objectivity. So I was very pleased with their funding. I really sensed they wanted to have this problem solved and they never gave me any suggestion, any hint of a suggestion about which side they were hoping we would come out on.

And you haven’t heard from them since you’ve gone public on these issues?

Oh, I actually have talked to them. And they appear to be very pleased.

Richard Muller’s new book is ‘Energy for Future Presidents.’ You can download the full version of this episode of Global Public Square on iTunes here.