JUDY WOODRUFF: Now to the debate over the magnitude of climate change, its impact, and the human role in it.
Typically, the battle plays out among prominent climate scientists and a vocal group of skeptics. But one skeptic's recent public conversion is adding new fuel to that fire and sparking criticism from both sides.
NewsHour correspondent Spencer Michels has the story.
SPENCER MICHELS: Physicist Richard Muller and his daughter, Elizabeth, a mathematician, are not exactly household names.
But in the world of climate change, where most scientists and a much smaller group of skeptics remain bitterly divided over their assessment of what's happening to the planet, Richard Muller has long been on the side of those who deny climate change is happening.
So, when he published an op-ed in The New York Times last month saying he was no longer a skeptic, it captured national attention and sparked angry reaction on both sides of the climate fence. Perhaps most disturbing to some of his former allies was this conclusion:
RICHARD MULLER, University of California, Berkeley: In our world, we attribute the warming from 1753 to the present essentially exclusively to humans -- not mostly, but exclusively.
SPENCER MICHELS: Even those skeptics who accept that the climate is changing attribute it to natural cycles, but Muller even claimed his study was more conclusive in that regard than any that came before.
RICHARD MULLER: We really are in some sense coming out with a stronger conclusion than the prior group had come out with.
SPENCER MICHELS: Working out of their house in Berkeley, where Muller is a physics professor at the University of California, the Mullers formed the Berkeley Earth Surface Temperature Project.
Using funds partly supplied by the Koch brothers, who have also funded skeptical organizations like the Heartland Institute, the Mullers had long analyzed temperature data others had collected. But, for years, they said they hadn't trusted that data.
RICHARD MULLER: I think many of the people working on this had convinced themselves that global warming was real and had lost some of their objectivity.
SPENCER MICHELS: But in their op-ed, the Mullers said that their latest research showed that the data from other climate change scientists was by and large correct.
ELIZABETH MULLER, Berkeley Earth Surface Temperature Project: We used all of the data, or essentially all of the data, five times more than any other group had done. And after having done all of that, we determined that the previous -- the previous studies on global warming had been about right. There was global warming of about one degree Celsius in the past 50 years. And that was a big surprise to us.
SPENCER MICHELS: The conclusion about a warming climate due to human actions matched what many other climate change believers have been saying, including William Collins, a senior scientist at Lawrence-Berkeley Laboratory. He acknowledges that natural warming and cooling periods have occurred for eons, but the warming occurring now is off rhythm.
WILLIAM COLLINS, Lawrence Berkeley National Laboratory: What we're seeing now is occurring much faster. Rather than happening over tens of thousands of years, we're seeing very rapid change occurring on just the time scale of a single century.
This timeline is showing how the temperature all over the globe has changed since the beginning of the 20th century. Look at how warm California has gotten, four or five degrees hotter than our historical climate.
SPENCER MICHELS: And, Collins concludes, man is a big contributor.
WILLIAM COLLINS: What man has been doing is enhancing the greenhouse effect by taking carbon dioxide that was formed over the last half-a-billion years and releasing that carbon dioxide, which is a greenhouse gas, back into the Earth's atmosphere.
SPENCER MICHELS: Yet, many of those believers were annoyed that Muller's conversion got more attention in the media than their reports have gotten in the past. They dismissed him as being publicity-hungry and adding nothing new to the debate.
Climate modeler and British Green Party member William Connolley called Muller's study rubbish, saying they hadn't added any knowledge to what had been done before. Skeptics were even more dismissive of Muller's work.
Judith Curry, professor of earth sciences at Georgia Tech, who suspects natural variability accounts for climate change, not human-produced CO2, said Muller's analysis is "way oversimplistic and not at all convincing."
Even former ally Anthony Watts thinks Muller got it wrong. Watts works five hours from Muller in Chico, California. There, he runs a company supplying data and display systems to television weather forecasters and private individuals. He was trained as a broadcast meteorologist, though he has authored some papers with academic researchers.
His blog, "Watts Up With That?," bills itself as the world's most viewed sight on global warming and climate change. Watts believes all climate warming data, Muller's included, is off because weather stations where temperatures are recorded have soaked up heat from their surroundings.
ANTHONY WATTS, Meteorologist: A brick building that's been out in the summer sun, you stand next to it at night, you can feel the heat radiating off of it. That's a heat sink effect. We have got more freeways, you know, more airports. We have got more buildings.
Yes, we have some global warming. It's clear the temperature has gone up in the last 100 years, but what percentage of that is from carbon dioxide and what percentage of that is from the changes in the local and measurement environment?
SPENCER MICHELS: He also thinks believers have a hidden agenda.
ANTHONY WATTS: Global warming has become essentially a business in its own right. There are whole divisions of universities that are set up to study this factor. And so there's lots of money involved. And so I think that there's a tendency to want to keep that going and not really look at what might be different.
SPENCER MICHELS: It's a charge climate change believers say is totally false. But many do agree with Watts' criticism of Muller for presenting his report in a newspaper, rather than in a scientific journal.
ANTHONY WATTS: He has not succeeded in terms of how science views, you know, a successful inquiry. His papers have not passed peer review.
RICHARD MULLER: In science, peer review means you give talks to the public. You send your papers to colleagues around the world. That's what I did. Before I wrote my op-ed, we put all of our papers available on the Web.
SPENCER MICHELS: But the fight over climate change is anything but academic. Whether the politicians listen to the 97 percent of scientists who say that it is real or they pay attention to the vocal community of skeptics will determine to a large extent what regulations and what laws get passed.
Neither presidential candidate is talking about climate change, but, in Congress, it's a different story; 74 percent of U.S. Senate Republicans publicly question the science of global warming, including Oklahoma Senator James Inhofe, who thinks it's a hoax.
SEN. JAMES INHOFE, R-Oklahoma: Those people who really believe that world is coming to an end because of global warming, and that's all due to manmade anthropogenic gases, we call those people alarmists.
SPENCER MICHELS: Polls show more than half the Republicans in the House are global warming skeptics. Many were elected with the Tea Party wave during the 2010 election.
In 2011, a Republican-dominated House committee defeated an amendment offered by Democrats simply acknowledging warming of the Earth.
Stanford University professor of communication and political science Jon Krosnick, who has polling on climate change for 15 years, thinks the skeptics are winning in Washington.
JON KROSNICK, Stanford University: The voices of skeptics on climate change are very loud in this country and particularly effective in Washington at the moment. But they're a very, very small group.
Less than 10 percent of Americans are confidently skeptical about climate change at the moment. And yet that group expresses its points of view so often and so vociferously that I believe they have got Washington confused at the moment.
SPENCER MICHELS: He says his polls, taken nationwide, show many Americans still worry about climate change.
JON KROSNICK: From the very beginning, we were surprised that large majorities, and in some cases huge majorities of Americans, expressed what you might call green opinions on the issue. They said they thought the planet had been gradually warming over the last 100 years. They thought human activity was responsible for it. And they supported a variety of government actions because they saw it as a threat.
SPENCER MICHELS: Krosnick says that neither storms nor the recent drought that has been affecting the Midwest affect his poll numbers, which have remained steady for more than a decade.
However, other polls showed a significant decline in the number of Americans saying there is solid evidence global warming is occurring, a drop of 20 percent between 2008 and 2010, when belief started rising again.
And polls conducted by Gallup and other news organizations suggest the issue ranks lower on voters' top priorities. Watts says polls can be manipulated by how the question is asked. He's worried that those who believe in manmade climate change will have their way in Washington.
ANTHONY WATTS: Some of the issues have been oversold. And they have been oversold because they allow for more regulation to take place. And so the people that like more regulation use global warming as a tool as a means to an end. And so, as a result, we might be getting more regulation and more taxes that really aren't rooted in science, but more in politics.
SPENCER MICHELS: But Muller and others think action is exactly what is needed.
RICHARD MULLER: I expect we will have considerable warming. And I think, depending on the growth of China, between 20 years and 50 years from now, we will be experiencing weather that's warmer than Homo sapiens ever experienced. And I tend to think that's going to be bad and we should do something about it and we can do something about it.
SPENCER MICHELS: Doing something about global warming raises a host of other issues, including new regulations and the costs of reducing greenhouse gases, issues that inflame an already contentious debate.
JUDY WOODRUFF: Online, Spencer talks to climate skeptic Anthony Watts about politics and global warming.