Climate skeptics' criticisms of the evidence for global warming make no difference to the emerging picture of a warming world, according to the most comprehensive, independent review of historical temperature records to date.
Scientists at the University of California, Berkeley, investigated several key issues that skeptics claim can skew global warming figures and found they had no meaningful effect on world temperature trends.
Researchers at the Berkeley Earth project compiled more than a billion temperature records dating back to the 1800s from 15 sources around the world and found that the average global land temperature has risen by around 1 degree Celsius (1.8 degrees Fahrenheit) since the mid-1950s.
This figure agrees with the estimate of global warming arrived at by major groups that maintain official records on the world's climate, including NASA's Goddard Institute for Space Studies in New York, the US National Oceanic and Atmospheric Administration (NOAA), and the Met Office's Hadley Centre, with the University of East Anglia, in the UK.
"My hope is that this will win over those people who are properly skeptical," Richard Muller, a physicist and head of the Berkeley Earth project, told the Guardian.
"Some people lump the properly skeptical in with the deniers and that makes it easy to dismiss them, because the deniers pay no attention to science. But there have been people out there who have raised legitimate issues," he said.
In the Berkeley Earth project, Muller sought to cool the debate over climate change by creating the world's largest open database of temperature records, with the aim of producing a transparent and independent assessment of global warming.
The initial reluctance of government groups to release all their methods and data, and the fiasco over emails from the University of East Anglia's Climatic Research Unit in 2009, gave the project added impetus.
The team, which includes Saul Perlmutter, joint winner of this year's Nobel Prize in Physics for the discovery that the universe is expanding at an increasing rate, has submitted four papers to the journal Geophysical Research Letters that describe their work to date.
Going public with results before they are peer-reviewed is not standard practice, but Muller said the decision to circulate the papers before publication was part of a long-standing academic tradition of sanity-checking results with colleagues. "We will get much more feedback from making these papers public before publication," he said.
Climate skeptics have criticized official global warming figures on the grounds that many temperature stations are poor quality, based largely in cities, and that data are tweaked by hand. However, the Berkeley study found that the so-called urban heat island effect, which makes cities warmer than surrounding rural areas, is locally large and real, but does not contribute significantly to average land temperature rises. This is because urban regions make up less than 1% of the Earth's land area.
And while stations considered "poor" might be less accurate, they recorded the same average warming trend.
"We have looked at these issues in a straightforward, transparent way, and based on that, I would expect legitimate skeptics to feel their issues have been addressed," Muller said.
Nevertheless, one prominent U.S. climate skeptic, Anthony Watts, claimed to have identified a "basic procedural error" concerning time periods used in the research, and urged the authors to revise the paper.
James Hansen, head of NASA's Goddard Institute for Space Studies, told the Guardian he had not read the research papers but was glad Muller was looking at the issue, describing him as "a top-notch physicist."
"It should help inform those who have honest skepticism about global warming," said Hansen. "Of course, presuming that he basically confirms what we have been reporting, the deniers will then decide that he is a crook or has some ulterior motive. As I have discussed in the past, the deniers, or contrarians, if you will, do not act as scientists, but rather as lawyers. As soon as they see evidence against their client (the fossil fuel industry and those people making money off business-as-usual), they trash that evidence and bring forth whatever tidbits they can find to confuse the judge and jury."
Peter Thorne at the Cooperative Institute for Climate and Satellites in North Carolina and chair of the International Surface Temperature Initiative, said: "This takes a very distinct approach to the problem and comes up with the same answer, and that builds confidence that pre-existing estimates are in the right ballpark. There is very substantial value in having multiple groups looking at the same problem in different ways.
"Openness and transparency is a must, particularly now with climate change being so politicized, but more to the point, with the huge socioeconomic decisions that rest on it."
Phil Jones, the director of the Climatic Research Unit at UEA who was at the center of the Climategate incident, said: "I look forward to reading the finalized paper once it has been reviewed and published. These initial findings are very encouraging and echo our own results and our conclusion that the impact of urban heat islands on the overall global temperature is minimal."
Some scientists were critical of the project and Muller's decision to release the papers before they had been peer reviewed. Peter Cox, professor of climate system dynamics at Exeter University said: "These studies seem to confirm the global warming estimated from the existing datasets, which is pleasing but not exactly a surprise to those of us who know how carefully the existing datasets are put together.
"It is surprising, however, that the authors believe that this news is so significant that they can't wait for peer review, especially when their conclusions aren't exactly revolutionary."
The Berkeley Earth project has been attacked by some climate bloggers, who point out that one of the funders is linked to Koch Industries, a company Greenpeace called a "financial kingpin of climate science denial."
Muller points out the project is organized under the auspices of Novim, a Santa Barbara-based nonprofit organization that uses science to find answers to the most pressing issues facing society and to publish them "without advocacy or agenda."
Other donors include the Fund for Innovative Climate and Energy Research (funded by Bill Gates), and the Department of Energy's Lawrence Berkeley Lab. The next phase of the project will focus on warming trends in the oceans.