A new climate study shows that since the mid-1950s, global average temperatures over land have risen by 0.9 degrees Celsius (1.6 degrees Fahrenheit), confirming previous studies that have found a climate that has been warming – in fits and starts – since around 1900.
Most climate scientists attribute warming since the mid-1950, at least to some degree, to carbon dioxide emissions from human activities – burning coal, oil, and to a lesser extent gas, and from land-use changes.
The latest results mirror those from earlier, independent studies by scientists at NASA's Goddard Institute for Space Studies in New York, the Hadley Center for Climate Prediction and Research in Britain, and the National Oceanic and Atmospheric Administration (NOAA).
These previous efforts, however, came under fire from some climate-change skeptics who said they had detected serious flaws in the analytical methods and temperature records the three groups used.
The new research, which has yet to be formally published but which appears in four papers posted on BerkeleyEarth.org, uses new analytical techniques and a much larger set of records than the previous studies did.
Indeed, the new approach to analyzing temperatures records allowed the team to make use of partial and older records previous studies had rejected as unusable, explains Richard Muller, a physicist at the Lawrence Berkeley National Laboratory who coordinated the effort.
In the end, the team's result shows that the earlier studies "were done carefully and that potential biases identified by climate-change skeptics did not seriously affect" the conclusions these studies reached, said Dr. Muller, who some climate activists have labeled a global-warming skeptic.
The approach embodied in the main work "is very valuable, but may also need some refinement," says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.
Besides confirming the temperature trend, the Berkeley group says it was able to rule out the urban heat-island effect as a significant contributor to global warming.
And it was able to show that even with a large number of critical US recording stations operating inaccurately, those stations still showed long-term trends that were consistent with more reliable stations.
In essence, any given measuring station may be off compared with surrounding stations. But if it's off by a consistent amount, long-term trends will still show up.
The study also highlighted the regional differences in temperature trends that can lead people to say: What global warming?
Over the past 70 years, the team found that about one-third of the measuring stations in its global sample indicated cooling trends. Two-thirds showed warming trends, with warm regions more than offsetting cool regions in developing a global average.
Money for the new study, dubbed the Berkeley Earth Surface Temperature project, came from five foundations, including one established by Microsoft founder Bill Gates and another from the Charles Koch Charitable Foundation, widely seen as a source of money for conservative organizations and initiatives that have fought efforts to curb greenhouse-gas emissions.
The work makes no attempt to attribute the rising temperatures to any particular cause. Nor does it include ocean temperatures, the subject of a future study.
Still, this confirmation could help move the discussion toward solutions, suggests Caspar Ammann, another climate scientist at NCAR.
With minor differences, trends in all four independent study groups' temperature records match up well from about 1900 on, with the Berkeley and NOAA analyses showing a slightly higher level for the mid-2000s than the NASA and Hadley analyses.
"The rather irrational doubt and claims of a hoax simply don't make sense, and this work might help restart the discussion about what is next," Dr. Ammann says.
The team Muller assembled is not built from the usual cast of climate-science suspects, although Judith Curry, who head's Georgia Tech's department of earth and atmospheric sciences, is a member of the team.
Instead, Muller says he drew heavily on scientists from astrophysics and particle physics with expertise in teasing convincing, reproducable evidence from enormous masses of hard-to-analyze data.
Among them: Saul Perlmutter, who earlier this month shared a Nobel Prize in Physics with two other scientists for discovering that the universe is expanding at an increasing pace. The data Dr. Perlmutter and his colleagues used to stun the world of cosmology – results that others later confirmed – were far less abundant and far harder to analyze than temperature records, Muller says.
The team's independence and its willingness to devise its own analytical methods to provide a reality check on the three other groups' results sold the Koch foundation on the project, Muller adds.
Beyond the immediate results, the group is trying to make such work more transparent to other scientists than critics say has been the case in recent climate science.
The team posted the four papers on the BerkeleyEarth.org website in advance of their publication, or even acceptance for publication, in peer-reviewed journals. In addition, the team posted the data and the computer programs used to process and analyze them.
That move has drawn criticism from some climate skeptics who intially supported the group's efforts, arguing that the team was more interested in publicity than in following proper scientific protocol of submitting to a journal and awaiting the verdict of anonymous reviewers.
But for more than a decade, researchers in physics, astrophysics, astronomy, and other disciplines have routinely posted papers-in-progress on public websites for review by colleagues. It's a way of getting an initial reality check on research before engaging the formal publication process.
Some researchers say they doubt this approach will work well for climate science, despite criticisms surrounding what some see as the difficulty of getting other researchers' data or access to the programs they used.
Although the approach can lead to overloaded e-mail inboxes, "I'm quite in favor of these new ways of getting work out and allow a broader set of eyes to provide feedback," says Ammann at NCAR.
[Editor's note: The original headline was changed to correct an inaccuracy about who funded the new climate study. Moreover, the original headline inferred that the Charles Koch Charitable Foundation may not have funded the research if it had known what the outcome would be, although there is no indication that is the case.]