Weather stations are giving a true picture of global warming, the group found The Earth's surface really is getting warmer, a new analysis by a US scientific group set up in the wake of the "Climategate" affair has concluded.
The Berkeley Earth Project has used new methods and some new data, but finds the same warming trend seen by groups such as the UK Met Office and Nasa.The project received funds from sources that back organisations lobbying against action on climate change.
"Climategate", in 2009, involved claims global warming had been exaggerated.
Emails of University of East Anglia (UEA) climate scientists were hacked, posted online and used by critics to allege manipulation of climate change data.
Fresh start The Berkeley group says it has also found evidence that changing sea temperatures in the north Atlantic may be a major reason why the Earth's average temperature varies globally from year to year.
The project was established by University of California physics professor Richard Muller, who was concerned by claims that established teams of climate researchers had not been entirely open with their data.
He gathered a team of 10 scientists, mostly physicists, including such luminaries as Saul Perlmutter, winner of this year's Nobel Physics Prize for research showing the Universe's expansion is accelerating.
Funding came from a number of sources, including charitable foundations maintained by the Koch brothers, the billionaire US industrialists, who have also donated large sums to organisations lobbying against acceptance of man-made global warming.
"We were concerned that the climate scientists were not putting all their data into the public domain, whether using Freedom of Information rules or anything else," he told BBC News.
"Science should be open, and data should be open, as a matter of principle."The group's work also examined claims from "sceptical" bloggers that temperature data from weather stations did not show a true global warming trend.
The claim was that many stations have registered warming because they are located in or near cities, and those cities have been growing - the urban heat island effect.
The Berkeley group found about 40,000 weather stations around the world whose output has been recorded and stored in digital form.
It developed a new way of analysing the data to plot the global temperature trend over land since 1800.
What came out was a graph remarkably similar to those produced by the world's three most important and established groups, whose work had been decried as unreliable and shoddy in climate sceptic circles.
The Berkeley group's record of global land temperature mirrors existing ones closely The third is a collaboration between the UK Met Office and UEA's Climatic Research Unit (CRU), from which the e-mails that formed the basis of the "Climategate" furore were hacked two years ago.
"Our biggest surprise was that the new results agreed so closely with the warming values published previously by other teams in the US and the UK," said Professor Muller.
"This confirms that these studies were done carefully and that potential biases identified by climate change sceptics did not seriously affect their conclusions."
Since the 1950s, the average temperature over land has increased by 1C, the group found.
They also report that although the urban heat island effect is real - which is well-established - it is not behind the warming registered by the majority of weather stations around the world.
They also showed that in the US, weather stations rated as "high quality" by Noaa showed the same warming trend as those rated as "low quality".
'Time for apology' Professor Phil Jones, the CRU scientist who came in for the most personal criticism during "Climategate", was cautious about interpreting the Berkeley results because they have not been published in a peer-reviewed journal.
"I look forward to reading the finalised paper once it has been reviewed and published," he said.
"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."
The Berkeley team has chosen to release the findings initially on its own website.
They are asking for comments and feedback before preparing the manuscripts for formal scientific publication.
In part, this counters the accusation made during "Climategate" that climate scientists formed a tight clique who peer-reviewed each others' papers and made sure their own global warming narrative was the only one making it into print.
But for Richard Muller, this free circulation also marks a return to how science should be done.
"That is the way I practised science for decades; it was the way everyone practised it until some magazines - particularly Science and Nature - forbade it," he said.
"That was not a good change, and still many fields such as string theory practice the traditional method wholeheartedly."
This open "wiki" method of review is regularly employed in physics, the home field for seven of the 10 Berkeley team.
Bob Ward, policy and communications director for the Grantham Institute for Climate Change and the Environment in London, said the warming of the Earth's surface was unequivocal.
"So-called 'sceptics' should now drop their thoroughly discredited claims that the increase in global average temperature could be attributed to the impact of growing cities," he said.
"More broadly, this study also proves once again how false it was for 'sceptics' to allege that the e-mails hacked from UEA proved that the CRU land temperature record had been doctored.
"It is now time for an apology from all those, including US presidential hopeful Rick Perry, who have made false claims that the evidence for global warming has been faked by climate scientists."
Ocean currents The Berkeley group does depart from the "orthodox" picture of climate science in its depiction of short-term variability in the global temperature.
The El Nino Southern Oscillation (ENSO) is generally thought to be the main reason for inter-annual warming or cooling.
But by the Berkeley team's analysis, the global temperature correlates more closely with the state of the Atlantic Multidecadal Oscillation (AMO) index - a measure of sea surface temperature in the north Atlantic.
There are theories suggesting that the AMO index is in turn driven by fluctuations in the north Atlantic current commonly called the Gulf Stream.
The team suggests it is worth investigating whether the long-term AMO cycles, which are thought to last 65-70 years, may play a part in the temperature rise, fall and rise again seen during the 20th Century.
But they emphasise that anthropogenic global warming (AGW) driven by greenhouse gas emissions is very much in their picture.
"Had we found no global warming, then that would have ruled out AGW," said Professor Muller.
"Had we found half as much, it would have suggested that prior estimates [of AGW] were too large; if we had found more warming, it would have raised the question of whether prior estimates were too low.
"But we didn't; we found that the prior rise was confirmed. That means that we do not directly affect prior estimates."
The team next plans to look at ocean temperatures, in order to construct a truly global dataset.
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