The study found that the Earth’s land has warmed by 1.5 degrees Celsius in the past 250 years. Photo: Justin McManus

THE Earth’s land has warmed by 1.5 degrees Celsius in the past 250 years and ”humans are almost entirely the cause”, according to a scientific study set up to address climate sceptic concerns about whether human-induced global warming is occurring.

Richard Muller, a climate sceptic physicist who founded the Berkeley Earth Surface Temperature (BEST) project, said he was ”surprised” by the findings. ”We were not expecting this, but as scientists, it is our duty to let the evidence change our minds.”

He said he considered himself a ”converted sceptic” and his views had received a ”total turnaround” in a short space of time.

”Our results show that the average temperature of the Earth’s land has risen by 2½ degrees Fahrenheit over the past 250 years, including an increase of 1½ degrees over the most recent 50 years. Moreover, it appears likely that essentially all of this increase results from the human emission of greenhouse gases,” Professor Muller wrote in an opinion piece for The New York Times.

The team of scientists based at the University of California, Berkeley, gathered and merged 14.4 million land temperature observations from 44,455 sites across the world dating back to 1753. Previous datasets created by NASA, the US National Oceanic and Atmospheric Administration, Britain’s Meteorological Office and the University of East Anglia’s Climate Research Unit had gone back only to the mid-1800s and used five times fewer weather station records.

The funding for the project included $US150,000 from the Charles G. Koch Charitable Foundation, set up by the billionaire US coal magnate who is a key backer of the climate sceptic Heartland Institute think tank. The research also received $US100,000 from the Fund for Innovative climate and Energy Research, created by Bill Gates.

Unlike previous efforts, the temperature data from various sources was not ”homogenised” by hand – a key criticism by climate sceptics – but, instead was ”completely automated to reduce human bias”. The BEST team’s findings, despite their deeper analysis, closely matched the previous temperature reconstructions, ”but with reduced uncertainty”.

Last October, the BEST team published results that showed the average global land temperature has risen by about one degree Celsius since the mid-1950s. But the team did not look for possible ”fingerprints” to explain this warming. The latest data analysis reached much further back in time but, crucially, also searched for the most likely cause for this rise in land temperature by plotting the upward temperature curve against suspected ”forcings”. It analysed the warming impact of solar activity – a popular theory among climate sceptics – but found that, over the past 250 years, the contribution of the sun is ”consistent with zero”.

Volcanic eruptions were found to have caused ”short dips” in the temperature rise in the period from 1750 to 1850, but ”only weak analogs” in the 20th century.

”Much to my surprise, by far the best match came to the record of atmospheric carbon dioxide, measured from atmospheric samples and air trapped in polar ice,” Professor Muller said. ”While this doesn’t prove that global warming is caused by human greenhouse gases, it is currently the best explanation we have found, and sets the bar for alternative explanations.” Professor Muller said his team’s findings went further and were ”stronger” than the latest report published by the Intergovernmental Panel on Climate Change.

In an unconventional move aimed at appeasing climate sceptics by allowing ”full transparency”, the results have been released before being peer-reviewed by the Journal of Geophysical Research. All the data and analysis may be freely scrutinised at the BEST website. This follows the pattern of previous BEST results.

Guardian News & Media