The earth 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 Bill Gates’ Fund for Innovative Climate and Energy Research.
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 1 degree Celsius since the mid-1950s. But the team did not look for possible “fingerprints” to explain this warming.
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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”mthan 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, none of which have yet been published in peer-reviewed journals.
When the BEST project was first announced last year, the prominent climate sceptic blogger Anthony Watts was consulted on the methodology. He stated at the time: “I’m prepared to accept whatever result they produce, even if it proves my premise wrong.” However, tensions have since arisen between Mr Watts and Professor Muller.
Early indications suggest that climate sceptics are unlikely to accept BEST’s latest results fully. Professor Judith Curry, a climatologist at the Georgia Institute of Technology who runs a blog popular with climate sceptics and who is a consulting member of the BEST team, told The Guardian that the method used to attribute human emissions to the warming was “way over simplistic and not at all convincing in my opinion”.
She added: “I don’t think this question can be answered by the simple curve fitting used in this paper, and I don’t see that their paper adds anything to our understanding of the causes of the recent warming.”
Professor Michael Mann, the Penn State paleoclimatologist who has faced hostility from climate sceptics for his famous “hockey stick ” graph showing a rapid rise in temperatures during the 20th century, said he welcomed the BEST results as they “demonstrated once again what scientists have known with some degree of certainty for nearly two decades”.
He added: “I applaud Muller and his colleagues for acting as any good scientists would, following where their analyses led them, without regard for the possible political repercussions. They are certain to be attacked by the professional climate change denial crowd for their findings.”
Professor Muller said his team’s analysis suggested that over land in the next 50 years, there would be one and half degrees of warming, but, if China continued its rapid economic growth and its vast use of coal, then that same warming could take place in less than 20 years.
“Science is that narrow realm of knowledge that, in principle, is universally accepted,” wrote Professor Muller. “I embarked on this analysis to answer questions that, to my mind, had not been answered. I hope that the Berkeley Earth analysis will help settle the scientific debate regarding global warming and its human causes. Then comes the difficult part: agreeing across the political and diplomatic spectrum about what can and should be done.”