The Earth’s land has warmed by 1.5C over the past 250 years and “humans are almost entirely the cause”, according to a scientific study set up to address climate change sceptics’ concerns about whether human-induced global warming is occurring.
Prof Richard Muller, a physicist and climate change sceptic 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 added that he now considers himself a “converted sceptic” and his views had undergone a “total turnaround” in a short space of time.
“Our results show that the average temperature of the Earth’s land has risen by 2.5F over the past 250 years, including an increase of 1.5 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,” 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 a collection of 14.4m land temperature observations from 44,455 sites across the world dating back to 1753. Previous data sets created by Nasa, the US National Oceanic and Atmospheric Administration, and the Met Office and the University of East Anglia’s climate research unit only went back to the mid-1800s and used a fifth as many weather station records.
The funding for the project included $150,000 from the Charles G Koch Charitable Foundation, set up by the billionaire US coal magnate and key backer of the climate-sceptic Heartland Institute thinktank. The research also received $100,000 from the Fund for Innovative Climate and Energy Research, which was created by Bill Gates.
Unlike previous efforts, the temperature data from various sources was not homogenised by hand – a key criticism by climate sceptics. Instead, the statistical analysis was “completely automated to reduce human bias”. The Best team concluded that, despite their deeper analysis, their own findings 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 1C 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 of the rise 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 has been “consistent with zero”. Volcanic eruptions were found to have caused short dips in the temperature rise in the period 1750–1850, but “only weak analogues” in the 20th century.