The Earth’s land has warmed by 1.5C over the past 250 years and “humans are almost entirely the cause”, a new study has suggested.
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 of the scientific study set up to address climate change sceptics’ concerns about whether human-induced global warming is occurring.
“We were not expecting this, but as scientists, it is our duty to let the evidence change our minds,” the Guardian quoted him as saying.
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.
For the study, 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.
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.
“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,” 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,” he added.
The study will be published in the Journal of Geophysical Research. (ANI)