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," said Muller. "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."