A formerly sceptical climate scientist says human activity is causing the Earth to warm, as a new study confirms earlier results on rising temperatures.

In a US newspaper opinion piece, Prof Richard Muller says: "Call me a converted sceptic."

Muller leads the Berkeley Earth Project, which is using new methods and some new data to investigate the claims made by other climate researchers.

Their latest study confirms the warming trend seen by other groups. The project received funds from sources that back organisations lobbying against action on climate change.

The team argues that the good correspondence between the new temperature record and historical data on CO2 emissions suggests human activity is "the most straightforward explanation" for the warming.

The paper reiterates the finding that the land surface temperature has risen 0.9C just in the last 50 years.

In a piece authored for the New York Times, Prof Muller, from the University of California, Berkeley, said: "Three years ago I identified problems in previous climate studies that, in my mind, threw doubt on the very existence of global warming.

"Last year, following an intensive research effort involving a dozen scientists, I concluded that global warming was real and that the prior estimates of the rate of warming were correct. I'm now going a step further: Humans are almost entirely the cause.".....

.....Prof Michael Mann, director of the Earth Science System Center at Penn State University, said that there was "a certain ironic satisfaction" in seeing a study funded by the Koch Brothers "demonstrate what scientists have known with some degree of confidence for nearly two decades: that the globe is indeed warming, and that this warming can only be explained by human-caused increases in greenhouse gas concentrations".

Prof Muller, meanwhile, describes his own change in standpoint as "a total turnaround".

He explained: "These findings are stronger than those of the Intergovernmental Panel on Climate Change [IPCC], the United Nations group that defines the scientific and diplomatic consensus on global warming."