Ex-sceptic says humans to blame for global warming

It's a road-to-Damascus moment. Long-standing climate sceptic Richard Muller, a physicist at the University of California at Berkeley, now says there is strong evidence that humans are indeed warming the planet.

His results are under attack from all sides. Climate sceptics slate him for his apparent betrayal, while mainstream climate scientists say his results are based on outdated methods.

Muller is a director of Berkeley Earth Surface Temperature (BEST). Made up of avowed climate sceptics, BEST aims to determine whether the many criticisms of temperature records are valid.

Last year, BEST released a vast database of temperature readings going back to 1800, which confirmed that Earth has indeed warmed. In a new paper posted online, Muller says the warming is best explained by humanity's release of greenhouse gases.

Muller's team found they could account for the temperature changes with just two factors: the cooling effects of large volcanic eruptions, and changes in levels of atmospheric carbon dioxide.

Climate sceptic bloggers have attacked the analysis, complaining that the correlation between CO₂ and temperature doesn't prove a causal relationship.

Climate scientists are equally unimpressed, and for the same reason. "It's not attribution," says Phil Jones of the University of East Anglia in Norwich, UK. Since the 1990s, climate researchers have been running climate models with and without rising levels of CO₂ and have found that the observed warming can only be replicated if the human influence is included.

However, Michael Mann of Penn State University says it at least demonstrates intellectual honesty. Writing on his Facebook page, Mann says: "I applaud Muller and his colleagues for acting as any good scientists would, following where their analyses led them."

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