A former climate-change skeptic now agrees most warming is caused by humans

In a remarkable New York Times Op-Ed, former climate change skeptic Richard Muller of the University of California, Berkeley, declares not only that global warming is real, but also that “humans are almost entirely the cause.” This is an even stronger statement than that of the Intergovernmental Panel on Climate Change, which in its 2007 report concluded only that “most” of the warming during the past half-century was attributable to human causes.

Muller’s Berkeley Earth group approached the problem by rigorously analyzing historic temperature reports. As he described their efforts,

We carefully studied issues raised by skeptics: biases from urban heating (we duplicated our results using rural data alone), from data selection (prior groups selected fewer than 20 percent of the available temperature stations; we used virtually 100 percent), from poor station quality (we separately analyzed good stations and poor ones) and from human intervention and data adjustment (our work is completely automated and hands-off). In our papers we demonstrate that none of these potentially troublesome effects unduly biased our conclusions.

Muller noted that their record of temperatures is long enough that they could search for the fingerprint of variability in the sun’s output reaching the earth. But Muller found no such fingerprint. Global warming is real.

So how much, if any, of this warming can truly be ascribed to human activity? The Berkeley Earth group found that the record of temperatures over the past 250 years fits the increasing emissions of carbon dioxide better than any other statistic they tried, and the magnitude of the change is entirely consistent with the known “greenhouse effect” of carbon dioxide. Muller cautions, “these facts don’t prove causality” and “shouldn’t end skepticism,” but they do raise the bar for any alternative explanation.

Muller’s concludes his New York Times article by declaring:

Science is that narrow realm of knowledge that, in principle, is universally accepted. I embarked on this analysis to answer questions that, to my mind, had not been answered. I hope that the Berkeley Earth analysis will help settle the scientific debate regarding global warming and its human causes.

Additional details can be found in Muller’s NY Times Op-Ed, and in technical papers available at the Berkeley Earth website.

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