A prominent scientist who was skeptical of the evidence thatclimate change was real, let alone that it was caused by humans, now says he has made a "total turnaround." Richard Muller, a professor of physics at the University of California, Berkeley, says he has become convinced that "the prior estimates of the rate of warming were correct," and that humans are "almost entirely the cause" of that warming.

Muller co-founded the Berkeley Earth Surface Temperature (BEST) team two years ago in order to independently assess what he viewed as questionable evidence of global warming. In a series of papers published last year, BEST presented their statistical analysis of 1.6 billion temperature reports spanning the last 200 years, controlling for possible biases in the data that are often cited by skeptics as reasons to doubt the reality of global warming.

Their analysis indicated that global warming is real — that the average global land temperature has risen by 2.5 degrees Fahrenheit (1.4 degrees Celsius) since 1750, including 1.5 degrees F (0.9 degrees Celsius) in the past 50 years. The numbers closely agree with the findings of past studies by the National Oceanic and Atmospheric Administration (NOAA), NASA and others; but finally, they were rigorous enough to satisfy Muller.

Now, in a brand new study that probed the causes of that warming, the BEST team says it has cleared from blame the natural variations in Earth's climate that so often get implicated by skeptics. Muller and his colleagues implicate carbon dioxide emissions by humans as essentially the sole cause of global warming.

"The carbon dioxide curve gives a better match than anything else we've tried," he wrote Saturday (July 28) in a New York Times editorial. "Its magnitude is consistent with the calculated greenhouse effect — extra warming from trapped heat radiation. These facts don't prove causality and they shouldn't end skepticism, but they raise the bar: To be considered seriously, an alternative explanation must match the data at least as well as carbon dioxide does."

That's a high bar indeed. In graphs released with the new study, a red line representing the atmoaspheric concentration of CO2 crawls across the decades almost exactly tracing the black line representing the observed warming of the Earth. [What Are Climate Change Skeptics Still Skeptical About?]

By comparison, the study found that natural variability, including variations in the solar cycle, El Niño events and the Atlantic multidecadal oscillation (shifts in sea-surface temperatures that run in cycles), could have accounted for no more than 0.17 degrees Celsius of temperature variation — either warming or cooling — during the past 150 years. These natural forces are much subtler than the warming seen during the same time period.