Call me a converted skeptic. 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, after an intensive research effort involving a dozen scientists, I concluded that global warming was real. I'm now going a step further: Humans are almost entirely the cause.
My total turnaround, in such a short time, is the result of careful analysis by the Berkeley Earth Surface Temperature project, which I founded with my daughter Elizabeth. Our results show that the average temperature of the Earth's land has risen 2.5 degrees Fahrenheit over the past 250 years, including an increase of 1.5 degrees over the most recent 50 years. Moreover, it appears likely that essentially all of this increase results from the human emission of greenhouse gases.
These findings are stronger than those of the Intergovernmental Panel on Climate Change, the U.N. group that defines the scientific and diplomatic consensus on global warming. In its 2007 report, the IPCC concluded only that most of the warming of the prior 50 years could be attributed to humans. It was possible, according to the IPCC consensus statement, that the warming before 1956 could be because of changes in solar activity, and that even a substantial part of the more recent warming could be natural.
Our Berkeley Earth approach used sophisticated statistical methods developed largely by our lead scientist, Robert Rohde, which allowed us to determine land temperature much further back in time. 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.
The historic temperature pattern we observed has abrupt dips that match the emissions of known explosive volcanic eruptions; the particulates from such events reflect sunlight, make for beautiful sunsets and cool the earth's surface for a few years. There are small, rapid variations attributable to El Niño and other ocean currents such as the Gulf Stream; because of such oscillations, the "flattening" of the recent temperature rise claimed by some people is not, in our view, statistically significant.
What has caused the gradual but systematic rise of two and a half degrees? We tried fitting the shape to simple math functions (exponentials, polynomials), to solar activity and even to rising functions like world population. By far the best match was to the record of atmospheric carbon dioxide, measured from atmospheric samples and air trapped in polar ice.
Just as important, our record is long enough that we could search for the fingerprint of solar variability, based on the historical record of sunspots. That fingerprint is absent. Although the IPCC allowed for the possibility that variations in sunlight could have ended the "Little Ice Age," a period of cooling from the 14th century to about 1850, our data argue strongly that the temperature rise of the past 250 years cannot be attributed to solar changes.
How definite is the attribution to humans? The carbon dioxide curve gives a better match than anything else we've tried. 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.
