Climate change skeptics for years counted UC Berkeley physicist Richard Muller in their ranks. Muller, a former McArthur award recipient, criticized the "hockey stick" graph of atmospheric CO2 versus time as a mathematical artifact (Muller 2004) and created a research institute, the Berkeley Earth Surface Temperature project, to examine the data more closely.
Based on his work, Muller now writes,
Our results show that the average temperature of the earth’s land has risen by two and a half degrees Fahrenheit over the past 250 years, including an increase of one and a half 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 United Nations group that defines the scientific and diplomatic consensus on global warming.
You can read Muller's op-ed, "Conversion of a climate change skeptic", in the New York Times and download his technical work at the BerkeleyEarth.org website.