Last month the Berkeley Earth Surface Temperature Project released the findings of its extensive study on global land temperatures over the past century. Physics professor Richard Muller, who led the study, heralded the findings with a number of controversial statements in the press, including an op-ed in this newspaper titled "The Case Against Global-Warming Skepticism." And yet Mr. Muller remains a true skeptic—a searcher for scientific truth. I congratulate Mr. Muller and his Berkeley Earth team for undertaking this difficult task in the realm of climate.

The Berkeley study reported a warming trend of about 1º Celsius since 1950, even greater than the warming reported by the U.N.'s Intergovernmental Panel on Climate Change (IPCC). I disagree with this result, which perhaps makes me a little more of a skeptic than Mr. Muller.

Mr. Muller has been brutally frank about the poor quality of the weather-station data, noting that 70% of U.S. stations involve uncertainties of between two and five degrees Celsius. One could interpret the Berkeley study's results as confirmation of earlier studies and of the IPCC's conclusions, despite the poor quality of the stations used. But perhaps the issue is that the Berkeley study and the ones that came before suffer from common errors. I suspect that the temperature records still are affected by the urban heat-island effect—a term given to any local warming, whatever its cause—despite efforts to correct for this. The urban heat-island effect could include heat produced not only in urban areas, but also due to changes in land use or poor station siting. Therefore, I suggest additional tests:

1. Disassemble the "global average" temperature to get a better picture of what's going on regionally. This could involve plotting both the IPCC's and the Berkeley study's data only for tropical regions, separating the northern and southern hemispheres and testing for seasonal variation and differences between day and night.

2. Better describe what we can think of as the demographics of weather stations, a major source of possible error. The IPCC used 6,000 stations in 1970 and only about 2,000 in 2000. Let's examine their latitude, altitude and possible urbanization, and see if there have been major changes in the stations sampled between 1970 and 2000. For example, it is very likely that airports were used as temperature stations in both 1970 and 2000, because airport stations are generally of high quality. But airports are likely warming rapidly because of increasing traffic and urbanization. So if the number of airport stations remained constant at, say, 1,200 over that 30-year interval, the warming observed there might have increased between 20% and 60% over the same period of time, thereby producing an artificial warming trend.