In the end, the team's result shows that the earlier studies "were done carefully and that potential biases identified by climate-change skeptics did not seriously affect" the conclusions these studies reached, said Dr. Muller, who some climate activists have labeled a global-warming skeptic.
The approach embodied in the main work "is very valuable, but may also need some refinement," says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.
Besides confirming the temperature trend, the Berkeley group says it was able to rule out the urban heat-island effect as a significant contributor to global warming.
And it was able to show that even with a large number of critical US recording stations operating inaccurately, those stations still showed long-term trends that were consistent with more reliable stations.
In essence, any given measuring station may be off compared with surrounding stations. But if it's off by a consistent amount, long-term trends will still show up.
The study also highlighted the regional differences in temperature trends that can lead people to say: What global warming?
Over the past 70 years, the team found that about one-third of the measuring stations in its global sample indicated cooling trends. Two-thirds showed warming trends, with warm regions more than offsetting cool regions in developing a global average.