 About 40% of the observed warming can be explained away using the urban heat island effect. So to try to summarize that, basically a lot of these thermometers were put out there in an open field in a rural area. A city was built up around it, and so it gets warmer, obviously, not because there's global warming, but because there's a city around the thermometer. The rest of the warming, they found, can be easily explained using changes in solar activity.