The more I look into the questionable foundations of climatology, the more nervous I get. One BIG assumption in the data for the last 60 years or so has been that "urban heat islands" are not artificially raising the measured temperatures. The urban heat island problem is pretty easy to understand: if you take the temperature of downtown Boston, you'll get a warming reading than if you take the same reading at Walden Pond (same latitude but a lot less people).
When you aggregate temperature readings from thousands of stations scattered all over the globe, you want to make sure that you are measuring the heat of the planet, not just the increased heat from islands of furnaces and air conditioners (both of which make things hotter, due to the miracle of the Second Law of Thermodynamics).
This article goes into a long email correspondence with one scientist inside the Climate Research Unit (CRU) who was clearly uncomfortable with the evidence other scientists relied on to discount the Urban Heat Island problem. You have to read all the way to the bottom to see his point, but its a good one.