I have previously pointed readers to ClimateAudit. This site is primarily dedicated to detailed analysis of long-term temperature reconstructions. The basic idea is to subject the claims of unprecedented warming and the attribution of this warming to human activity to careful scrutiny. The level of statistical rigor in these analyses is often excellent, which is why I like it so much. However, this can be off-putting to some readers.
One recent theme of the site, however, is pretty accessible without a lot of math background.
A major analytical complaint about rising temperatures used to be what is called the Urban Heat Island effect. The basic idea is that the record of temperature increase over the past several decades has been distorted by increasing urbanization near the temperature sensors. A major paper by Jones in 1990 asserted that analysis of urban vs. rural sensors demonstrates this is not a real problem.
Recently, a number of scientists have started to look at the quality of the temperature measurement sensors themselves. Teams have begun to photograph the sensors. Look at this post, and scroll about a third of the way down to see one of my favorites. Unlike my assumption that these are pretty high tech, what you see is a wooden box about 5 feet from a trash burn barrel. Or go here to see an exhibit from a recent paper by Roger Pielke and a large number of coauthors. This looks like a box stuck on the back of a suburban house – directly above a BBQ grill. Gee, do you think these might have some temperature pollution?
Now, I think that there really has been an increase in global temperatures, but this does not give me huge confidence in the underlying data from ground sensors.