The study by University of Maryland scientists involved computer monitoring of over 2000 weather stations nationwide and comparing them to balloon and satellite data from the U.S. National Centers for Environmental Prediction and the National Center for Atmospheric Research.
The researchers are currently extending their methodology from a national to global scope. “We expect to extend our study to obtain global results later this year, but these findings for the United States already suggest that land use changes may account for between a third and a half of the observed surface global warming,” said Eugenia Kalnay.
You seem to have neglected the reports during the last several weeks about the study that the Little Ice Age lasted from 1300 to 1900. Of course we’ve been warming up since 1900, because we were colder than usual.
Boston Globe story.
PDF report.
And if you look around a little, you can find some data sets on the Internet and you can look at some such data yourself. Charting the temperature variations over the past 4,000 years from one study’s data was interesting, even by only using a spreadsheet.