The Climategate files revealed that the Hadcrut surface temperature series was fraudulent, and cast doubt on the Giss surface temperatures. Small Dead Animals and Watts up with that, and more soon discovered fraud in the Giss surface temperature series, and now fraud in the GHCN series.
Which is all the surface temperature series that support global warming. All of three them. Each one is fraudulent. Twentieth century warming has not been replicated. Excluding all temperatures series based on fraud, the remaining evidence indicates that the 1930s were as warm or warmer than the present.