PJ Media

http://a-sceptical-mind.com

has the best references to the falsify/corrupted data truth.

The Global Historical Climatology Network which represents most of the data uses an opaque data adjuestment system- measurements are changed without good justification. Yes adjustment is necessary because of many reasons including the atrocious site and measuring station management and local thermal effects. By the way, local thermal effects are more than thermal urban heat island, its also nighttime raised temperatures due to cities causing mixing surface and troposphere air and fields are cooler than forests due to reflection.