The global warming blogs have done an analysis of the surface temperature readings, from which noconsensus quotes:
leading meteorological institutions in the USA and around the world have so systematically tampered with instrumental temperature data that it cannot be safely said that there has been any significant net “global warming” in the 20th century.
Temperature readings have had “value added” to their data, with the “raw” surface station histories mysteriously changing from one download to the next, and surface stations that reported cooler readings have been mysteriously dropped.