The GISS temperature record can no longer be trusted.

On or around about Nov. 14, 2009, GISS began to use a new global temperature data set that deleted temperature records relating to the late 19th and early 20th centuries.  Not only that, but the temperature data that remained in use were adjusted downward for dates preceding the year 2000 (increasingly downward the farther back in the temperature record they go), thereby creating or exaggerating “warming” trends for individual locations.

A discussion thread at [1] http://wattsupwiththat.com details the nature of the data fiddling.

From that discussion thread:

…See under “What’s New”:
[2] http://data.giss.nasa.gov/gistemp/graphs/

“Nov. 14, 2009: USHCN_V2 is now used rather than the older version 1. The only visible effect is a slight increase of the US trend after year 2000 due to the fact that NOAA extended the TOBS and other adjustment to those years.
Sep. 11, 2009: NOAA NCDC provided an updated file on Sept. 9 of the GHCN data used in our analysis. The new file has increased data quality checks in the tropics. Beginning Sept. 11 the GISS analysis uses the new NOAA data set. ”

The “visible effect” of the new, manipulated data set is far greater than the quoted note implies.  The data changes were without a doubt manufactured to create a warming trend where none exists or to exaggerate possibly existing warming trends.

This is not the first time that the gate keepers of climate data were caught in the act of large-scale falsifying of existing data.