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from: http://data.giss.nasa.gov/gistemp/

GISS Surface Temperature Analysis What's New
Posted November 2006 data and updated new animations (Dec. 14, 2006)
(July 11, 2005)
The NASA GISS Surface Temperature Analysis (GISTEMP) provides a measure of the changing global surface temperature with monthly resolution for the period since 1880, when a reasonably global distribution of meteorological stations was established. Input data for the analysis, collected by many national meteorological services around the world, is the unadjusted data of the Global Historical Climatology Network (Peterson and Vose, 1997 and 1998) except that the USHCN station records up to 1999 were replaced by a version of USHCN data with further corrections after an adjustment computed by comparing the common 1990-1999 period of the two data sets. (We wish to thank Stephen McIntyre for bringing to our attention that such an adjustment is necessary to prevent creating an artificial jump in year 2000.)

These data were augmented by SCAR data from Antarctic stations not present in GHCN. Documentation of our analysis is provided by Hansen et al. (1999), with several modifications described by Hansen et al. (2001). The GISS analysis is updated monthly.

We modify the GHCN/USHCN/SCAR data in two stages to get to the station data on which all our tables, graphs, and maps are based: in stage 1 we try to combine at each location the time records of the various sources; in stage 2 we adjust the non-rural stations in such a way that their longterm trend of annual means is as close as possible to that of the mean of the neighboring rural stations. Non-rural stations that cannot be adjusted are dropped.

Our analysis includes results for a global temperature index as described by Hansen et al. (1996). The temperature index is formed by combining the meteorological station measurements over land with sea surface temperatures obtained primarily from satellite measurements, the HadISST data. Any uses of the temperature index data, i.e., the results including sea surface temperatures, should credit Reynolds, Rayner, Smith, et.al (2002). (See references.)

We limit our analysis to the period since 1880 because of the poor spatial coverage of stations prior to that time and the reduced possibility of checking records against those of nearby neighbors. Meteorological station data provide a useful indication of temperature change in the Northern Hemisphere extratropics for a few decades prior to 1880, and there are a small number of station record s that extend back to previous centuries. However, we believe that analyses for these earlier years need to be carried out on a station by station basis with an attempt to discern the method and reliability of measurements at each station, a task beyond the scope of our analysis. Global studies of still earlier times depend upon incorporation of proxy measures of temperature change. References to such studies are provided in Hansen et al. (1999).
Annual Summations
NASA news releases about the GISS surface temperature analysis are available for 2006, 2005, and 2004.
We also provide here discussions of global surface temperature trends for 2005, 2004, 2003, 2002, and 2001.
Table Data: Global and Zonal Mean Anomalies dTs
Anomaly values indicate difference from the corresponding 1951-1980 means.
Table of global-mean monthly, annual and seasonal dTs based on met.station data, 1880-present, updated through most recent month
Table of N. Hemi-mean monthly, annual and seasonal dTs based on met.station data, 1880-present, updated through most recent month
Table of S. Hemi-mean monthly, annual and seasonal dTs based on met.station data, 1880-present, updated through most recent month
Table of global-mean monthly, annual and seasonal land-ocean temperature index, 1880-present, updated through most recent month
Table of zonal-mean annual dTs, 1880-present, updated through most recent completed year
Table of zonal-mean annual land-ocean temperature index, 1880-present, updated through most recent completed year
Gridded Monthly Maps of Temperature Anomaly Data
Users interested in the entire gridded temperature anomaly data may download the three basic binary files from our ftp site. Also available there are two sample FORTRAN programs, "SBBX_to_1x1.f" and "sbbx2nc.f", which demonstrate how you can extract gridded anomaly files for any month and year from the larger datasets.
Data files for individual years may be obtained from the ftp site's subdirectories: bin for binary format, txt for ASCII text, and netcdf for netCDF. However, these files are updated irregularly and the latest year or two of annual data might not be available.

Anomalies and Absolute Temperatures
Our analysis concerns only temperature anomalies, not absolute temperatures. The temperature anomaly tells us how much warmer or colder than normal it is at a particular place and point in time, the 'normal temperature' being the mean over many (30) years (same place, same time of year). It seems obvious that to find the anomaly, you first have to know the current and normal absolute temperatures. This is correct for the temperature at one fixed spot (the location of one thermometer), but not true at all for regional mean temperatures.
Whereas the individual reading represents just this spot but can be very different from nearby readings, the anomaly computed from those readings is much less dependent on location, elevation, wind patterns etc; it turns out to be representative for a region that covers several square miles. Hence we can combine anomalies from various stations to find regional mean anomalies. Regional absolute temperatures however cannot be obtained from observations alone. For a more detailed discussion, see The Elusive Absolute Surface Air Temperature.

References
Please see the GISTEMP references page for citations to publications related to this research.
Copies of many of our papers are available in the GISS publications database. Re-prints not available there may be obtained by request from Dr. James Hansen.
Contacts
Please address scientific inquiries about the GISTEMP analysis to Dr. James Hansen.
Please address technical questions about these GISTEMP webpages to Dr. Reto Ruedy.
Also participating in the GISTEMP analysis are Dr. Makiko Sato and Dr. Ken Lo.

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GISS Website Curator: Robert B. Schmunk
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Page updated: 2007-08-07