GISS Surface Temperature Analysis

Background of the Analysis

The basic GISS temperature analysis scheme was defined in the late 1970s by James Hansen when a method of estimating global temperature change was needed for comparison with one-dimensional global climate models. The scheme was based on the finding that the correlation of temperature change was reasonably strong for stations separated by up to 1200 km, especially at middle and high latitudes. This fact proved sufficient to obtain useful estimates for global mean temperature changes.

Temperature analyses were carried out prior to 1980, notably those of Murray Mitchell, but most covered only 20-90°N latitudes. Our first published results (Hansen et al. 1981) showed that, contrary to impressions from northern latitudes, global cooling after 1940 was small, and there was net global warming of about 0.4°C between the 1880s and 1970s.

The early analysis scheme went through a series of enhancements that are listed and illustrated on the History Page.

Documentation and Assessment of Results

The analysis method was fully documented in Hansen and Lebedeff (1987), including quantitative estimates of the error in annual and 5-year mean temperature change. This was done by sampling at station locations a spatially complete data set of a long run of a global climate model, which was shown to have realistic spatial and temporal variability. This however only addresses the error due to incomplete spatial coverage of measurements. A more complete uncertainty analysis was published in Lenssen et al. (2019) and forms the core of the Uncertainty website.

As there are other potential sources of error, such as urban warming near meteorological stations, many other methods have been used to verify the approximate magnitude of inferred global warming. These methods include inference of surface temperature change from vertical temperature profiles in the ground (bore holes) at many sites around the world, rate of glacier retreat at many locations, and studies by several groups of the effect of urban and other local human influences on the global temperature record. All of these yield consistent estimates of the approximate magnitude of global warming, which reached about 0.8°C in 2010, twice the magnitude reported in 1981.

Further affirmation of the reality of the warming is its spatial distribution, which has largest values at locations remote from any local human influence, with a global pattern consistent with that expected for response to global climate forcings (larger in the Northern Hemisphere than the Southern Hemisphere, larger at high latitudes than low latitudes, larger over land than over ocean).

An updated documentation (Hansen et al. 2010) compares alternative analyses and addresses questions about perception and reality of global warming; various choices for the ocean data are tested; it is also shown that global temperature change is sensitive to estimated temperature change in polar regions, where observations are limited. A multi-year smoothing is applied to fully remove the annual cycle and improve information content in temperature graphs. Despite large year-to-year fluctuations associated with the El Niño-La Niña cycle of tropical ocean temperature, the conclusion could be made that global temperature continued to rise rapidly in the 21st century, new record heights being reached in every decade.

GISS Homogenization (Urban Adjustment)

One of the improvements — introduced in 1998 — was the implementation of a method to address the problem of urban warming: The urban and peri-urban (i.e., other than rural) stations are adjusted so that their long-term trend matches that of the mean of neighboring rural stations. Urban stations without nearby rural stations are dropped. This preserves local short-term variability without affecting long term trends. Originally, the classification of stations was based on population size near that station; the current analysis uses satellite-observed night lights to determine which stations are located in urban and peri-urban areas.

Monthly Updates

Graphs and tables are updated around the middle of every month using the current adjusted GHCN data. The new file incorporates reports for the previous month as well as late reports and corrections for earlier months.

We maintain a running record of any modifications made to the analysis on our Updates to Analysis page.

See Also

+ History of GISTEMP
+ GISTEMP Frequently Asked Questions
+ GISTEMP References
+ GISTEMP Uncertainty Quantification

Return to GISTEMP homepage