

This cartoon is a good representation of many of the problems involved in taking the temperature of the Earth. The most obvious problem, even without reference to the cartoon, is that there is not a single temperature for the Earth but an enormous number of local temperatures that span the range from the frozen Antarctic to the tropical jungles and arid deserts of Africa. How this array of local temperatures is combined to give a single measure of the planetary temperature is the difficult task that is briefly outlined here.

Among other problems associated with taking the Earth’s temperature are:

different instruments used by different observers

differences in locations from potential interferences such as heat sources

differences in height above the surface where the temperatures are taken

differences in when the readings are taken and which are recorded

Further complicating the issue is that there are many locations on the globe where there are no temperature measurements or where there have been only sporadic ones, such as over the oceans. Satellite measurements of atmospheric temperature beginning in 1979 have helped with coverage, particularly over the oceans, but the data are indirect measures based on black body radiation and require interpretation to convert to temperatures for comparison with data from direct thermometric instruments.

To take the Earth’s temperature and see how it is changing with time, three groups of scientists (two in the United States and another in the United Kingdom) analyze compilations of data from thousands of instrumental records over about the past 150 years. The groups are:

the National Aeronautics and Space Administration (NASA), Goddard Institute for Space Studies (GISS) at Columbia University, whose results are often labeled GISS or GISTEMP

the National Oceanic and Atmospheric Administration (NOAA), National Climatic Data Center (NCDC), whose results are usually labeled as NOAA or NCDC

a cooperative effort between the Hadley Centre for Climate Prediction and Research and the University of East Anglia’s Climatic Research Unit (CRU), whose results are usually labeled with some variation of HadCRU.

All three groups provide results for land and sea temperatures separately as well as for the entire land-sea globe. Recently, another analysis of land temperatures only from a University of California group, the Berkeley Earth Surface Temperature (BEST) study, was announced. All four analyses, shown superimposed in this figure, come to the same conclusion—the Earth’s land temperature has warmed by 0.9 °C in the past half century.