The AMS Glossary defines “surface temperature” as
“1. In meteorology, the temperature of the air near the surface of the earth; almost invariably determined by a thermometer in an instrument shelter. 2. In oceanography, the temperature of the layer of seawater nearest the atmosphere.”
The term “global-averaged surface temperature” is not defined by the Glossary.
This climate metric is important since it is the basis for so much of the discussion of climate change as discussed in the 2005 National Research Council report (e.g., see the text that begins at http://www.nap.edu/books/0309095069/html/19.html). As stated there,
“The concept of radiative forcing is based on the hypothesis that the change in global annual mean surface temperature is proportional to the imposed global annual mean forcing, independent of the nature of the applied forcing.”
Figure 1-4 of the National Research Council report shows that surface temperature change is central to the climate policy framework.
This temperature, of course, is actually a derived concept based on the global mean radiative forcing. However, in seeking to determine the change of this surface temperature over time, the procedure has been to use surface air and sea surface temperatures to measure this quantity. As shown most recently in our paper, Pielke Sr., and Matsui, 2005: Should light wind and windy nights have the same temperature trends at individual levels even if the boundary layer averaged heat content change is the same?, temperature trends are often a function of height near the surface. Which near surface or surface temperature change, if any of them, should be used? No single level is adequate.
Clearly, the concept of basing climate policy on such an ambiguously measured climate metric as a globally-averaged surface temperature change is inadequate with respect to actual human- and natural-caused climate change. We cannot actually measure such an average directly. Despite its extensive use and long pedigree in the literature and it use in assessments such as in the IPCC reports, a global-averaged surface temperature change based on surface air measurements is not a quantitatively accurate way to communicate climate science to policymakers.
A more appropriate metric for policymakers, for global warming, for instance, would be the global-averaged and regional-averaged patterns of changes in heat content in units of Joules as discussed in the weblogs of September 25th entitled “Is Global Warming Spatially Complex?” and of October 20th “Is Global Warming the Same as Climate Change?.” As recommended in the 2005 National Research Council report, we need to develop new climate metrics to accurately communicate to policymakers and move the science community beyond globally-averaged surface temperature change.