In our blog of July 11, we introduced the concept of moist enthalpy (see also Pielke, R.A. Sr., C. Davey, and J. Morgan, 2004: Assessing “global warming” with surface heat content. Eos, 85, No. 21, 210-211. ). This is an important climate change metric, since it illustrates why surface air temperature alone is inadequate to monitor trends of surface heating and cooling. Heat is measured in units of Joules. Degrees Celsius is an incomplete metric of heat.
Surface air moist enthalpy does capture the proper measure of heat. It is defined as CpT + Lq where Cp is the heat capacity of air at constant pressure, T is air temperature, L is the latent heat of phase change of water vapor, and q is the specific humidity of air. T is what we measure with a thermometer, while q is derived by measuring the wet bulb temperature (or, alternatively, dewpoint temperature).
To illustrate how important it is to use moist enthalpy, we can refer to the current heat wave in the southwest United States. The temperatures in Yuma, Arizona, for example, have reached 110°F (43.3°C), but with dewpoint temperatures around 32°F (0°C). In terms of moist enthalpy, if the temperature falls to 95°F (35°C) but the dewpoint temperature rises to 48°F, the moist enthalpy is the same. Temperature by itself, of course, is critically important for many applications. However, when we want to quantify heat in the surface air in its proper units in physics, we must use moist enthalpy.
In terms of assessing trends in globally-averaged surface air temperature as a metric to diagnose the radiative equilibrium of the Earth, the neglect of using moist enthalpy, therefore, necessarily produces an inaccurate metric, since the water vapor content of the surface air will generally have different temporal variability and trends than the air temperature.
There are quite a few other issues with using the global-averaged surface temperature to characterize climate change (see NRC 2005, Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties). The realization that temperature is an incomplete measure of heat adds another problem to its use.