Several years ago, we proposed the use of surface moist enthalpy as the prefered metric to diagnose the heat of the near surface air;
Pielke Sr., R.A., C. Davey, and J. Morgan, 2004: Assessing “global warming” with surface heat content. Eos, 85, No. 21, 210-211.
As we write in that paper [highlight added]
“Surface air temperature alone does not capture the real changes in surface air heat content of the Earth system. Even using the limited definition of the term “global warming,”the moisture content of the surface air must be included. Future assessments should include trends and variability of surface heat content in addition to temperature.”
In our follow-up paper
Davey, C.A., R.A. Pielke Sr., and K.P. Gallo, 2006: Differences between near-surface equivalent temperature and temperature trends for the eastern United States – Equivalent temperature as an alternative measure of heat content. Global and Planetary Change, 54, 19–32.
There is currently much attention being given to the observed increase in near-surface air temperatures during the last century. The proper investigation of heating trends, however, requires that we include surface heat content to monitor this aspect of the climate system. Changes in heat content of the Earth’s climate are not fully described by temperature alone. Moist enthalpy or, alternatively, equivalent temperature, is more sensitive to surface vegetation properties than is air temperature and therefore more accurately depicts surface heating trends. The microclimates evident at many surface observation sites highlight the influence of land surface characteristics on local surface heating trends. Temperature and equivalent temperature trend differences from 1982–1997 are examined for surface sites in the Eastern U.S. Overall trend differences at the surface indicate equivalent temperature trends are relatively warmer than temperature trends in the Eastern U.S. Seasonally, equivalent temperature trends are relatively warmer than temperature trends in winter and are relatively cooler in the fall. These patterns, however, vary widely from site to site, so local microclimate is very important.
A new paper has appeared which examines this issue on a global scale [which I read about on Climate Abyss in a comment from Peter Thorne in response to my comment to John Nielsen-Gammon on his latest post on the Texas drought and heat]
Peterson, T. C., K. M. Willett, and P. W. Thorne (2011), Observed changes in surface atmospheric energy over land, Geophys. Res. Lett., 38, L16707, doi:10.1029/2011GL048442
with the abstract
“The temperature of the surface atmosphere over land has been rising during recent decades. But surface temperature, or, more accurately, enthalpy which can be calculated from temperature, is only one component of the energy content of the surface atmosphere. The other parts include kinetic energy and latent heat. It has been advocated in certain quarters that ignoring additional terms somehow calls into question global surface temperature analyses. Examination of all three of these components of atmospheric energetics reveals a significant increase in global surface atmospheric energy since the 1970s. Kinetic energy has decreased but by over two orders of magnitude less than the increases in both enthalpy and latent heat which provide approximately equal contributions to the global increases in heat content. Regionally, the enthalpy or the latent heat component can dominate the change in heat content. Although generally changes in latent heat and enthalpy act in concert, in some regions they can have the opposite signs.”
The Peterson et al article is an effective examination with the current data analyses from the HadCRUH land dataset and Global Historical Climatology Network Monthly (GHCN-M) Version 3. It should, of course, be reevaluated when all of the uncertainties and biases we have identified, for example, in Pielke et al 2009, Klotzbach et al 2009 and Fall et al 2011 are remedied.
They show that global warming, as diagnosed from surface measurements over land, is actually larger than that diagnosed from the dry bulb temperature trends alone (as we also found in the Davey et al 2006 study). The analysis of moist enthalpy should also be extended into the lower troposphere in order to see if the divergence between the surface and lower tropospheric temperature trends that we identified in Klotzbach et al 2009 can be explained when the trends in water vapor are included.
I disagree, however, with the following text in Peterson et al
The heat content of the upper ocean has become a heavily utilized metric of global climate change [e.g., Palmer et al., 2010]. Some authors argue that the heat content of the surface atmosphere should also be a key metric. Indeed, the “concept of ‘global warming’ requires assessments of units of heat (that is, Joules)” according to Pielke et al. . Davey et al.  argue that global surface temperature is not a “proper” measure of the heat content of the Earth’s climate system; which is true as it is just a measure of temperature. But Pielke et al.  go even further to claim that “ignoring concurrent trends in surface air absolute humidity therefore introduces a bias in the analysis of surface air temperature trends” and that we “need to include absolute humidity in order to describe observed temperature trends.”
Temperature and humidity are distinctly different physical parameters as implied by their units of K and g kg−1, and they are measured by different instrumentation. Therefore, we do not understand how ignoring humidity could bias an analysis of temperature trends or why an assessment of humidity would be required in order to describe trends in temperature. We do, however, have concerns about the potential for the general public to misinterpret heat content analysis. Figure 1 shows that heat content tends to be decreasing in Australia despite increases in surface temperature. Presenting heat content as the primary metric for global warming could lead lay readers to erroneously perceive Australia as cooling – after all, its heat (content) is decreasing. Our concern is not just nomenclature. Heat content by any other name if used as a global warming metric has the potential to imply cooling even in places with increasing temperature simply because the location is becoming dryer.
The Peterson et al paper is incorrect that temperature and humidity are distinctly different physical parameters. They both are associated with heat as they clearly show in their equation 3 ; i.e.
H= C pT + L q
where C p is the heat capacity at constant pressure and L is the heat of vaporization.
A more important issue is that they have not recognized that the use of surface measurements to diagnose the radiative imbalance of the climate system requires the identification of where the heat from this imbalance goes to. Their paper is quite effective at bringing in the concept of the Bowen ratio, but they are not understanding that we need to monitor both types of heat (sensible; i.e. dry bulb and latent heat) in order to diagnose the heat content changes in the surface air, and to properly interpret the observed trends in the dry bulb temperatures.
Perhaps this would be clearer to them if instead of writing that we
“need to include absolute humidity in order to describe observed temperature trends.”
if we wrote that we
“need to include absolute humidity in order to correctly explain observed temperature trends.”
Finally, they write
“Presenting heat content as the primary metric for global warming could lead lay readers to erroneously perceive Australia as cooling – after all, its heat (content) is decreasing.”
However, if the heat content is decreasing because of drying, even though the dry bulb temperature trend is positive, it IS cooling in terms of Joules per kilogram of air! There is less heat energy in this air that when the moist enthalpy was higher.
I suggest that both dry bulb temperature and moist enthalpy trends and anomalies be presented in climate analyses and in model predictions. This will provide a more complete diagnosis of climate (and surface global warming) of the atmosphere than using the dry bulb temperatures alone.