Temperature is not the best metric to assess global warming. There are several reasons to use temperature when discussing global warming. The first is that most lay people can understand what a temperature is. Energy is a more difficult concept to grasp, even though they are essentially the same thing. To get the change in energy from a change in temperature, assuming nothing else changes, one need only multiply by a constant.

But as Dr. Pielke points out on his blog What Does Moist Enthalpy Tell Us?, there can actually be a decrease in surface temperature but an increase in surface energy if the water vapor content increases. He uses the example of Yuma, Arizona.

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.

In this post I will include the effects of moist enthalpy when calculating the globally averaged surface heat content. The procedure for this is quite simple. I download gridded temperature and specific humidity data from the NCEP/NCAR reanalysis. I then obtain a globally averaged temperature and specific humidity. Because the area of the grid cells decreases near the poles, an area average was used. From these two values, it is possible to calculate the near surface heat content changes.

This figure shows the change in near surface heat content (dQ) in blue. There are many wiggles in the time series, but there is an overall trend towards higher values. Notice that the large El Nino in 1998 is still plainly visible. Also, the current La Nina is visible in the large drop in energy at the very end of the time series. The calculated linear trend in the surface heat content is 2.827 Joules per gram of atmosphere per century. If we assume that the mass of the atmosphere is roughly constant at 5E18 kilograms, the trend in surface energy is about 1.4E20 J per year.

I’ve also added the thick red line, which shows a smoothed version of the blue line. The thin red line is the linear trend. Also, the purple dots are the decadal-averaged heat content. Note that all four of the graphical representations show an increase in surface heat content even when the changes in water vapor content are accounted for.

But instead of using the specific humidity to calculate the change in energy, we could also have calculated a equivalent temperature that accounts for the changes in water vapor content.

T_e = T + ^L/_cp * ^w/_(1-w)

where T is the measured temperature in Kelvin, L the latent heat of vaporization, Cp the specific heat of dry air at constant pressure, and q the specific humidity (from The Use of Equivalent Temperature to Analyze Climate Variability).

From this graph, it is clear that the surface equivalent temperature is rising, just like the actual surface temperature. In this analysis, the surface air temperature has a trend from 1948 to present of 0.110 C per decade (not shown). Thus when the changes in specific humidity are taken into account, there is an increase in the temperature trend; there is a larger change in the energy in the surface environment than diagnosed with just the surface air temperature.

Finally, we can compare the two terms of the surface heat content to see which one is larger.

The top panel of the above graph shows the heat content changes due only to changes in temperature. The middle panel shows heat content changes due only to changes in water vapor content. And the bottom panel shows the total changes in heat content (same as blue line in the first figure).

The trend in the sensible heat (CpT) is 1.27 g/J per century and the trend in latent heat (Lq) is 1.557 g/J per century. This means that not only in the surface water vapor content increasing, the energy from this is of the same order of magnitude as the sensible heat. Put another way, if global warming were to be framed as a change in surface energy as opposed to surface temperature, the degree of warming would be twice as large.

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