Influence Of Water Vapor Trends On Multi-Decadal Surface Temperature Trends – Part IV From Our JGR Paper

This weblog continues the discussion of the issues that we raised in our paper

Pielke Sr., R.A., C. Davey, D. Niyogi, S. Fall, J. Steinweg-Woods, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, R. Mahmood, S. Foster, R.T. McNider, and P. Blanken, 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Res., 112, D24S08, doi:10.1029/2006JD008229.

Weblogs on the first three sections of our JGR paper have already appeared (see, see see, and see),  along with an excellent follow up on the first section by Lucia Liljegren (see February 18, 2008 Guest weblog).

The current weblog discusses the role of surface air water vapor trends in terms of how surface air temperatures trends, used to diagnose global warming and cooling, can be misinterpreted. As we write in our paper

“Near-surface air temperature trends are also significantly influenced by trends in surface air absolute humidity over the same time period since even with the same amount of heat within the near-surface air, the heat would be distributed differently between sensible and latent heat of the air. This issue has not been investigated in the assessment of multidecadal surface air temperature trends…….Ignoring concurrent trends in surface air absolute humidity therefore introduces a bias in the analysis of surface air temperature trends.”

The equation that combines the effect of temperature and humidity on the heat content of surface air can be described as follows from Pielke et al. 2004:


(Thanks to Dan Hughes for pointing out the error in the previous version of the above figure!) 

An example of how T and TE often vary on the hottest summer afternoons in Colorado is illustrated in the figure below [Figure 11 in Pielke et al. 2005]


What is informative about the above figure is that the time of day with the largest amount of heat (in Joules per kilogram of air) measured at the surface is in the morning, not the afternoon! In this case, which is typical of many location where drier air is mixed downward from higher in the atmosphere during the afternoon, if the goal is to use surface temperature data to diagnose global warming, then using the maximum temperature during the day without including the amount of water vapor in the air will, in this case, result in a bias.

The neglect of including water vapor in the assessment of global warming is a serious oversight. There are the following consequences of neglecting this contribution to the heat content of surface air in the warm season at mid- and high-latitudes and all year in the tropics. For example, neglecting the effect of the change in surface albedo [which is a large effect also],

  1. When land is converted such as due to deforestation, during the growing season the maximum near-surface air temperature is likely to be higher yet the near-surface is water vapor content is lower since transpiration is reduced, compared to what it was previously.  As a result, the near-surface air heat content in Joules per kilogram is lower.
  2. When arid and semi-arid land is coverted to irrigation, during the growing season, them maximum temperature is likely to be lower yet the water vapor content will be higher than it was before the landscape conversion due to the added transpiration. The near-surface heat content is  higher.

 The effect of the albedo change would further reduce the near-surface  heat content in #1, since, except for dark soils, deforested surfaces generally have a higher albedo, and this a larger reflection of solar insolation.  With #2, the opposite typically occurs, as irrigated landscapes have lower albedos than the natural arid and semi-arid landscapes; thus thus the solar insolation absorbed at the surface (and this the near-surface heat content) is even higher.

What this means is that assessing the  global warming by using temperature alone will introduce a significant bias in the diagnosis. Since, for instance, the land conversion by deforestation in the tropics is much larger in areal extent than the conversion to irrigation in the tropics, the result is a yet another warm bias in the diagnosis of global warming if the near surface air temperature is used by itself. Even the extent of vertical mixing of drier air and warmer air downward during sunny afternoons (and thus the value of the daily maximum temperature) will result in a bias if this vertical mixing changes over time.

Clearly, this is yet another issue with respect to the diagnosis of global warming using a global temperature trend, which the IPCC has ignored.

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