Pan Evaporation Trends And Its Relation To The Diagnosis of Global Warming – Comments On A New Article By Roderick Et Al 2009

There is a new paper on long term trends of evaporation from land areas [thanks to Steve Short and Benny Pesier for alerting us to it].

The paper is

Roderick, Michael L. , Michael T. Hobbins and Graham D. Farquhar, 2009: Pan Evaporation Trends and the Terrestrial
Water Balance. I. Principles and Observations
. Geography Compass 3/2 (2009): 746–760, 10.1111/j.1749-8198.2008.00213.x

The abstract reads

“Pan evaporation is just that – it is the evaporation rate of water from a small dish located at the ground-surface. Pan evaporation is a measure of the evaporative demand over terrestrial surfaces. Declines in pan evaporation have now been reported in many regions of the world. The trends vary from one pan to the next, but when averaged over many pans, they are typically in the range of −1 to −4 mm a−2 (mm per annum per annum). In energetic terms, a trend of −2 mm a−2 is equivalent to −0.16 W m−2 a−1 and over 30 years this is a change of −4.8 W m−2. For comparison, the top-of-atmosphere forcing due to doubled CO2 is estimated by the Intergovernmental Panel on Climate Change (IPCC) to be ~3.7 W m−2. Hence, the magnitude of the pan evaporation trend is large. What is of even greater interest is the direction – a decline – given the well-established warming of the last 30–50 years.

In this article, the first in a two part series, we describe the underlying principles in using and interpreting pan evaporation data and then summarise the reported observations from different countries. In the second article, we describe the interpretation of the trends in terms of changes in the terrestrial water balance.”

The conclusion of the paper has the text

“Analyses of the pan evaporation data averaged over many pans from many different countries has revealed declines that are typically in the range of −1 to −4 mm a−2 (Table 1). In energetic terms, a trend of say −2 mm a−2 is equivalent to −0.16 W m−2 a−1. For comparison, estimates of the top of the atmosphere radiative forcing over the last 40 years are an order of magnitude lower at about 0.02 W m−2 a−1 (Hansen et al. 2005). Clearly, the pan evaporation trend is large, and combined with the fact that it has, on average, declined, warrants further and detailed investigation.”

This very interesting paper suggests that water fluxes from the land surface into the atmosphere have decreased despite reports of an increase in near surface air temperatures. It also further highlights the need to consider water vapor trends as well as dry bulb air temperature trends when using land surface observations as part of the diagnosis of T’ as discussed on page 21 in NRC (2005),  

                                                         dH/dt = f – T´/λ   (1)

where H is the heat content in Joules of the climate system, f is the radiative forcing at the top of the tropopause, T´ is the change in surface temperature in response to a change in heat content, and λ is the climate feedback parameter [which more accurately should be called the “temperature feedback parameter”. T´ is determined from land and ocean observations of surface air dry bulb temperature trends.

However, if T´based only on the dry bulb temperature is used in this equation, than an important aspect of diagnosing λ and calculating  dH/dt in response to the radiative forcing is missing, since trends in water vapor will affect the magnitude of T´.

As we show in

Pielke Sr., R.A., C. Davey, and J. Morgan, 2004: Assessing “global warming” with surface heat content. Eos, 85, No. 21, 210-211

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


Fall, S., N. Diffenbaugh, D. Niyogi, R.A. Pielke Sr., and G. Rochon, 2009:  Temperature and equivalent temperature over the United States (1979 – 2005). Int. J. Climatol., submitted

                                                           h = CpT + Lq   (2)

where h is the moist enthalpy, Cp is the specific heat at constant pressure, T is temperature, L is the latent heat of vaporization and q is the specific humidity. The units of h can be expressed as Joules per kilogram of air.

The radiative forcing in equation (1) affects both T and q. Thus, both T´ and q´need to be evaluated in order in order to ascertain if some of the radiative forcing (i.e. the term “f” in equation 1) has gone into evaporation of water (i.e. latent heat) rather than into T´ (the sensible heat).  If q´< 0, than T´ will overstate the actual warming in equation 1, while if q´> 0, than T´will understate the warming.

From the Roderick et al 2009 paper,  q´could be positive if the reduction of the evaporation is due to a moister atmosphere (i.e. higher average dew point temperatures) within the surface boundary layer (thus reducing the turbulent vertical flux divergence of water vapor), or q´ could be negative if the actual absolute humidities (i.e. the dew point temperatures) have decreased over time.  The answer to this question is needed.


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