May 11, 2010

Pan Paradox

Filed under: Droughts, Precipitation

One of the ongoing debates in the climate change world involves the popular prediction of more droughts, longer droughts, and droughts of greater intensity. The underpinnings of this prediction are easy to follow, so this is definitely a strong pillar in the climate alarmist camp. As the temperature increases, potential evapotranspiration (PET) will certainly increase. There are many equations describing the relationship between PET and temperature, and they all indeed show PET would increase should the temperature increase. The physics here is solid. So if PET increases, actual evaporation will increase in areas with even a small amount of soil moisture, and in the absence of some compensating increase in rainfall, soil moisture will be depleted. The combination of increasing temperatures and decreasing precipitation should all but guarantee the place will become drier thereby yielding the increase in drought duration, intensity, and frequency. There is always a drought somewhere on the planet to point to as evidence that this is really happening, will likely get worse in the future, and all the rest. We’ve all heard it a million times … “If we don’t act know, ______ will happen” (fill in the blank, but today, we will focus on droughts).

A search in the Technical Summary of the latest IPCC assessment includes the following summary statements regarding drought. IPCC states “More intense and longer droughts have been observed over wider areas, particularly in the tropics and subtropics since the 1970s. While there are many different measures of drought, many studies use precipitation changes together with temperature. Increased drying due to higher temperatures and decreased land precipitation have contributed to these changes [emphasis added].” Furthermore, they write “Although precipitation has increased in many areas of the globe, the area under drought has also increased. Drought duration and intensity has also increased. While regional droughts have occurred in the past, the widespread spatial extent of current droughts is broadly consistent with expected changes in the hydrologic cycle under warming. Water vapour increases with increasing global temperature, due to increased evaporation where surface moisture is available, and this tends to increase precipitation. However, increased continental temperatures are expected to lead to greater evaporation and drying, which is particularly important in dry regions where surface moisture is limited. [emphaisis added]” In both of these statements, we see the IPCC suggesting that greater evaporation in continental areas will lead to more and more droughts in the future.

The latest article on this subject was produced by five Chinese scientists with major institutions in China; the work was supported financially by the Chinese National Key Program, Key Projects in the National Science and Technology Pillar Program, and the Natural Science Foundation of China. Given the scientists involved and the financial support they received, it is obvious that the drought issue is taken quite seriously in China.

Zheng et al. begin their article noting that “Evaporation plays an important role in the global water and energy cycle. It has been previously reported that panevaporation has decreased in several regions of the world since the 1950s, along with a significant increase of air temperature; this inverse relationship has become known as the ‘panevaporation paradox’.” They are absolutely correct … despite all the predictions for increased drying, pan evaporation (that is, water evaporated from a large open container, or “pan”) measurements from many parts of the world continue to show decreases as temperatures have risen. There are many competing theories on why this is happening, and Zheng et al. state that “researchers ascribed the decreasing panevaporation to the decrease in diurnal temperature ranges, the decrease in solar radiation and the vapor pressure deficit, or the decrease in wind speed.” Once again, what seemed so simple (i.e., higher temperatures will increase PET rates) winds up being a lot more complex in the real world.

Zheng et al. capitalize on an extensive network of meteorological measurements, including pan evaporation measurements, located throughout the Haihe River Basin in China. If your geography is a bit rusty, the authors tell us “Located in northern China, the Haihe River Basin (HRB; 112ºE~120ºE and 35ºN~43ºN) has a total area of more than 318 × 103 km2, including two megacities, Beijing and Tianjin. The HRB is one of the most developed areas in China, with a population accounting for about 10% of the nation’s total population.” The map (Figure 1) should “orient” you to the study area as well as show the extensive network of meteorological stations in the basin.

Figure 1. Map of the Haihe River Basin (HRB) (from Zheng et al., 2009)

Much of the Zheng et al. article deals with equations that can translate standard meteorological measurements into estimates of PET. After establishing the equations, they compared estimates of PET with actual pan evaporation measurements taken in the study area. They showed an excellent relationship between actual and estimated PET values with 95% shared variance overall for the entire basin.

Figure 2 shows the variation and trend in annual pan evaporation measurements, annual pan evaporation estimates, and actual measurements by season. All of the plots show a marked decrease over the 1957 to 2001 study period, and the decrease is highly statistically significant in all but the autumn and winter seasons (when PET is low).

Figure 2. Long-term changes in annual PET measurements (Epan) and estimates (ETref) for the HRB.

Zheng et al. next analyzed the relationship between the PET values and temperature, wind speed, solar radiation, and vapor pressure, and to some extent, they solved the paradox in their study area. Vapor pressure (water in the air) has been increasing while solar radiation and wind speed have decreased; all three of these trends can cause the decrease in PET, irrespective of what’s happened to the temperature in China. The authors conclude “In this study, however, we found that increasing temperature indeed led to the increase of panevaporation, but this effect was offset by changes in other climate factors. The decreasing wind speed and solar radiation and the increasing vapor pressure resulted in the decrease of panevaporation in the HRB.” And finally, they state “The increasing mean temperature in the HRB could have resulted in the increase of panevaporation; however, this effect has been offset by a decrease of wind speed, a decrease of solar radiation, and an increase of vapor pressure in this region. Wind speed was the dominant factor in decreasing panevaporation in the HRB.”

Once again, there is always more to the story than what first meets the eye. Predictions for increased drying in continental areas need to be reassessed!


Zheng, H., X. Liu, C. Liu, X. Dai, and R. Zhu. 2009. Assessing contributions to panevaporation trends in Haihe River Basin, China. Journal of Geophysical Research, 114, D24105, doi:10.1029/2009JD012203.

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