January 20, 2010

Hydrocycle Looking Better than Ever

Filed under: Droughts, Floods, Precipitation

Of the many pillars that support the alarmist view of global warming is that droughts will increase in many parts of the world. This prediction is fairly straightforward, for if temperatures increase, potential evapotranspiration (ETo) should increase as well. If precipitation stays the same in the future and ETo increases with higher temperatures, the area would see a reduction in soil moisture and a trend toward drought. Of course should precipitation be reduced while ETo rates increase, the trend toward drought could be severe. In the ultimate alarmist view, ETo increase and extreme precipitation increases, and the area would then see an increase in both floods and droughts. We have heard it all before and we have covered these topics in many essays, but the beat goes on and on.

A very interesting paper appeared recently in Theoretical and Applied Climatology written by a team of scientists from the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences and the Policy Research Center for Environment and Economy, Ministry of Environmental Protection of the People’s Republic of China. The authors note that “This paper was financially supported by the National Natural Science Foundation of China, the President Fellowship of Chinese Academy of Sciences in 2007, and by the Knowledge Innovation Program of the Chinese Academy of Sciences.

The Yin et al. team basically reviewed the issue of climate change and its potential impact on ETo. They note that “In addition, one of the expected consequences of global warming is that the air near the surface should be drier, which should result in an increase in the rate of evaporation from terrestrial open water bodies”. The rate of evaporation from open water bodies is a decent measure of potential evapotranspiration, and throughout the world, ETo is estimated from evaporation pans like the one pictured below. A sentence in the Yin et al. article might come as a surprise to some who would insist that the warming world is producing higher rates of ETo. They authors reveal that “Some studies have reported significant decreasing trends in pan evaporation or ETo in recent decades over worldwide regions including European Russia, the western and eastern United States, Siberia, India, Israel, Australia and New Zealand”. So it appears something is happening in many parts of the world that is lowering ETo rates when the alarmists are looking for an increase in ETo.

Figure 1. Common evaporation pan

Yin et al. collected meteorological data, including pan evaporation rates for 595 stations throughout China for the period 1961 to 2008 (Figure 2). The global warming crusade will jump and cheer as they see maximum temperatures increasing at a rate of 0.22ºC per decade while minimum temperatures increased at a rate of 0.34ºC per decade. However, despite the warm-up in China, potential evapotranspiration has decreased significantly at a rate of 8.56 mm (a third of an inch) per decade. In their own words, Yin et al. note “Potential evapotranspiration has decreased significantly (at the 90% level of confidence) in China as a whole during the past 48 years. The trend of annual ETo was −8.56 mm/10a on average with a relative decrement of −5.04% compared to 48-year average value.”

Figure 2. Trends of potential evapotranspiration and meteorological variables (ETo, wind, sunshine, maximum temperature, minimum temperature, and relative humidity) since 1960s in China (from Yin et al., 2009)

In commenting on the decline in ETo, the team states “For China as a whole, the decreasing trend of ETo is primarily attributed to wind speed due to its significant decreasing trend and high sensitivity. Relative humidity is the highest sensitive variable; however, it has negligible effect on ETo for its insignificant trend. The positive contribution of temperature rising to ETo is offset by the effect of wind speed and sunshine duration.” Further commenting on the decline in wind speed, w, Yin et al. write “Although w is not the highest sensitive variable, it has declined significantly at a rate of −0.09 m s−1/10a in China with the relative change of −23.74%. Therefore, it was found to be the primary contributor which would cause ETo to decrease by −6.41% in the past 48 years.” They conclude “Increasing temperature is able to enhance ETo as generally expected. However, the contribution of temperature rising to ETo was offset, to a large extent, by the impact of the wind speed, according to its significant decrement and high sensitivity with ETo. Therefore, ETo has declined while climate warming has increased.”

Equally good news comes from a recent piece in the Journal of Climate from two scientists at Columbia University; their research was funded my NASA. Liepert and Previdi begin their article noting that “Changes in the global hydrological cycle associated with greenhouse gas–induced warming are one of the most important aspects of anthropogenic climate change. The common assumption is that global precipitation P will increase in a warmer world”.

The first few sentences of the abstract of their complex research seems like music to our ears. They state “Recently analyzed satellite-derived global precipitation datasets from 1987 to 2006 indicate an increase in global-mean precipitation of 1.1%–1.4% decade-1. This trend corresponds to a hydrological sensitivity (HS) of 7% K-1 of global warming, which is close to the Clausius–Clapeyron (CC) rate expected from the increase in saturation water vapor pressure with temperature. Analysis of two available global ocean evaporation datasets confirms this observed intensification of the atmospheric water cycle. The observed hydrological sensitivity over the past 20-yr period is higher by a factor of 5 than the average HS of 1.4% K-1 simulated in state-of-the-art coupled atmosphere–ocean climate models for the twentieth and twenty-first centuries.”

We get the message from these two articles – we are getting more rain and we are seeing a decline in potential evapotranspiration. The drought alarmists must be dismayed, but the “more flooding” crowd must rejoice. However, on balance, learning that potential evapotranspiration is trending downward and rainfall is increasing sounds to us at World Climate Report like a good thing for forests, grasslands, and agricultural crops throughout the world.


Liepert, B.G., and M. Previdi. 2009. Do models and observations disagree on the rainfall response to global warming? Journal of Climate, 22, 3156-3166.

Yin, Y, S. Wu, G. Chen, and E. Dai. 2009. Attribution analyses of potential evapotranspiration changes in China since the 1960s. Theoretical and Applied Climatology. DOI 10.1007/s00704-009-0197-7.

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