November 27, 2006

Dimming Fights Drought?

A recent article in Geophysical Research Letters by Rutgers’ scientists Alan Robock and Haibin Li addresses the issue of global warming and reduced soil moisture levels in important agricultural areas. Every popular global warming presentation lays out the case that higher temperatures in the future will cause higher levels of evaporation that will overwhelm any changes in precipitation and force soil moisture levels to drop. Of course, crops will fail, we will have more frequent and severe droughts of longer duration, and it will have all been caused by elevated carbon dioxide (CO2) levels. You’ve heard the story a 1,000 times by now.

Robock and Li note “Potential soil moisture changes from global warming, especially desiccation in growing seasons, are a grave threat to food security on which human society relies. Numerical models have been utilized to explore how water storage will change with global warming. Many models predict a decline of soil moisture over the midlatitudes of the Northern Hemisphere.”

But it was the first sentence of their abstract that grabbed our attention at World Climate Report as they drop the bomb “Summer soil moisture increased significantly from 1958 to the mid 1990s in Ukraine and Russia. This trend cannot be explained by changes in precipitation and temperature alone.” Oh no, just where the soil moisture should be drying up due to global warming, we find that soil moisture levels are going up. They write “The reported upward summer soil moisture trends for many Former Soviet Union stations are consistent with the decrease of pan evaporation around the same period for the same region, as pan evaporation can be thought as a direct measurement of the atmospheric evaporative demand.” (This work is a follow-on to an earlier examination of soil moisture trends in the Ukraine conducted by Robock and colleagues a few year ago, see here for our take on that one).

So what is causing soil moisture to increase? How about solar dimming and upward CO2 concentrations? Robock and Li note “Both ground-based observations and satellite measurements reveal a widespread reduction of solar irradiance from 1950s to 1990s and a gradual recovery afterwards, known as the ‘global dimming’ phenomenon. Increasing atmospheric aerosol loading from rapid industrialization is believed to be the culprit.” The argument is that global dimming has reduced solar radiation and “In the context of the hydrological cycle, the reported decline of shortwave radiation over such a long period may potentially increase water storage over land by damping evaporative demand from the atmosphere.”

There’s even more, and it just gets better and better! Robock and Li recall that “Many plant species tend to reduce stomatal openings with increasing atmospheric CO2. The concurrent higher canopy resistance reduces water loss through plant transpiration and thus may have profound impacts on the hydrological cycle” and that “A recent study suggests such CO2 effects are to a large extent responsible for continental runoff increases for the 20th century.”

To investigate the role of dimming and CO2 fertilization on soil moisture trends, Robock and Li “conducted experiments with a sophisticated land surface model.” The figure below shows the simulated and observed soil moisture in summer. The team wrote “The observed soil moisture for Ukraine exhibits an increase from 1958 to the early 1980s and then starts to level off afterwards. So does the soil moisture in the control but with much smaller magnitude. Imposed dimming brings additional soil moisture increase compared to the control. The stronger the dimming, the higher the increase in soil moisture” (Figure 1). And in terms of the effect of CO2, they found “The elevated CO2 caused very small soil moisture increases in contrast with the constant CO2 scenario. This is no surprise since in the elevated CO2 cases, evapotranspiration decreased by only about 0.1%.”


Figure 1. Observed and simulated June through August plant available soil moisture for 1958–2002. Correlation coefficients between simulations and observations for the time period of the observations.

They warn that “In spite of the relatively small CO2 effects as indicated from our study on decadal scales, increasing CO2 may be one of the most important modifiers of the water cycle for the past century and the effects may become more conspicuous if CO2 and concentrations continue to increase. To that end, these human-induced external forcings have to be better understood to reduce uncertainties in future predictions and for better water availability assessments.” They finish the article claiming that “reanalysis systems and the latest Intergovernmental Panel on Climate Change Fourth Assessment Report models cannot capture the magnitude of the observed soil moisture increase for Ukraine and Russia.”

Alan Robock and Haibin Li should be commended for their sophisticated work, their courage to criticize the models, and the good sense to examine empirical data that have a trend opposite to what is expected. We at World Climate Report say thanks for the interesting article!

Reference

Robock, A., and H. Li, 2006. Solar dimming and CO2 effects on soil moisture trends, Geophysical Research Letters, 33, L20708, doi:10.1029/2006GL027585.




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