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Water Limitations of Terrestrial Plants in a CO2-Enriched and Warmer World: Perception vs. Reality
Gerten, D., Schaphoff, S. and Lucht, W. 2007. Potential future changes in water limitations of the terrestrial biosphere. Climatic Change 80: 277-299.

Climate-alarmists typically predict increasing drought for many parts of the world as the air's CO2 content continues to climb; and they claim that this phenomenon will have huge negative consequences for earth's terrestrial plants. However, that seemingly simple conclusion ignores some important mitigating circumstances and feedback phenomena that suggest a significantly different outcome.

What was done
The authors of this intriguing paper "explore the impacts of projected atmospheric CO2 concentration increase and climate change on global soil moisture and, to appraise the ecological importance of soil moisture changes, on the degree to which they are actually felt by vegetation," which latter phenomenon is parameterized as LTA: "an index that quantifies the degree to which transpiration and photosynthesis are co-limited by soil water shortage," where high values of LTA indicate low water limitation. This assessment was conducted with the Lund-Potsdam-Jena dynamic global vegetation model that enables the "quantitative assessment of transient changes in vegetation and land surface hydrology in response to variations in climate and atmospheric CO2 concentration," as per Sitch et al. (2003) as modified by Gerten et al. (2004), with effects on plant water limitation being assessed as the differences between two 30-year time slices (1961-1990 and 2071-2100).

What was learned
After completing all of their analyses, Gerten et al.'s "main finding" was that "the final CO2 effect on LTA is a positive one," which they note "is in accordance with extensive experimental evidence (summarized e.g. by Amthor, 1995) and with results from other macro-scale modeling studies (e.g. Cao and Woodward, 1998; Kergoat et al., 2002; Levis et al., 2000; Betts et al., 2004)," which led them to further conclude that "LTA is a better proxy for limitation of plant growth and net primary production than soil moisture itself or precipitation.

What it means
In discussing the significance of their findings, the three researchers from the Potsdam Institute for Climate Impact Research say "the frequently opposite direction of change in soil moisture and LTA suggests that decreases in soil moisture are not necessarily felt by actual vegetation, which is attributable mainly to the physiological vegetation response to elevated CO2," and that "without this beneficial effect, the sign of change in LTA would be reversed from predominantly positive to predominantly negative." Consequently, even in a world of declining soil moisture content - which is itself just the opposite of what is observed in the real world (see Soil (Water Status - Field Studies) in our Subject Index) - the physiological benefits provided by the ongoing rise in the air's CO2 content tend to overpower whatever negative effects global warming might have on earth's plant life; and this is one of the reasons why the Greening of the Earth continues unabated over the entire planet.

Amthor, J.S. 1995. Terrestrial higher-plant response to increasing atmospheric [CO2] in relation to the global carbon cycle. Global Change Biology 1: 243-274.

Betts, R., Cox, P.M., Collins, M., Harris, P.P., Huntingford, C. and Jones, C.D. 2004. The role of ecosystem-atmosphere interactions in simulated Amazonian precipitation decrease and forest dieback under global climate warming. Theoretical and Applied Climatology 78: 10.1007/s00704-004-0050-y.

Cao, M.K. and Woodward, F.I. 1998. Dynamic responses of terrestrial ecosystem carbon cycling to global climate change. Nature 393: 249-252.

Gordon, W.S., Famiglietti, J.S. 2004. Response of the water balance to climate change in the United States over the 20th and 21st centuries: results from the VEMAP Phase 2 model intercomparisons. Global Biogeochemical Cycles 18: 10.1029/2003GB002098.

Kergoat, L., Lafont, S., Douville, H., Berthelot, B., Dedieu, G., Planton, S. and Royer, J.-F. 2002. Impact of 2 x CO2 on global scale leaf area index and evapotranspiration: conflicting stomatal and LAI responses. Journal of Geophysical Research 107: 10.1029/2001JD001245.

Levis, S., Foley, J.A. and Pollard, D. 2000. Large-scale vegetation feedbacks on a doubled CO2 climate. Journal of Climate 13: 1313-1325.

Sitch, S., Smith, B., Prentice, I.C., Arneth, A., Bondeau, A., Cramer, W., Kaplan, J.O., Levis, S., Lucht, W., Sykes, M.T., Thonicke, K. and Venevski, S. 2003. Evaluation of ecosystem dynamics, plant geography and terrestrial carbon cycling in the LPJ dynamic global vegetation model. Global Change Biology 9: 161-185.

Reviewed 30 May 2007