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The authors emphasize the importance of land-cover change in forecasting future freshwater …


Biology Articles » Bioclimatology » Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends

Abstract
- Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends

Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends

Shilong Piao,* Pierre Friedlingstein,* Philippe Ciais,* Nathalie de Noblet-Ducoudré,* David Labat, and Sönke Zaehle*
*Institut Pierre Simon Laplace, Laboratoire des Sciences du Climat et de l'Environnement, Commissariat à l'Énergie Atomique, 91191 Gif sur Yvette, France; and
Laboratoire de Mécanisme de Transfert en Géologie, Unité Mixte de Recherche 5563, Centre National de la Recherche Scientifique/Institut de Recherche pour le Développement/Université de Paris Sud 14, Avenue Edouard Belin, 31400 Toulouse, France
To whom correspondence should be addressed. E-mail: pierre.friedlingstein@lsce.ipsl.fr
Communicated by Inez Y. Fung, University of California, Berkeley, CA, August 3, 2007.
Author contributions: S.P., P.F., and P.C. designed research; S.P., P.F., P.C., N.d.N.-D., D.L., and S.Z. performed research; S.P., P.F., and P.C. analyzed data; and S.P., P.F., P.C., N.d.N.-D., D.L., and S.Z. wrote the paper.
Received October 25, 2006.

Freely available online through the PNAS open access option. Proc Natl Acad Sci U S A. 2007 September 25; 104(39): 15242–15247.

 

Abstract

 
The significant worldwide increase in observed river runoff has been tentatively attributed to the stomatal “antitranspirant” response of plants to rising atmospheric CO2 [Gedney N, Cox PM, Betts RA, Boucher O, Huntingford C, Stott PA (2006) Nature 439: 835–838]. However, CO2 also is a plant fertilizer. When allowing for the increase in foliage area that results from increasing atmospheric CO2 levels in a global vegetation model, we find a decrease in global runoff from 1901 to 1999. This finding highlights the importance of vegetation structure feedback on the water balance of the land surface. Therefore, the elevated atmospheric CO2 concentration does not explain the estimated increase in global runoff over the last century. In contrast, we find that changes in mean climate, as well as its variability, do contribute to the global runoff increase. Using historic land-use data, we show that land-use change plays an additional important role in controlling regional runoff values, particularly in the tropics. Land-use change has been strongest in tropical regions, and its contribution is substantially larger than that of climate change. On average, land-use change has increased global runoff by 0.08 mm/year2 and accounts for ≈50% of the reconstructed global runoff trend over the last century. Therefore, we emphasize the importance of land-cover change in forecasting future freshwater availability and climate.
 
Keywords: atmospheric CO2, water cycle, climate change, land cover change

 

 

Climate change and human intervention are expected to strongly alter the global hydrological cycle in the coming decades (15). Previous reconstruction of global runoff data suggests that global river runoff increased significantly during the 20th century (2). However, it is difficult to estimate whether this trend in runoff is caused by natural or anthropogenic factors, because the characteristics and dynamic properties of the hydrological cycle depend on many interrelated links among climate, atmosphere, soil, and vegetation dynamics. Long-term changes in runoff depend on the balance of precipitation and evapotranspiration. The latter term is not only driven by climatic factors, such as temperature, wind speed, surface humidity, and solar radiation, but also is modulated by physiological (e.g., stomatal) and structural [e.g., leaf area index (LAI)] components of vegetation. It is well known that stomata respond to elevated atmospheric CO2 concentrations by partial closure (6). Accordingly, a recent modeling analysis suggested that the rising atmospheric CO2 concentration is the main driver of the observed increase in continental runoff (1). Nevertheless, the results of the Gedney et al. (1) study should be viewed with caution because only the direct effect of atmospheric CO2 concentrations on stomatal conductance was considered. Structural changes in vegetation in response to increased productivity under higher atmospheric CO2 levels, particularly changes in LAI (7), were not taken into account in their study.

Land use is another key factor controlling the water balance of ecosystems and the associated river runoff. Land use changes already impacted the terrestrial water cycle and will continue to do so in the next century (811). However, globally comprehensive analyses of the impacts of land-use change on runoff are scarce, especially compared with studies relating the effects of land-use change on the global carbon cycle. Here we investigate how historical changes in cropland establishment and abandonment, combined with atmospheric CO2 and climate change, have modified the regional and global runoff patterns. We used a process-based terrestrial biosphere model, organizing carbon and hydrology in dynamics ecosystems (ORCHIDEE) (12), to separately quantify the hydrological contribution of the driving factors.

 


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