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*
Pierre Simon Laplace, Laboratoire des Sciences du Climat et de
l'Environnement, Commissariat à l'Énergie Atomique, 91191 Gif sur
Yvette, France; and
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
Communicated by Inez Y. Fung, University of California, Berkeley, CA, August 3, 2007.
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.
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 (1–5).
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.
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 (8–11).
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.