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. 2007 Sep 25;104(39):15242-7.
doi: 10.1073/pnas.0707213104. Epub 2007 Sep 18.

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

Shilong Piao  1 , Pierre Friedlingstein, Philippe Ciais, Nathalie de Noblet-Ducoudré, David Labat, Sönke Zaehle
Affiliations

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

Shilong Piao et al. Proc Natl Acad Sci U S A. .

Abstract

The significant worldwide increase in observed river runoff has been tentatively attributed to the stomatal "antitranspirant" response of plants to rising atmospheric CO(2) [Gedney N, Cox PM, Betts RA, Boucher O, Huntingford C, Stott PA (2006) Nature 439: 835-838]. However, CO(2) also is a plant fertilizer. When allowing for the increase in foliage area that results from increasing atmospheric CO(2) 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 CO(2) 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/year(2) and accounts for approximately 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.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Change in global runoff anomalies from 1901 to 1999. (A) Comparison of global runoff change between that reconstucted by Labat et al. (2) and that modeled in simulation E3, including climate, atmospheric CO2, and land-use change. (B) Interannual variation and trend in modeled global runoff resulting from the effects of increased atmospheric CO2 (simulation E1), climate change (simulations E2–E1), land-use change (simulations E3–E2), and a decrease in stomatal conductance associated with rising atmospheric CO2 (simulation E4), respectively. The ORCHIDEE-simulated global runoff in simulation E3 shows a significant increasing trend with a rate of 0.17 mm/year2 during the last century, which is close to the estimation of 0.18 mm/year2 by Labat et al. (2). The global runoff trend derived from simulation E4 is estimated to be ≈0.16 mm/year2 for the period from 1960 to 1999, which is close to the study of Gedney et al. (1), who estimated a trend of ≈0.2 mm/year2 in response to the reduced leaf-level stomatal conductance because of rising atmospheric CO2 since 1960.
Fig. 2.
Fig. 2.
Change in the runoff anomalies during the 20th century in the different continental regions. All continents show that the runoff derived from simulation E3 is significantly correlated with that estimated by Labat et al. (2) (P < 0.02). Because of the small number (seven) of observing stations that were available in Africa since 1983, the trend in Africa runoff is calculated based on the period from 1901 to 1982. Trend_ob, runoff trend reconstructed by Labat et al. (2); Trend_climate, the modeled runoff trend because of climate change (simulations E2–E1); Trend_CO2, the modeled runoff trend because of increasing atmospheric CO2 (allowing LAI changes) (simulation E1); Trend_land, the modeled runoff trend because of land use change (simulations E3–E2). The sum of Trend_climate, Trend_CO2, and Trend_land reflects the runoff trend derived from simulation E3 that consider all factors change.
Fig. 3.
Fig. 3.
Spatial distribution of the trend in modeled runoff (A–D), precipitation (E), and fraction of agriculture area (F) over the last century. (A–D) Runoff trend because of the combined effects of climate, land use, and atmospheric CO2 (simulation E3) (A); increase in atmospheric CO2 (allowing LAI changes) (simulation E1) (B); climate change (simulations E2–E1) (C); and land use change (simulation E3-E2) (D).
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References

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