Thanks to Valentine Anantharaj of the Geosystems Research Institute at Mississippi State University for alerting us to this very interesting new paper.
Shilong Piao, Pierre Friedlingstein, Philippe Ciais, Nathalie de Noblet-Ducoudre, David Labat, and Sönke Zaehle, 2007: Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends. PNAS, vol. 104, no. 39, 15242-15247.
The abstract reads
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.
The conclusions include the text
“Process-based simulations of global runoff using a terrestrial biosphere model suggest that the observed significant increase in global runoff in the 20th century was mainly a consequence of climate change and widespread deforestation. We estimate that the secular rise in atmospheric CO2 caused a small but significant decrease in global runoff because of the antagonistic responses of leaf-level processes and vegetation dynamics. On the basis of our findings, it seems overoptimistic to assume that rising CO2 could cause water savings in soil and thereby further promote vegetation productivity at a scale large enough to affect continental runoff. The results presented here not only provide insights for large-scale field experiments but also highlight the importance of biosphere feedbacks on the water balance of land surfaces …. The roles of vegetation growth feedbacks and land-use change cannot be ignored when projecting future changes in hydrologic processes and climate.”
The last sentence in the above extract from the conclusions that
“The roles of vegetation growth feedbacks and land-use change cannot be ignored when projecting future changes in hydrologic processes and climate”,
and the conclusion in the abstract that
“Land-use change has been strongest in tropical regions, and its contribution [to runoff] is substantially larger than that of climate change”
reinforces the conclusions presented, for example, in
Pielke Sr., R.A., 2005: Land use and climate change. Science, 310, 1625-1626.