Rockstrom et al. estimate the order of magnitude of freshwater consumption needed to feed a population of 9 billion people by 2050 and the amount of water needed to realize the planet's full biomass carbon sequestration potential. They analyze these uses of freshwater in a framework of "planetary boundaries" within which the Earth system is resilient; beyond the boundaries, abrupt and irreversible change could take place. For instance, river ecosystems can collapse if water levels become too low due to water withdrawal for human uses.
On a global level, they find that achieving global food security and maximizing carbon sequestration together would require increasing water consumption by 3,250 cubic kilometers (780 cubic miles) per year. Combined with the current use of 2,600 cubic kilometers (624 cubic miles) per year, this would be above the safe planetary boundary level of about 5,000 cubic kilometers (1,200 cubic miles) per year.
They stress the need for societies to recognize the trade-offs between using water for food production and for increasing carbon sequestration through biomass. They conclude that since food production is essential, large-scale carbon sequestration through increasing biomass might not be realistic as a major mode of climate change mitigation.
Geophysical Research Letters, doi: 10.1029/2012GL051688
Title: The planetary water drama: Dual task of feeding humanity and curbing climate change
Authors: J. Rockstrom: Stockholm Resilience Centre, Stockholm University, Stockholm, Sweden;
M. Falkenmark: Stockholm Resilience Centre, Stockholm University, Stockholm,
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American Geophysical Union