1. Met Office Hadley Centre, Fitzroy Road, Exeter EX1 3PB, UK
2. School of Engineering, Computing and Mathematics, Exeter University, Exeter EX4 4QF, UK
3. Met Office, Joint Centre for Hydro-Meteorological Research
4. Centre for Ecology and Hydrology, Maclean Building, Wallingford, Oxfordshire, OX10 8BB, UK

Correspondence to: Richard A. Betts¹ Correspondence and requests for materials should be addressed to R.A.B. (Email: richard.betts@metoffice.gov.uk).

Abstract

In addition to influencing climatic conditions directly through radiative forcing, increasing carbon dioxide concentration influences the climate system through its effects on plant physiology¹. Plant stomata generally open less widely under increased carbon dioxide concentration², which reduces transpiration^{1, 3, 4, 5, 6} and thus leaves more water at the land surface⁷. This driver of change in the climate system, which we term 'physiological forcing', has been detected in observational records of increasing average continental runoff over the twentieth century⁸. Here we use an ensemble of experiments with a global climate model that includes a vegetation component to assess the contribution of physiological forcing to future changes in continental runoff, in the context of uncertainties in future precipitation. We find that the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by 6 per cent relative to pre-industrial levels; an increase that is comparable to that simulated in response to radiatively forced climate change (11  6 per cent). Assessments of the effect of increasing carbon dioxide concentrations on the hydrological cycle that only consider radiative forcing^{9, 10, 11} will therefore tend to underestimate future increases in runoff and overestimate decreases. This suggests that freshwater resources may be less limited than previously assumed under scenarios of future global warming, although there is still an increased risk of drought. Moreover, our results highlight that the practice of assessing the climate-forcing potential of all greenhouse gases in terms of their radiative forcing potential relative to carbon dioxide does not accurately reflect the relative effects of different greenhouse gases on freshwater resources.

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