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Global change

The water cycle freshens up

Rivers are delivering increasing amounts of fresh water to the ocean. The cause seems to be the influence that higher concentrations of atmospheric carbon dioxide are having on water use by plants.

Measurements of stream flow around the world have documented an increase in the amount of water that runs off the continents and returns to the ocean1. This trend has been occurring since the beginning of the century, yet changes in precipitation over land do not sufficiently account for this increase. On page 835 of this issue, Gedney et al.2 identify an important contributor to increasing global runoff — decreased evaporation resulting from the influence of elevated atmospheric carbon dioxide on plant physiology (Fig. 1, overleaf).

Figure 1: Plants, CO2 and the global water cycle.
figure1

The balance between precipitation (P) and evaporation (E) over land determines the surface runoff (R), which returns water from the continents to the oceans. Plant photosynthesis plays an integral role in the global water cycle, by mediating the transfer of water from the land surface to the atmosphere. Elevated CO2 can lead to closure of leaf stomata, which reduces leaf water loss and thereby decreases overall continental evaporation. Gedney et al.2 show that this process, initiated by increased atmospheric CO2, can account for the increases in surface runoff observed over the past century.

Credit: EYE OF SCIENCE/SPL

Carbon dioxide is the currency of plant photosynthesis: plants take up CO2 from the atmosphere and incorporate it into their tissues in the form of organic carbon compounds. This uptake of CO2 is accomplished through plant stomata — small openings in the surface of leaves (pictured above) that open and close to allow the exchange of CO2 and other gases with the atmosphere. During gas exchange, water is inevitably lost to the atmosphere, again through stomatal openings. This is the process of plant transpiration, which, on a global scale, mediates the transfer of water from the soil into plant tissues, and out through stomata to the atmosphere. On vegetated land, plant transpiration can make a substantial contribution to total surface evapotranspiration, which represents the sum of plant transpiration and other surface evaporation.

Plants can regulate the opening and closing of stomata in response to changing environmental conditions; in a high-CO2 atmosphere they are more efficient in their use of soil moisture. The stomata do not open as much or for as long, and less water is lost from leaves to the atmosphere3. As a consequence, plants acquire enough carbon through their stomata with less water uptake from the soil. The result is that continental evapotranspiration is reduced, more moisture is left in the soil, and this additional surface water can lead to increased continental runoff4.

Using a technique known as ‘optimal fingerprinting’ (also known as ‘detection and attribution’), Gedney et al.2 show that this direct effect of elevated CO2 on plant transpiration is the dominant contributor to observed increases in continental runoff. Optimal fingerprinting is simply a statistical regression in which a model simulation is compared with observations to isolate which processes in the model are consistent with the observed data. If a model simulation is consistent with observations, the process that drives the model trend is said to be ‘detected’ in the observations; if the observed trend is also inconsistent with other plausible explanations, then the trend can be ‘attributed’ to a specific cause.

Gedney et al. investigated four plausible contributors to observed increases in runoff: climate change leading to changes in temperature and precipitation; land-use change and consequent changes in vegetation cover; so-called ‘solar dimming’, resulting from an increasingly hazy atmosphere; and the direct effect of CO2 on plant transpiration. The effects of each of these on surface runoff were simulated using a sophisticated land-surface and vegetation model, and the results of the model were compared with historical observations of continental runoff. The authors' analysis shows that model-simulated runoff trends are consistent with the observed trend only when the direct effect of CO2 on transpiration is included in the simulation. So they attribute increases in continental runoff over the past century to the physiological effect of elevated atmospheric CO2.

Detection and attribution has been widely used in climate science to attribute observed climate trends to both natural and anthropogenic causes5. Recent increases in surface temperature have been successfully attributed to increases in CO2 and other greenhouse gases6, as have changes in other climate variables such as ocean heat content7 and sea-level pressure8. Gedney and colleagues' research represents the first time that detection-and-attribution techniques have been used to successfully link changes in the functioning of terrestrial ecosystems to human influences on the atmosphere.

As with any statistical analysis, these results are only as sound as the model used, the experimental design and the quality of observations. As our understanding of the terrestrial biosphere and our ability to model and monitor it improves, contributors to observed runoff trends that were not considered in this study may well be identified. Gedney and colleagues' findings are nonetheless an important step forward in our understanding of the diverse and complex ways in which human activities are affecting the global climate system. The findings have implications for future surface warming and freshwater availability, both of which could increase if CO2 continues to affect surface-water fluxes as demonstrated here. This research also opens some fascinating avenues of investigation — such as the possibility of using records of river runoff to monitor the functioning of terrestrial ecosystems in response to climate change, or of studying how changes in runoff induced by elevated CO2 might affect ocean circulation.

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Matthews, D. The water cycle freshens up. Nature 439, 793–794 (2006). https://doi.org/10.1038/439793a

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