More and more water is pouring into the sea from the continents. A consequence of global warming? Definitely - but in an unexpected way.
Everything seems to be clearly regulated in our planet's water cycle: Evaporated water from soil, bodies of water and plants forms clouds that unload their cargo at the same or different locations. The local difference between precipitation and evaporation ultimately ends up in the surface and underground water veins of the continents and flows into the oceans. However, according to level measurements over the last few decades, they are filling the oceans' bathtubs full and full. What increases the outflow?
A popular suspect in such questions is global warming, which is known to change precipitation patterns. However, reduced solar radiation due to aerosols may also have an effect here: it would lead to a reduction in evaporation from open water surfaces. Not to forget the influence of land use and irrigation, which, among other things, affect the reflection properties of the land surface and thus the energy balance, or also help determine the depth of the soil at which plant roots can still get hold of the precious water.
One observation, one unknown cause, many possible factors - only models and statistics can help to solve the puzzle. So Nicola Gedney of the Hadley Center for Climate Prediction and Research and her colleagues fed the data to a simulation of the land surface hydrological cycle, keeping a different parameter constant in each run. With a sophisticated analysis, they then tracked down the culprit.
It's the usual suspect - climate change - but not in the expected way. Because the change in precipitation patterns was reflected in the changed discharge data on all continents, but did not explain the long-term increase over the decades. The other factors - reduced solar radiation from aerosols and land use - did not provide suitable results either.
But the scientists had integrated another factor that at first glance might seem negligible: less perspiring plants. Because with increasing carbon dioxide concentrations, these close their respiratory pores, the stomata or stomata, more often - after all, they are now being supplied with an excess of the basic material for photosynthesis. However, when they close, they also lose less water vapor, which is why they need less moist replenishment from the ground. What is saved above is left over below and winds its way down winding paths into the groundwater and ultimately into the rivers.
Only if this microscopic effect is taken into account, as Gedney and her colleagues found, does their simulation correctly reproduce the actually observed global increase in runoff. However, the researchers do not claim to have found the only solution to the riddle - after all, there are other influences that they had not taken into account. Obviously, this direct influence of CO2 on the metabolism of plants should not be forgotten when it comes to future forecasts.
Speaking of future forecasts: To what extent does the increased stomata closure possibly affect the global climate beyond the runoff effect? Since the energy flow also changes with the reduced evaporation, stopping sweating could turn out to be an unexpectedly central cog in other areas of the climate mechanism. Apart from that, forecasts of fresh water supplies and thus the drinking water supply are also related to it. And a whole new field of research may open up for scientists: how long-term data from river levels can be used to explain the effect of global warming on vegetation.