Hidden Soil Moisture Effects Drive Land Carbon Uptake

The year-to-year change in the amount of carbon taken up by Earth's land ecosystems is primarily driven by variations in soil moisture which is affecting air humidity. This has been found by an international team of scientists including Markus Reichstein and Martin Jung from Max Planck Institute for Biogeochemistry in Jena, Germany. However, variations in soil moisture affect land carbon uptake mostly indirectly, through mechanisms coupling land ecosystems and atmospheric properties.

Earth's land ecosystems absorb a large portion of carbon dioxide emissions produced by human activities, helping to slow global warming. On average, the plants and soil take up, or fix, about 30 percent of human emissions. But from one year to the next, that number can be as high as 40 percent or as low as 20 percent. Climate scientists aim to pin down exactly what produces this variability so they can account for it and create the most accurate models for predicting future climate.

There has been significant debate within the climate modeling community over what exactly causes this so-called interannual variability. One side argues that these changes are mostly driven by atmospheric properties, such as temperature and air humidity near the surface, while the other side favors soil moisture.

A new study published in the April 1 issue of the journal Nature resolves this debate, showing that soil moisture is indeed in the driver's seat in terms of how much carbon dioxide is taken up by land ecosystems. But soil moisture acts only mostly indirectly, by affecting the air humidity and temperature, which in turn affect plants' ability to fix carbon. "Soil moisture is the driver, and temperature and humidity are the lever," says Vincent Humphrey, lead author on the new paper from California Institute of Technology (Caltech), USA.

To analyze the importance of soil moisture, the researchers ran simulations using climate models that fully integrate what is known about Earth’s land, oceans, and atmosphere. They simulated two different worlds: a reference planet with normal Earth conditions, and a hypothetical world that never experiences extremes in soil moisture - no droughts nor floods. “In the reference simulation, we saw the expected variability in carbon uptake by the land over time” says Martin Jung, group-leader at Max Planck Institute for Biogeochemistry, “but in the case of the hypothetical world, the year-to-year changes surprisingly disappeared.” This means that under stable soil moisture conditions, plants always fixed approximately the same percentage of human emissions. "We can therefore say with confidence that soil moisture plays a dominant role in the year-to-year change we see in the amount of carbon taken up by the land." says Humphrey.

But the researchers also realized that in the hypothetical world without drought or floods, there were far fewer events with elevated temperatures or reduced humidity than in the reference. This, they found, was due to a set of processes called land–atmosphere feedbacks, when land characteristics strongly control the atmosphere near Earth's surface. As an example, a drought produces hotter and drier air, since trees have less water available for transpiration to cool the atmosphere.

"Soil moisture significantly impacts near-surface temperatures and atmospheric humidity” confirms Markus Reichstein, Director at Max Planck Institute for Biogeochemistry. “Dry soils make extreme events such as heat waves more harmful, because the plants cannot re-moisten and cool the land surface”, he adds.

The scientists were surprised by the importance of these land–atmosphere feedbacks in terms of their effect on global carbon uptake. It turned out that the direct impact of changing soil moisture only accounted for about a quarter of the interannual variability. A surprising 75 percent came indirectly, as a product of changes to the temperature and air humidity. This means that during droughts, plants are unable to fix carbon not so much because there is less water in the soil but mainly because the atmosphere has quickly become hotter and drier as a result of the drought.

“These results add another piece to our notion that changes in the hydrological cycle are likely much more important for ecosystems than merely the global warming.” concludes Reichstein.

(Text adopted from a Caltech version by Kimm Fesenmaier)

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