29 February 2024

Soil moisture rises despite climate change

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Precipitation, not temperature, explains soil moisture trends, a Harvard University study has found. It is critical to improve forecasts of long-term changes in rainfall in response to climate change

by Matteo Cavallito

 

Climate change is not making soils drier. Instead, it is promoting an increase in rainfall, which in turn leads to rising soil moisture. This is the conclusion of a study conducted by three Harvard University researchers and published in the journal Nature Water.

“Climate model simulations and different indices of aridity generally indicate a decrease in summer soil surface moisture in the continental United States as a consequence of anthropogenic climate change,” the research states. “However, soil moisture observations from ground probes and satellites from 2011 to 2020 indicate positive summertime trends across 57% of the continental USA.”

The investigation

The researchers compared ground observations conducted between 2011 and 2020, which is the short period of time for which adequate soil moisture measurements are available in the United States. They then combined this information with satellite data.

“To evaluate the mechanisms driving these trends, we have developed a two-layer land surface model that predicts surface temperature and soil moisture on the basis of observed variations in precipitation, solar radiation, vapour pressure and snowmelt,” the authors explain. “Of these four model forcings, we found that internal precipitation variability explains the largest fraction of the observed soil moisture trends from 2011 to 2020.”

Precipitation is crucial for soil

The increase in moisture that accompanies global warming may seem counterintuitive. The explanation for the phenomenon, however, is quite convincing and is related to the overall effect of all the forces at play. Increased temperature, for example, promotes soil drying. This phenomenon, however, “was largely balanced by CO2 fertilization, which allows plants to use water more efficiently,” says a statement from the university.

Both of these effects, in short, tend to cancel each other out, leaving precipitation as the main driver of soil moisture.

“Surface air warming and the response of plants to increasing atmospheric CO2 also influence the soil moisture trends, but these effects are small at decadal timescales and partly compensate for one another,” the study explains. “Looking forwards, our results indicate that internal precipitation variability will dictate decadal soil moisture trends and that centennial soil moisture trends will primarily depend on changes in precipitation that are currently highly uncertain.”

The threat of drought

The key role of rainfall implicitly emphasizes the problem of water scarcity. A recent study by Beijing Normal University published in the journal Climate and Atmospheric Science, in particular, pointed out that drought phenomena characterized by low soil moisture levels and large vapor pressure deficit pose “pose significant threats to terrestrial carbon sink and agricultural production.” Limiting, more than expected, the ability of vegetation to absorb the element.

Great uncertainty

Harvard researchers, meanwhile, highlight the importance of improving predictions of long-term changes in precipitation in response to climate change. Especially, the authors point out, in relation to food production. “We don’t have very accurate measurements of long-term soil moisture, but the consequences of high temperatures for agricultural yields have a lot to do with water availability,” explains Lucas Vargas Zeppetello, lead author of the research.

Plants, he adds, “are generally less sensitive to temperature if they have sufficient water, but in dry conditions they can get in big trouble.”

According to Peter Huybers, a professor in Harvard’s School of Engineering and Applied Sciences. the study’s results “suggest that reduced surface soil moisture is far from a foregone conclusion given the uncertainty in precipitation trends around the globe.” This uncertainty makes it “virtually impossible to predict soil moisture in the coming decades.” Making the development of sound crop water management strategies increasingly important as a result.