As global warming progresses, factors that promote drought and aridity will outweigh a gentle rise in precipitation, scientists predict, leading to a net increase in aridity over Earth’s landmasses. However, recent research suggests that the calculations behind these predictions may overestimate future dryness because they rely too much on indirect atmospheric factors.
Instead, some scientists have called for predictions based directly on projected changes in the water cycle itself, such as changes in runoff and soil moisture. To help resolve this issue, Berg et al. recently analyzed the future of soil moisture as predicted by 25 climate models from Phase 5 of the Coupled Model Intercomparison Project (CMIP5).
Although previous studies have focused mostly on soil moisture down to a depth of 10 centimeters, Berg and coauthors note that such analyses may be incomplete because, for example, many plants soak up water at depths of 2–3 meters. For a more comprehensive analysis, they examined previously simulated soil moisture at different depths down to 3 meters.
The researchers used data from such prior simulations of monthly changes in soil moisture around the world between 1976–2005 and 2070–2099. They found that soil moisture changes in the modeled data followed a vertical gradient, with surface layers losing more moisture than deeper layers. In northern middle to high latitudes, some regions (such as the northern United States, central Canada, and central Asia) showed annual moisture losses from surface soils, whereas total soil moisture increased annually.
The scientists interpret the vertical gradient seen in the models as a reflection of seasonal factors. In the summer, evaporation and thirsty plants soak up water from the surface, whereas deeper soil moisture is affected only by plants. In the winter, precipitated moisture infiltrates deeper layers, and much of this moisture remains throughout the year.
Overall, the findings support the idea that predictions of extensive drying based on atmospheric variables agree well with projected decreases in surface soil moisture but overestimate total soil moisture losses. The results highlight the importance of considering vertical water transport in soil when predicting the effects of global warming on water resources, agriculture, and ecosystems. (Geophysical Research Letters, https://doi.org/10.1002/2016GL071921, 2017)
—Sarah Stanley, Freelance Writer