Quantifying human contribution to terrestrial drying

Historical drying trends have been demonstrated to occur over the lands surface, mostly in the subtropics and midlatitudes. Such drying trends are also widely projected to continue during the twenty-first century, especially under high greenhouse gas emission pathways.

The causes of terrestrial drying can be understood in terms of the effects of natural climate variability patterns, natural solar variability and volcanic eruptions, and human-induced greenhouse gases and aerosol emissions. Previous studies have effectively separated the causes of terrestrial drying by using formal detection and attribution (D&A) methods based on Earth system model (ESM) predictions with different combinations of forcing agents turned on and off. However, these studies focused on either meteorological drought, agricultural drought, or surface aridity indicators that did not reveal seasonal variations or vertical differences in moisture across soil layers.

To fill this research gap, LLNL scientist Celine Bonfils and collaborators at Oak Ridge National Laboratory used a Standardized Soil Moisture Index (SSI)—calculated from multi-source merged data sets—and a variety of simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to detect and attribute the changes in latitudinal terrestrial soil moisture between 1971–2016. For this, a pattern-based D&A method previously developed at LLNL was applied separately for each month of the year and each soil layer. The merged data sets use a comprehensive list of observations, reanalysis, and offline model simulations to reduce the potential biases and uncertainty caused by the limited sampling of source data.

The research team found greenhouse gases contributed significantly to the decrease in surface soil moisture (0–10 cm) from August to November and in root-zone soil moisture (0–100 cm) from September to April, with more rapid drying in 0–10 cm than 0–100 cm. Their findings demonstrate clear vertical, zonal, and seasonal patterns and highlight predominant human contributions to terrestrial drying, providing a basis for drought and flood risk management.

[Y. Wang, J Mao, F.M. Hoffman, C. Bonfils, H. Douville, M. Jin, P.E. Thornton, D.M. Ricciuto, X. Shi, H. Chen, S.D. Wullschleger, Quantification of human contribution to soil moisture-based terrestrial aridity, Nature Communications (2022), doi: 10.1038/s41467-022-34071-5.]