Detecting an externally forced signal in observed terrestrial water storage

The response of terrestrial freshwater storage to anthropogenic climate forcing is a fundamental yet poorly constrained aspect of global hydrological change. Detection and attribution studies have identified human influence in several components of the hydrological cycle, including precipitation and runoff (e.g. Zhang et al., 2007; Marvel et al., 2019; Gudmundsson et al., 2021). However, attribution of observed changes in terrestrial water storage (TWS) has remained elusive due to the short length of observational records, substantial internal climate variability, and the confounding influence of direct human water management.

Here we use observations from NASA’s Gravity Recovery and Climate Experiment (GRACE), which provide a uniquely robust, spatially explicit measure of terrestrial water storage change, together with a formal detection and attribution framework (Santer et al., 2013) informed by simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6). We show that the observed GRACE TWS record contains a spatially coherent signal that exceeds the range of simulated unforced variability, strengthens over time, and is robust across alternative fingerprint constructions and GRACE processing choices.

The detected fingerprint is characterised by large-scale wetting and drying patterns broadly consistent with modelled responses to anthropogenic forcing across many regions. Regional deviations are primarily concentrated in intensively irrigated and groundwater-dependent areas, indicating the superimposed influence of direct human water use and remaining model limitations. Additional analyses using reanalysis products and observationally constrained climate model simulations provide complementary context for interpreting the emergence of this signal. Together, these results provide the first fingerprinting evidence of anthropogenically forced change in global terrestrial water storage and establish continental freshwater storage as a detectable and attributable component of the climate system.

References

Zhang, X. et al. Detection of human influence on twentieth-century precipitation trends. Nature 448, 461–465 (2007).

Marvel, K. et al. Twentieth-century hydroclimate changes consistent with human influence. Nature 569, 59–65 (2019).

Gudmundsson, L. et al. Globally observed trends in mean and extreme river flow attributed to climate change. Science 371, 1159–1162 (2021).

Santer, B. D. et al. Identifying human influences on atmospheric temperature. PNAS 110, 26–33 (2013).