WET Hydrology & Hydrogeology

November 20, 2024 - GRACE project scientists' report evidence from data gathered by the system that Earth’s total amount of freshwater dropped abruptly starting in May 2014 and has remained low ever since. Reporting in Surveys in Geophysics, the researchers suggested the shift could indicate Earth's continents have entered a persistently drier phase.

As observed by the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow On (GRACE-FO) missions, global terrestrial water storage (TWS), excluding ice sheets and glaciers, declined rapidly between May 2014 and March 2016. By 2023, it had not yet recovered, with the upper end of its range remaining 1 cm equivalent height of water below the upper end of the earlier range. Beginning with a record-setting drought in northeastern South America, a series of droughts on five continents helped to prevent global TWS from rebounding. While back-to-back El Niño events are largely responsible for the South American drought and others in the 2014–2016 timeframe, the possibility exists that global warming has contributed to a net drying of the land since then, through enhanced evapotranspiration and increasing frequency and intensity of drought. Corollary to the decline in global TWS since 2015 has been a rise in barystatic sea level (i.e., global mean ocean mass). However, we find no evidence that it is anything other than a coincidence that, also in 2015, two estimates of barystatic sea level change, one from GRACE/FO and the other from a combination of satellite altimetry and Argo float ocean temperature measurements, began to diverge. Herein, we discuss both the mechanisms that account for the abrupt decline in terrestrial water storage and the possible explanations for the divergence of the barystatic sea level change estimates.

There was a time when the world’s greatest cities were located on the banks of rivers. Water stored beneath the earth’s surface is a resource that has made life viable in many dry places across the planet. Groundwater consumption has become a critical element of urban development and the expansion of human populations into regions that would be otherwise uninhabitable. Much of the world’s groundwater reserves come from “fossil” aquifers. That means that water last replenished the aquifer a very long time ago: in the range of 10,000-20,000 years, when our planet was in its last glacial period. In many cases, if we use this water today, it won’t be recharged during our lifetimes. So there is a finite amount of water for use presently, and excessive withdrawal becomes unsustainable.