Shifts in Water Cycle

 

Luis Samaniego, Permanent shifts in the global water cycle.Science387,1348-1350(2025).DOI:10.1126/science.adw5851

Ki-Weon Seo et al. , Abrupt sea level rise and Earth’s gradual pole shift reveal permanent hydrological regime changes in the 21st century.Science387,1408-1413(2025).DOI:10.1126/science.adq6529

Below are excerpts from the above cited articles

According to a recently published peer reviewed study, global water storage within the land (as groundwater, stream flow and lakes), the source of most fresh water storage,  has been steadily declining in this century. The study found a substantial loss of approximately 1.6 gigatonnes of water  in the early 21st century. Regions in East and Central Asia, Central Africa, and North and South America have shown substantial depletion in soil moisture over this period. The authors also claim the findings  indicate that this lost water within the land area has not recovered to previous levels. This ongoing and persistent decline suggests that the negative shift in soil moisture may be irreversible because of prolonged drought conditions and reduced precipitation in certain regions.

Generally, droughts begin with precipitation deficits, like we are having here in Northern Virginia. If this condition persists it leads to an overall depletion of terrestrial water storage, including soil moisture (SM), groundwater, and water in streams and lakes. However, there are huge challenges in measuring terrestrial water storage, especially the measurement of groundwater and root-zone soil moisture. Also, we had a limited understanding of terrestrial water storage depletion at continental scales until the development of satellite gravity missions. Observations from the Gravity Recovery and Climate Experiment, GRACE, (May 2002 to May 2017) and the  GRACE Follow-On (June 2018 to present) missions provide continental-scale observations of terrestrial water storage changes. These changes serve as a proxy indicator of hydrological drought.

The data from the GRACE missions measured a gradual global depletion of terrestrial water storage from 2005 to 2015 of approximately 1287 gigatonnes (Gt) of water. Since water on earth is neither created nor destroyed the loss of terrestrial water is found in the seas. Seo et al found that the water loss from the land was equivalent to about 3.52 mm of global mean sea level (GMSL) rise.

It is not yet certain that the terrestrial water storage loss is linked to decadal climate variations or to longer-term changes associated with a warming climate. The certain data from the GRACE missions is not long enough the data has only been available since 2002. Since the late 1990s, there have been reports of considerable declines in evapotranspiration associated with decreasing soil moisture and increasing atmospheric vapor pressure declines. Further study is necessary to determine the cause of these observances.

Seo et al. built a predictive model to create a comprehensive analysis of how global terrestrial water storage has changed over the past four decades. Three independent datasets were integrated into the model to try to validate the soil water depletion: terrestrial water storage anomalies from Gravity Recovery and Climate Experiment (GRACE), global mean sea level from satellite altimetry, and a century-long dataset on the movement of Earth’s rotational axis  (polar motion which was found to responds to shifts in terrestrial water storage).

This work is a good start. It asks questions that will require additional GRACE data or improved models to answer. Though it tries to use other data sources to validate and understand the trend, this work is an analysis of global terrestrial water variations over the past two decades only. It is still necessary to understand the interplay of a broad range of factors that influence precipitation and the transfer of water from land to atmosphere through evaporation and transpiration. Advanced land surface and hydrological models that can accurately represent these factors under the influence of changing climate would be necessary to observe and understand the evolution of terrestrial water storage.

Developing next-generation models that incorporate anthropogenic influences such as farming, large dams, and irrigation systems (water diversions and use) is essential. The ongoing advancements of a land surface modeling system by the European Centre for Medium-Range Weather Forecasts represents a step forward. These improvements could reduce uncertainties and enhance our understanding of the impacts of climate change on the global water cycle.