Water evaporates from the ocean, rains over land, and eventually flows back to the oceans through rivers in what is commonly referred to as the "global water cycle.” Because changes in the water cycle can drive more or less water from land to ocean in given years, this cycling affects global sea level.
When more water is transported to land, it can remain in soils and forests for several years, causing an apparent drop in sea level. Similarly, when it rains less over land, more water stays in the ocean, causing an apparent rise in sea level. This year-to-year variability in the water cycle drives year-to-year changes in sea-level rise.
Additionally, human activities on land such as the pumping of ancient groundwater from deep aquifers drives more water into the ocean and causes additional sea-level rise. This process tends to increase sea level over the long term, as a result of growing human agricultural development. Finally, the historic construction of dams and reservoirs on river systems around the world has impeded the delivery of water to the ocean, contributing to a decrease in sea level.
Several processes combine to drive changes in land water storage which can then impact sea level. Due to the range of processes involved, many different observation systems are required to understand changes in land water storage. The GRACE and GRACE-FO satellites provide observations of global mass change using satellite gravimetry. These observations have shown that natural climate variability in the global water cycle can impact the rate of sea level change on shorter timescales, while also providing estimates of regional groundwater depletion.
Data from GRACE and GRACE-FO has provided a means to monitor continental water storage and groundwater changes globally. These observations offer a complement to large-scale hydrological models to estimate groundwater declines and the combination of models with the GRACE data can be used to verify model accuracy.
In addition to the GRACE and GRACE-FO satellites, several other observation systems provide information that improves our understanding of the global water cycle: Soil Moisture Active Passive (SMAP) , CloudSat, Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS), Atmospheric Infrared Sounder (AIRS), Microwave Limb Sounder (MLS).
In order to improve current estimates and monitor changing human and climate influences, it is important to continue monitoring the terrestrial water cycle, including surface water storage and groundwater storage, from space. The GRACE-FO mission will extend the observational record into the next decade for global mass change. Monitoring efforts from satellites coupled with in situ studies will provide a longer record and allow better determination of uncertain terms in the movement of water around the globe. Additionally, longer records of regional groundwater changes in response to changing water supply and demand will allow for better projection of these effects into the future.
In the next decade, the availability of large ensemble simulations of earth system models will allow for improved determination of human forcing and its influence on sea-level contributions associated with hydrology. Improvements in the resolution and complexity of land-surface models will also allow for better understanding of changing regional hydrology, including the inclusion of human activities and water consumption. These model developments will be supported by future observing platforms. For example, the Surface Water and Ocean Topography (SWOT) mission will provide the first observational estimate of global continental river discharge, thus improving our understanding of global and regional water cycles. Continued studies of terrestrial hydrology, river discharge, and the effects of river plumes on coastal ocean dynamics will deepen insight into the complex set of processes that can influence regional sea-level variability.