Land Water Storage and Sea level

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Land Water Storage and Sea level

Land water storage (also referred to as terrestrial water storage) tells us how much water is stored on land, in sources including soil moisture, groundwater, rivers and reservoirs.

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Observed and projected global sea-level 1900-2100

Observed and projected global sea-level 1900-2100

Observed global mean sea level change (from the 1995-2014 average) for 1900-2018 and projected global mean sea level change for 2020-2100 for three future scenarios based on IPCC AR6 WG1. The solid lines are the central estimates for the observations and projections and the shaded area indicates uncertainty.

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Observed and projected land water storage component of sea-level 1900-2100

Observed and projected land water storage component of sea-level 1900-2100

Observed global mean contribution of land water storage to sea level change (from the 1995-2014 average) for 1900-2018 and projected global mean sea level change for 2020-2100 for three future scenarios based on IPCC AR6 WG1. The solid lines are the central estimates for the observations and projections and the shaded area indicates uncertainty.

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Contributions to sea level rise (in mm/year)

Contributions to sea level rise (in mm/year)

Contributions to sea level rise (in mm/year) for two time period, 2006-2018 and 1901-2018. The individual contributions are ocean warming, glaciers, antarctic ice sheet, greenland ice sheet and land water storage. The sum of these is shown (with its uncertainty) and compared to the observed global mean sea level (with its uncertainty). Note that the sum of the contributions and the observed global mean sea level are consistent with one another.

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Why is Land Water Storage important for understanding sea level change?

Water on land is important for the climate system as it sums multiple components, including soil moisture and water stored in vegetation, which affect plant life, as well as water channelled, retained, and removed by human activities, such as rivers, reservoirs, and groundwater. Land water storage saw a peak in the 1970s which was sufficient to slow down the global mean sea level rise in that period. Although land water storage is a small component of the sea level budget, it is important for ensuring that this budget ‘closes’, meaning that, within uncertainties, the sum of the contributors to sea level matches the observed sea level rise.

How has the amount of water stored on land changed?

Land Water Storage varied over the 20th century but is now contributing to rising sea levels. The most notable feature in the land water storage time series is the drop in sea level contribution in the 1970s. This was caused by increased reservoir building, thus retaining more water on land. Estimates of past land water storage come with large uncertainties, and there aren’t uniform uncertainties across the different components of land water storage. For this reason, the IPCC considers the contribution of Land Water Storage to sea level change as low confidence.

Why has Land Water Storage changed?

The IPCC states that changes in land water storage can be caused by human influence e.g., as a result of the building of reservoirs, extracting groundwater or deforestation, but that climate variations e.g., changes in the amounts of water in lakes and permafrost are also contributors. However, they do note that these climate variations can also be indirectly affected by human influence.

How is Land Water Storage projected to change?

Future land water storage is uncertain, but it is projected to have a small positive contribution to global mean sea levels over the coming century. The contribution will be smaller if all the dams currently planned are built, as these have a lowering effect on global mean sea level, while groundwater extraction makes a positive contribution to global mean sea level as the majority of groundwater extracted for use will eventually end up in the oceans.

How is Terrestrial Water Storage measured?

The Land Water Storage observations come from Frederikse et al., 2020 which sums a number of different components: the natural component, a water impoundment component, and a groundwater depletion component.

The natural component is obtained using a statistical model trained on GRACE satellite data and inputs from observational and reanalysis datasets ( Humphrey and Gudmundsson, 2019).

The rate of water impoundment in reservoirs is obtained from Chao et al., 2008 which itself takes data from the International Commission on Large Dams (ICOLDs). In their estimates Chao et al., account for small reservoirs, water seepage from reservoirs, and the fact that reservoirs will not always be filled to capacity. Frederikse et al., take these same considerations into account.

Groundwater depletion estimates for Frederikse et al., come from the GRACE satellite observations, hydrological models and well observations ( Wada et al. 2012, Döll et al., 2014). Their estimates also take into account the findings of Wada et al., 2016 who show that not all extracted groundwater will accumulate in the oceans e.g., some will be used for irrigation and return to being groundwater.

How are Land Water Storage projections made?

Land water storage projections are determined based on the known relationships between population size and groundwater depletion and population size and impoundment of water using dams. These projections are therefore more linear than the projections of other components and vary little by scenario. The projected relationship up to 2040 is adjusted for recent estimates of dam construction. Climate variability is not included in land water storage projections as its future changes are either not well quantified or are considered negligible.

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References

AR6 Projections

Terrestrial Water Storage

AR6 SSP1-2.6

AR6 SSP2-4.5

AR6 SSP5-8.5

Frederikse et al.

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Sea Level

Terrestrial Water Storage