Guest essay by Eric Worrall

A JPL led study has suggested construction of water reservoirs in the 1970s held back so much water from running into the sea it paused sea level rise.

Climate change: Dams played key role in limiting sea level rise

By Matt McGrathEnvironment correspondent

The construction of large-scale dams has played a surprising role in limiting rising seas, say scientists.

Over the past century, melting glaciers and the thermal expansion of sea water have driven up ocean levels.

But this new study finds that dams almost stalled the rising seas in the 1970s because of the amount of water they prevented from entering the oceans.

Without them, the annual rate of rise would have been around 12% higher.

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Read more: https://www.bbc.com/news/science-environment-53836018

The abstract of the study;

Published: 19 August 2020

The causes of sea-level rise since 1900

Thomas Frederikse, Felix Landerer, Lambert Caron, Surendra Adhikari, David Parkes, Vincent W. Humphrey, Sönke Dangendorf, Peter Hogarth, Laure Zanna, Lijing Cheng & Yun-Hao Wu 

The rate of global-mean sea-level rise since 1900 has varied over time, but the contributing factors are still poorly understood¹. Previous assessments found that the summed contributions of ice-mass loss, terrestrial water storage and thermal expansion of the ocean could not be reconciled with observed changes in global-mean sea level, implying that changes in sea level or some contributions to those changes were poorly constrained^2,3. Recent improvements to observational data, our understanding of the main contributing processes to sea-level change and methods for estimating the individual contributions, mean another attempt at reconciliation is warranted. Here we present a probabilistic framework to reconstruct sea level since 1900 using independent observations and their inherent uncertainties. The sum of the contributions to sea-level change from thermal expansion of the ocean, ice-mass loss and changes in terrestrial water storage is consistent with the trends and multidecadal variability in observed sea level on both global and basin scales, which we reconstruct from tide-gauge records. Ice-mass loss—predominantly from glaciers—has caused twice as much sea-level rise since 1900 as has thermal expansion. Mass loss from glaciers and the Greenland Ice Sheet explains the high rates of global sea-level rise during the 1940s, while a sharp increase in water impoundment by artificial reservoirs is the main cause of the lower-than-average rates during the 1970s. The acceleration in sea-level rise since the 1970s is caused by the combination of thermal expansion of the ocean and increased ice-mass loss from Greenland. Our results reconcile the magnitude of observed global-mean sea-level rise since 1900 with estimates based on the underlying processes, implying that no additional processes are required to explain the observed changes in sea level since 1900.

Read more: https://www.nature.com/articles/s41586-020-2591-3

A few mm / year of sea level rise is no threat to anybody’s wellbeing. But I find it fascinating that constructing a few dam projects was apparently enough to stall this allegedly serious climate threat.

If building a few dams in the 1970s was enough to stall sea level rise, there are plenty of other gigantic water projects on the drawing board which would likely stall sea level rise for a few more decades, such as the Egyptian Qattara Depression Project, the CSIRO Three Rivers Northern Catchment Scheme, or one of the many variations of the Bradfield Scheme. China has a large area of useless wasteland which could conceivably be filled with water. Even filling these natural depressions with sea water in many cases could improve the local microclimate.

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