THIS POST IS A CRITICAL REVIEW OF A PAPER PUBLISHED IN NATURE ON THE IMPACT OF DAMS ON SEA LEVEL RISE.

CITATION: Frederikse, T., Landerer, F., Caron, L. et al. The causes of sea-level rise since 1900. Nature 584, 393–397 (2020). https://doi.org/10.1038/s41586-020-2591-3

ABSTRACT: The rate of global-mean sea-level rise since 1900 has varied over time, but the contributing factors are still poorly understood. 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. 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.

Estimating the sea-level budget: To obtain estimates of changes in global ocean mass (barystatic changes), we combine estimates of mass change for glaciers, ice sheets and terrestrial water storage (TWS). For the TWS estimate, we consider the effects of natural TWS variability, water impoundment in artificial reservoirs and groundwater depletion. For 2003–2018, we use observations from the Gravity Recovery and Climate Experiment (GRACE)29 to quantify the barystatic changes. We estimate changes in sea level due to global thermal expansion (thermosteric changes) from insitu subsurface observations30–32 over the period 1957–2018, and com-bine these estimates with an existing thermosteric reconstruction15. To obtain an estimate of GMSL changes and their accompanying uncertain-ties, we combine tide-gauge observations with estimates of local VLM from permanent Global Navigation Satellites System (GNSS) stations and with the difference between tide-gauge and satellite-altimetry observations.Each tide-gauge and VLM record is affected by glacial isostatic adjustment (GIA) and by the effects of gravity, rotation and deforma-tion (GRD) from contemporary surface-mass redistribution due to changes in ice mass and TWS. Owing to the irregular spatial distribution of tide-gauge sites, these effects could bias reconstructed global-mean and basin-mean sea-level changes33. To avoid this bias, we remove the local sea-level and VLM imprints from GIA and contemporary GRD effects from each tide-gauge and VLM record before computing basin-mean and global-mean sea-level changes from the tide gauges9.We propagate the uncertainties and associated covariances in the sea-level observations, in the contributing processes, and in the GIA and contemporary GRD effects into the final estimates of sea-level changes and the contributing processes. To this end, we generate an ensemble of 5,000 realizations of global-mean and basin-mean sea-level changes and all of the contributing processes. For processes for which multiple estimates are available, such as GIA, we randomly select one of these estimates when computing each individual ensemble member. For processes for which an estimate of the uncertainty is available, such as GNSS observations, we sample the estimate assuming a Gaussian distribution of the stated uncertainty about the corresponding mean. Then, we compute global-mean and basin-mean sea-level changes and the contributing processes for each ensemble member. We use the ensemble mean and spread to estimate all basin-mean and global-mean sea-level contributions and the associated confidence intervals. See Extended Data Fig.1 and Methods for a detailed description of our approach.Global-mean sea levelOur GMSL estimate (Fig.1a) shows a trend of 1.56±0.33mmyr−1 (90% confidence interval) over 1900–2018. It is also characterized by sub-stantial multidecadal variability, with higher rates of sea-level rise during the 1940s and since the 1990s, and lower rates around 1920 and 1970. The higher rates at the turn of the millennium are in good agreement with independent satellite-altimetry observations34. The observed trend over 1900–2018 is consistent with the sum of the esti-mated thermal expansion and changes in ocean mass, which sum to 1.52±0.33mmyr−1 (90% confidence interval). This consistency holds not only for the trends over the full study period, but also over the past 50 years (Table1), and for the pattern of multidecadal variability (Fig.1c), except for the low rates of sea-level change around the 1920s and early 1930s.Thermosteric and barystatic sea-level changes show similar multidec-adal variability patterns to the GMSL changes, although the amplitude of barystatic variability is larger than that of thermosteric variabil-ity, and barystatic variability is the main cause of multidecadal GMSL variability (Fig.1c). The barystatic variability is not dominated by a single process (Fig.1d). The above-average rate of GMSL rise in the 1940s is largely attributable to above-average contributions from glaciers and the Greenland Ice Sheet, whereas the high rate of barys-tatic sea-level rise since 2000 is attributable to both the Greenland and Antarctic ice sheets and to TWS. The low rates around 1970 are dominated by the TWS term (Fig.1d). This negative contribution is caused predominantly by reservoir impoundment.

CRITICAL COMMENTARY

Here, the assumption is that dam reservoirs sequester water from the land-ocean system such that land water that would have flowed to the ocean is captured and removed from the earth’s hydrological dynamics. If that were true we would have removed a cumulative amount of 300 gigatons of water by 1970 and 600 by 2010 as seen in the chart above. That corresponds to about 0.9mm of SLR equivalent by 1970 and 1.8 mm of SLR by 2010 or an annual SLR sequestration rate of 0.02mm of SLR lost to cumulative dam sequestration on an annual basis in the period 1930-2010.

However, dam reservoirs do not permanently remove water from the earth-ocean hydrological system but cause a delay in the transfer of the water through hydroelectricity and irrigation systems before the water is returned to the hydrological cycle and thereby to the ocean. However, the dams do delay the return of the water.

We estimate that the physical holdup in the reservoir is in the order of 12 days per billion cubic meters of reservoir capacity, but the the hydrological dynamics in the use of the water in terms of power generation, irrigation, and water supply to communities may delay the return of the water but this delay is unlikely to be more than seasonal or annual and certainly unlikely to be decadal or longer. More importantly, it is not possible for these effects to be cumulative.

However, it can be argued that at any given time the amount of water in the dams would otherwise have been in the ocean if it were not for dams and that they do represent an amount of sea level and that growth in dam reservoir capacity represents an equivalent sea level rise.

The diagram below shows that the effect of growth in dam reservoir capacity is estimated as 1.7mm of sea level over a period of 80 years. The corresponding rate of sea level rise is 0.02mm per year. This effect does not seem significant in the context of 3mm/year of AGW sea level rise particularly so in terms of a large uncertainty band of the SLR estimate in terms of global eustatic sea level in a dynamical system.