Sea Level Investigation and Management (SLIM) studies whether sea level rise can be slowed by storing water on land. Sea level is projected to rise 5 to 6 mm per year through 2050 under both low and high emissions scenarios, and 340 million people live on land projected to be below annual flood levels by 2050. Reducing greenhouse gas emissions is essential for addressing sea level rise over the long term. But substantial sea level rise is already locked in for the coming decades. This initiative examines the capacity, costs, and risks of methods that could potentially limit sea level rise – including reduced over-pumping of aquifers, aquifer recharge, and filling of low-lying depressions and reservoirs – and estimates the social cost of adding a cubic meter of water to the ocean.
Reducing greenhouse gas emissions is essential over the long term, but substantial sea level rise is locked in for the coming decades. The Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) gives a central estimate of 6 mm per year between 2020 and 2050 under a high emissions scenario and 5 mm per year under a very low emissions scenario. The greenhouse gases already in the atmosphere continue to heat the planet under either pathway. Under the low emissions scenario, the median projection continues at about 5 mm per year out to 2150, long after temperatures stabilize.
Sea level already responds to how much water the land holds. Over-pumping of aquifers adds about 0.4 mm per year. Dams built between 1950 and 2000 slowed sea level rise by about 0.5 mm per year. In 2010, global sea level fell at a rate of 4 mm per year, driven foremost by heavy rainfall retained in Australia’s interior.
We ask whether deliberate storage of water on land can have an effect of similar size, sustained year on year. Our first objective is a credible, costed plan for storing water at the scale of Greenland’s annual ice loss, several hundred cubic kilometers per year. Our research covers six areas:
Injection and storage technology: We measure what it costs to put a cubic meter of water underground and how that cost changes with rate and scale.
Geologic storage: We map where fresh and saline water can be stored at useful rates and volumes.
Freshwater aquifers: We estimate how much water reduced pumping, managed recharge, and managed flooding could retain on land, together with local benefits such as water supply and reduced subsidence.
Value of a stored cubic meter: We estimate the damages avoided by keeping a cubic meter of water out of the ocean, in analogy with the social cost of carbon. A first calculation gives 3 to 34 cents per cubic meter at a 2% discount rate, depending on how coastal communities adapt.
Emergency capacity: We assess reservoirs and low-lying depressions that could be filled rapidly if ice sheet loss accelerates.
Politics, law, and ethics: We study the consent, liability, water rights, and governance required before storage could scale.
Each project reports the size of the opportunity, its cost, its practicality, and the technical, economic, and political barriers to deployment. Modifying surface and subsurface reservoirs involves difficult trade-offs, and we work with experts in policy and ethics from the outset. Site-specific work involves researchers and institutions from the countries concerned.
SLIM is housed in the Harvard University Center for the Environment, a center of the Salata Institute for Climate and Sustainability. SLIM will fund research through a call for proposals, which will be announced soon. A steering committee advises on the scope of the research and the selection of projects, and reviews progress with investigators at workshops held at Harvard.
Sea level management is the intentional confinement of water on or within the land to counteract sea level rise. It supplements reductions in greenhouse gas emissions and adaptation along the coast.
Sea level rose at an average rate of 3 mm per year between 1993 and 2022, and 4 mm per year between 2010 and 2022. The main contributions to sea level rise are the melting of glaciers and ice sheets and the thermal expansion of a warming ocean. The Intergovernmental Panel on Climate Change (IPCC) projects that by 2100 sea level will be 50 to 100 cm above 1950 levels, with a small chance of being more than 150 cm above 1950 levels if ice sheets become unstable.
One millimeter of sea level is 3.6 × 1011 cubic meters of water. Spread over a year, that is a flow of 11,000 cubic meters per second, slightly more than the average discharge of the Mackenzie River.
We define the social cost of seawater as the net present cost of adding one cubic meter of water to the global ocean. A cubic meter spread across the ocean’s 3.6 × 1014 square meters raises sea level by 2.8 femtometers. Estimated damages from a meter of sea level rise this century run into the tens of trillions of dollars. A back-of-the-envelope calculation at a 2% discount rate gives 34 cents per cubic meter if coastal communities do not adapt and 3 cents per cubic meter under an idealized adaptation strategy.
We use the social cost of seawater as a benchmark, with a working value of about 10 cents per cubic meter. A storage method that costs less per cubic meter than the damages it averts has positive net value.
Pacala and Socolow argued in 2004 that a portfolio of existing technologies, or wedges, could stabilize greenhouse gas concentrations. We apply the same idea to sea level. Holding sea level at 2025 levels under a low emissions pathway requires wedges that together store 5 mm per year.
Irrigation draws an estimated 0.6 to 0.8 × 1012 cubic meters of groundwater per year, equivalent to about 2 mm of sea level. Most of this is replaced by natural recharge. Over-pumping is the extraction in excess of recharge, which depletes aquifers. About 80% of the depleted water reaches the ocean and adds about 0.4 mm per year to sea level rise. Bringing pumping into balance with recharge would remove that contribution. Stable aquifers also sustain local water supplies and avoid the compaction that causes land to subside. Groundwater extraction lowered parts of Shanghai by about a meter.
Sands, sandstones, and carbonates within 2,000 meters of sea level hold pore space roughly equal to the volume of all ice sheets and glaciers, the equivalent of 70 meters of sea level. Managed aquifer recharge uses spreading basins, riverbank filtration, in-channel structures, and injection wells, and some systems operate at a cost per cubic meter within the range of the social cost of seawater. Seawater can also be stored in deep saline formations. Where fresh water is scarce, desalination adds about $1 per cubic meter. The limits are the supply of water, the rate at which rock accepts it, and the risks of salinization and induced seismicity.
Some inland basins lie below sea level. Filling one with seawater stores a fixed volume, typically the equivalent of a few millimeters of sea level, and the new lake then loses water to evaporation at a rate set by its surface area. Salt accumulates in the basin, and seawater can enter the surrounding groundwater. The effects on soils and aquifers determine whether a site is viable.
Water impounded behind dams slowed sea level rise by about 0.5 mm per year between 1950 and 2000. Dam construction has since slowed, which accounts for part of the acceleration in sea level rise after 2000. Reservoirs that have filled with sediment can be dredged and refilled. Hydropower, flood control, and local environmental disruption all enter the assessment of any new reservoir.
Every wedge carries environmental and social costs. Injected or diverted seawater can salinize soil and freshwater aquifers. Filling aquifers and reservoirs can trigger earthquakes. Large storage projects affect land use, water rights, and nearby communities, and their concerns must be addressed before any project proceeds.
Sea level management may prove technically impossible, economically infeasible, or too costly to society. Existing knowledge does not yet permit that judgment. Our research is designed to inform it.
With the Salata Institute for Climate and Sustainability, Harvard University is mobilizing to meet the global climate challenge.
