While there are many troubling consequences of the global warming we are imposing on ourselves, one of the most inexorable is the increase in ocean volume that leads to rising seas around the world. These changes are already bringing coastal flooding with increasing frequency, loss of coastal land, and related trouble such as salt-water incursion into groundwater, and much worse is in store. Stefan Rahmstorf at RealClimate recently reviewed several recent papers on sea level rise and among other concerns noted that even if we manage to limit warming to just 2 degrees Celsius, the expected eventual sea-level rise is 25 meters (82 feet) over thousands of years. Part of this increase in volume comes from the melting of land-bound ice: mountain and polar glaciers and the major icesheets of Greenland and Antarctica. The other part of the increase comes from the slow steady heating of the ocean as a whole - warmer water occupies more volume than colder water. As long as our planet is subject to climate forcings that bring it away from the pre-industrial energy balance, the ocean will continue warming even if the surface temperature starts to cool again once we have brought CO2 under control. Even while the surface of Earth has warmed by about 1 degree C since pre-industrial times so far, the temperature increase in the deep ocean has been measured at only one or two hundredths of a degree; eventually the deep ocean should catch up to the surface so we will see on the order of 100 times present sea level rise from that cause - unless we can return surface temperatures to pre-industrial levels first.
Sea level rise (SLR) has accelerated over the past few centuries - see this post by Tamino on analysis of satellite and tide-gauge measures. Presently sea level is rising at close to 3 mm/year - 1 foot per century. The IPCC has warned of up to 1 meter of sea level rise by 2100, implying a rate increasing to 10 mm/year or more. James Hansen in a recent paper argued for the possibility of several meters in 50 to 100 years due to a major acceleration in ice-sheet loss: this would imply a rate on the order of 30-50 mm/year of SLR. Even at 3 mm/year the inexorable rise is troubling; 10 times that amount would be devastating. Is there anything we can do about it? What does a look at some of the numbers in this problem tell us?
First the magnitude of the problem: Earth has 335 million square km of ocean (and 149 million square km of land). So 1 meter of SLR spread over that ocean area corresponds to 335 trillion cubic meters of water with a mass of 335 trillion metric tons. At the present rate the yearly increase is 0.003 meters, or 1 trillion tons per year. IPCC expects the rate to grow to at least 3 times as much; Hansen 10 times or more. Is it possible we could alter the water cycle to retain that quantity of water on land, rather than add it to the ocean, to stop or at least reduce SLR?
Since ocean area is about 2.25 times that of land on Earth, retaining 1 m of SLR on land means adding 2.25 m of water averaged over all the land surface. Obviously we don't want to put the water everywhere, so the actually inland water rise in any affected area would be significantly more than that in reality. One recent proposal looked at pumping sea water onto the top of the Antarctic ice sheet where it would freeze and stay without the need for constructing any sort of containment. Given that Antarctica's area is only 14 million square km (and the area where this sort of scheme would work significantly less than that) it would imply adding over 20 meters of ice for every prevented meter of SLR. Given the icesheets are already thousands of meters thick, this isn't out of the bounds of reason, though the study linked above indicated it would require an enormous additional energy expenditure to accomplish it.
Looking at the numbers there may be something much simpler we could do that would not require huge energy expenditures in itself: retain more of the naturally precipitated water on continental land. Annual precipitation depth over most land areas of the world is on the order of 1000 mm/year. If we just divert 0.3 to 1% of that rainfall to prevent it from returning to the world's oceans we could stop SLR until whatever storage capacity was involved became full. This could have significant additional benefits. Increasing the world's reserves of fresh water could help alleviate the droughts expected under climate change. Large water reservoirs close in horizontal distance but significantly separated vertically could provide new pumped hydro-electric energy storage that would be the perfect complement to increased solar and wind resource use. Refilling underground aquifers would reverse the salt-water incursion and stop the land subsidence problems that have plagued some parts of the world in recent years. What are the potential total capacities of these systems?
From this USGS page we can get a picture of current capacities. Currently lakes (fresh and salt), rivers, and swamps hold almost 200,000 cubic km or 200 trillion cubic meters of water - that is on the same order but less than the 335 trillion cubic meters associated with 1 meter of SLR. Increasing total inland water storage in the form of lakes by a few percent could take care of a few years worth of SLR, but doing more than that, or keeping it up on an ongoing basis, seems daunting. Could we double total Earth surface lake volume by 2100 (handling roughly what the IPCC expects for SLR by then)? Significant land area would be lost to new inland water resources, but we would surely apply this in undeveloped regions (deserts? mountain areas?) so that, given coastal land preserved, the net benefit was positive. Artificial lakes of average 100 m depth would require about 2% of Earth's land surface area to hold the volume associated with 1 m SLR, so land area loss might be kept relatively low with this approach. It certainly becomes highly impractical, though, if we need to handle the 25 meters of SLR that is eventually expected.
Groundwater may be a much more promising and possibly long-term solution. The total volume of groundwater on earth from the USGS reference is about 23 million cubic km, or over 100 times the surface water volume. 1 meter of SLR could be prevented with a less than 2% expansion of groundwater storage over the next century. Raising water tables everywhere by several meters would not be practical, but there are certainly many locations where the water table is 100 ft (30 meters) or more below the surface, and finding a way to retain water in those land areas (building new artificial lakes for example!) would gradually raise subsurface water levels as well. Water recharge wells can also be used to more directly bring the water to deeper underground layers.
This is not in any way intended to imply we can happily continue to burn fossil fuels. We need to stop burning them as soon as possible; the Paris agreement is a good start. But whatever we manage to do about carbon, if we intend to retain our developed coastal land for the next few hundred years then finding some way to handle sea level rise is imperative. Unless we somehow manage to pull gigatons of carbon out of the carbon cycle and bury it again, or unless there is some other major geo-engineering effort to bring Earth's energy balance back to pre-industrial levels, we are facing an inexorable expansion of ocean volume through thermal expansion and the melting of land ice, both of which processes will continue for thousands of years into the future. Diverting just a small percentage of continental rainfall into long-term above-ground or underground storage, instead of letting it all flow to the ocean, has the potential to give us at least a few hundred years of breathing room. It won't be easy, and it may not be nearly as feasible as I think, but it may also have some other benefits along the way so it should certainly be studied in detail as another option for dealing with some aspects of our climate change problem.