Can we stop the seas from rising? Yes, but less than you think.

November 1, 2012

One of the main concerns with climate change is that it's causing the oceans to advance. Global sea levels have risen about seven inches over the past century and that pace is accelerating. Not only does this threaten coastal regions, but it also makes storm surges much worse — both for huge hurricanes like Sandy and for smaller storms too.


We can hold back some of the tide, but not all of it. (Amanda Lucier/The Washington Post)

And the oceans are likely to keep creeping up. Scientists project that if we keep warming the planet at our current pace, sea levels could rise between two and seven feet by 2100, particularly as the world's glaciers and ice caps melt. So that raises the question: Is there anything we can do to stop sea-level rise? How much would cutting greenhouse-gas emissions help?

As it turns out, reducing our emissions would help slow the rate of sea-level rise — but at this point, it's unlikely that we could stop further rises altogether. That's the upshot of a recent study from the National Center on Atmospheric Research (NCAR). The study estimated that aggressive steps to cut emissions could reduce the amount of sea-level rise by somewhere between 6 and 20 inches in 2100, compared with our current trajectory. That's quite a bit. But sea levels will keep rising for centuries no matter what we do. We can't stop it entirely. We can only slow the pace.

As NCAR's Gerald Meehl, a co-author of the study, explained to me by e-mail, it's a lot easier to stabilize global temperatures by cutting carbon emissions than it is to stabilize sea-level rise. The carbon-dioxide that we've already loaded into the atmosphere will likely have effects on the oceans for centuries to come. "But with aggressive mitigation," Meehl added, "you can slow down the rate of sea level rise, which buys time for adaptation measures."

There are two ways that global warming causes sea levels to rise. First, as carbon-dioxide traps more heat on the planet, the oceans get warmer and expand in volume. Second, ice caps in Greenland and Antarctica as well as other glaciers start melting, pouring more water into the oceans. Once these processes get underway, they won't stop quickly, even if we ceased putting carbon-dioxide into the atmosphere tomorrow.

The NCAR paper estimated that if emissions go unchecked, we could warm the planet 4°C over pre-industrial levels by 2100, causing sea levels to rise between two and five feet. By contrast, if we get really proactive at cutting emissions, we could probably keep the temperature increase below 2°C. But sea levels would still rise by between 11 inches and 3.5 feet. (The wide range is due to the uncertainties in modeling the behavior of glaciers and ice sheets—if the ice sheets destabilize, a bigger rise is possible.) That's progress, but not total victory.

In both scenarios, sea-levels continue to rise through 2300, though at very different rates. The graph below shows the projected thermal expansion of the oceans (this doesn't factor in glaciers and ice sheets, which are more difficult to model). The red line is the "don't stop polluting" scenario. The blue line is the "aggressive carbon-cutting" scenario. The green line is a less aggressive cut:

Other studies and modeling work have come to similar conclusions, albeit with somewhat different numbers (see here and here). The basic idea: Cutting emissions can make a modest difference in sea-level rise in the near term, but the real impact comes after 2100.

A few takeaways from these studies:

1) We're going to need to adapt to sea-level rise no matter what we do on carbon emissions. Even the "optimistic" scenario in the NCAR paper still envisions sea-levels rising roughly 11 inches by 2100. That's assuming we cut emissions drastically and the ice sheets don't do anything too unpredictable. Even then, New York City will have a bigger flood zone than it does today. Storm surges on the coasts will be much larger. Low-lying areas will be at greater risk. In Bangladesh, for instance, the area prone to severe flooding would increase by 69 percent (pdf) with just a foot of sea-level rise. 

2) That said, cutting emissions can make a significant difference this century. Keeping sea-level rise a foot or two lower than it otherwise might be is nothing to sneeze at. As this map of New York City shows, the flood zone increases dramatically with each additional foot of sea-level rise. A city like Norfolk, Va. could get swamped entirely by a Category 3 hurricane if ocean levels rose by two to five feet. Florida's adaptation costs go up by billions of dollars with each additional foot of sea-level rise. Every little bit helps.

3) Sea-level rise is likely a much bigger problem for future generations. Not to get too morbid, but I'll probably be dead by 2100. So will most people reading this blog. So the main question at issue here is whether we want to leave our descendants a relatively stable coastline or an unstable one. 

According to NCAR projections, sea levels could rise as much as 34 feet, or nine meters, by 2300 if emissions continue unchecked (though modeling projections that far out have very large uncertainties, so don't take this as a definitive number). To get a sense of what a nine-meter rise would look like, check out this interactive map. South Florida would be underwater. So would New Orleans. And Shanghai. And the Netherlands. And Bangladesh. But this is also 200 years in the future. That's a big reason why climate change is such a difficult problem to deal with.

Further reading:

 — Credit due to Roger Pielke Jr. for asking this question in the first place. His post cites a study showing a much smaller effect on sea-level rise by 2100 if we cut emissions, though that study doesn't looking at the impacts from melting glaciers and ice caps. The newer NCAR study tries to include those effects (though, as noted, that increases the uncertainty).

 — A look at why the United States is unprepared to adapt to climate disasters like sea-level rise.

 — Why Hurricane Sandy should get us thinking more seriously about climate change, sea levels, and storm surges.

 — A list of cities expected to get hit hardest by rising sea levels.

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Dylan Matthews | November 1, 2012