The Next 500 Years of Sea Level Rise

Climate change is raising global sea levels. This is not a good thing, but at least it's all going to stop by the end of this century, right? Every news report about the topic talks about what will happen by 2100, or during the 21st century, or over the next hundred years. So do educational websites and so does the 800-lb. gorilla of climate science, the U.N. Intergovernmental Panel on Climate Change (IPPC). 

The bad news: scientists didn't pick 2100 because they think that's when sea-level rise will stop. They picked it because their computer models of the climate get less reliable the farther out you go into the future. 

Oceans don't care about climate models, though: the sea just does its thing. And as greenhouse gases continue to build up in the atmosphere, its thing is to keep rising, right past 2100 and on upwards for hundreds of years beyond, at the very least. And unlike many other sea-level studies, a new report that's been accepted for publication in the journal Global and Planetary Change  goes out on a limb by projecting just how high the sea will rise, not by 2100, but all the way out to 2500, five centuries from now.

Greenland holds enough ice to raise global sea level 23 feet – 
or to fill the Lower 48 states 2,940 feet high, like a bathtub.

"We feel comfortable projecting that far," says co-author Aslak Grinsted, a glaciologist at the University of Copenhagen, citing a relatively new method of tying greenhouse-gas emissions to sea level. The most likely numbers, say Grinsted and his co-authors: about 2.5 feet of sea-level rise by 2100, and about 6.5 feet by 2500. (Some locations could see more of an increase in sea levels than that, due to variations in ocean currents and other factors.) These numbers don't reflect a worst-case scenario where we just keep burning fossil fuels such as coal and oil like there's no tomorrow, thereby pumping more climate-warming gases into the air. If we did that, say the authors, the numbers would be more like 3.5 feet by 2100 and a horrifying 18 feet by 2500.

But even if the world moves aggressively to limit fossil-fuel burning, the oceans will still continue to rise. The reason is that every ounce of carbon dioxide (CO2), a key climate-warming greenhouse gas, we pump into the atmosphere stays up there for decades, trapping the Sun's heat and warming the planet. Most of the CO2 floating above us right now has been there for many decades already, and every ton of coal and gallon of gasoline we burn today just gets piled on top of that. The heat trapped by greenhouse gases raises sea levels in three ways. First, it raises ocean temperatures and makes seawater expand. Second, it melts the ice sitting on land in places like Greenland and Antarctica. Third, it can make that land-based ice flow more rapidly to the sea, even if it doesn't melt.

These things take time, though. It takes years for trapped heat to work its way into the oceans, which means that the expansion of seawater lags behind rising temperatures. The melting of ice, meanwhile, depends not just on how warm it gets, but on how long it stays that way. An ice cube can survive a 500-degree oven, after all, if you only leave it in there for a few seconds. Leave it in a 100-degree oven for 20 minutes, and it will melt away. 

All that is well known. What makes the new study different is that the authors are willing to go out 400 years further into the future than usual with actual numbers. The reason for their confidence is that existing models tend to look at a number of different factors independently: how much CO2 is in the air, how much the Earth warms as a result, how much that warming affects seawater expansion, how much it affects the melting of ice, how much it affects the movement of ice, something scientists don't really understand so well yet, and so on. Each of these factors has some uncertainty, and all that uncertainty adds up.

In this case, by contrast, Grinsted and his colleagues looked mainly at two factors: how much CO2 and other pollutants have been in the air over the past couple of centuries (not much uncertainty there) and how sea level responds. By cutting out all the intermediate steps, they've chopped away at the uncertainty. They haven't reduced it to zero, admits Grinsted. "We think our method is more correct," he says, "but it's good to have other, independent estimates." That, he argues, is the best way to reduce uncertainty about future sea-level rise to a minimum.

The one thing that's clear, he says, "is that that sea level will continue 
to rise for hundreds of years no matter what emissions path we choose to take."