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The concept of sea level rise for much of the public may suggest an image of the world’s oceans as something like a bathtub, with melting glaciers as the faucet.
“The media have this idea in mind that sea level due to climate change is a uniform, global thing,” says Glenn Milne, a geophysicist at the University of Ottawa and lead author for a recent review in the journal Nature Geoscience on the causes of sea level change.
“That’s the Waterworld kind of misconception, ” he says, “and in reality it’s going to be very different than that.”
Melting ice from mountain glaciers, ice caps, and ice sheets is, in fact, only one of the key drivers of sea level rise, which is anything but uniform from region to region.
A second major contributor is thermal expansion, also known as thermosteric sea level rise. It is caused by the basic physics of water molecules expanding as they warm. If climate patterns cause an increase in water temperature for a given swath of ocean, its volume increases, and with it sea level.
Melting and thermal expansion are the undisputed main drivers, accounting for the vast majority of the sea level rise measured at the global scale over the past century, and, to differing degrees, of the various predictions of sea level rise such as the 0.18 to 0.59 meters forecast for the 21st Century in the latest report by the Intergovernmental Panel on Climate Change.
But other phenomena make at least small contributions globally, and with regional impacts. For instance, freshwater from melting ice sheets on land and from melting sea ice decrease the overall salt content of the ocean. Lower salinity makes water less dense, or compacted, leading to a measurable sea level increase as ocean salinity decreases.
Sea Level Rise Acts Locally
Though sea level rise is most often reported as a global average, the actual sea rise seen at a given coastal locale can be very different from that mean as a result of various factors.
One of the most important local effects is called glacial rebound. As a glacier melts, its weight decreases, so the glacier applies less pressure to the solid ground below it. As a result, the land beneath the glacier and for some distance around it, actually rises over time, which can cancel out some or all sea level rise in that particular region.
Farther away it’s a different story. Picture a long plank resting on two concrete blocks about a quarter of the way in from either end. Stand in the center of that plank and it hangs lower. But that bend makes the ends stick up higher. Step off and those ends drop down.
The same thing happens with the land far away from a melting ice sheet, and that lowering due to rebound will magnify the sea level rise at such sites.
Glacial rebound, as you might guess, proceeds at glacial speeds, so the effects play out for millennia. This is exactly what’s happening in places like the Chesapeake and Hudson Bay regions, which are still transforming as a result of the last deglaciation nearly 10,000 years back.
In the New Orleans region, a different but related phenomenon dominates. There, weight from the build up of sediment around the Mississippi Delta over the long term has caused compaction that magnifies sea level rise.
Considering the Gravity of the Situation
Glaciers play yet another significant role in controlling sea level rise because of their measurable gravitational effect. Like everything else with mass, glaciers exert a certain amount of gravitational pull. As they melt, that pull on the ocean water around them decreases, allowing more water to move away and lowering sea level locally.
It all means that less water is pulled toward the glacier, which translates to less sea level rise. But that water has to go somewhere, and models suggest that somewhere is generally the opposite side of the globe. If Greenland’s glaciers melt, then the Southern Hemisphere takes the hit, and if Antarctica melts, we in the north do. Of course, in reality, there could be melting at both ends for combined effects, or the ice at certain locations could even grow as a result of increased precipitation.
The Challenge of Assessing Relative Roles
Figuring out the relative contributions of all these regional and global-scale controls on sea level rise, not to mention the degree to which human activities vs. natural patterns influence these drivers, poses challenges for researchers. But new technologies coming online over the past 15 years have vastly improved their findings.
To estimate sea level rise thousands of years back or more, scientists analyze geological deposits. Corals and certain microorganisms generally live within certain depth and salinity boundaries, so studying where they are found in the deposits, and then dating them, gives an estimate of sea level at a given time. For the past century or so, tide gauges have been measuring sea level rise, but these provide measures only for a given location, and there are great gaps in the areas where such gauges have been in place.
Satellite altimeters now gauge the absolute level of the ocean at points around the globe, which are integrated to establish the planet’s mean sea level. But there is no good way to separate out the relative contributions of thermal expansion and glacier melting using such data. To do that, researchers turn to newer instruments such as the Gravity Recovery and Climate Experiment (GRACE) satellites launched in 2002, and a network of drifting ocean monitoring devices known as Argo.
Because the total amount of water remains unchanged, thermal expansion doesn’t affect gravity, but freshwater coming from glaciers does because of the water mass it adds. Subtracting GRACE measurements from altimeter readings can therefore separate the components. Because Argo buoys measure temperature and salinity, their data can be used to estimate thermal expansion, though there are significant gaps such as the absence of temperature data for the deep sea beyond the 2,000-meter reach of Argo buoys.
Analyzing all these data is allowing researchers to pick apart those relative contributions. However, because each measurement has its own substantial uncertainty, uncertainty in the relative contributions established remains.
The Big Picture
Despite that uncertainty in the details, researchers are confident that the broader picture is clear. Going back thousands of years, sea level rise was in the range of tenths of millimeters per year. Over the last century, that rate increased to an average of about 2mm per year, but is now approaching 4mm per year.
These increases have tracked well with increasing temperatures, which drive the major sea level contributors. “There’s no obvious other mechanism to explain what we’re seeing,” says the University of Ottawa’s Milne.
A useful resource addressing 20th Century data for some 2,000 sites: Permanent Service for Mean Sea Level.
Mark Schrope is a freelance science writer living in Melbourne, Fla.