Sea level is rising. Period. Full stop.
Or is it?
Well, the short answer is “yes”, global sea level is rising, by somewhere on the order of 0.12 inches per year, or 1 foot in 100 years. And it is rising faster each year: recent projections from the IPCC indicate we might get that foot of sea-level rise by 2050 – only a little more than a 30-year-mortgage away!
But – and here is where it gets interesting – sea level is NOT rising everywhere. In some places such as the northern coast of Canada, it is actually falling, and has been for >10,000 years. Of course, if sea level; is rising globally by an average of 0.12 inches/year, and falling along some coasts, then it must be rising faster than 0.12 inches/year elsewhere. And, indeed it is: in the United States we have areas experiencing sea-level rise at rates of ~0.25 inches/year in coastal Virginia, and as high as nearly 0.5 inches/year in parts of the Mississippi Delta of Louisiana. You can explore this more at the NOAA Tides & Currents Sea-Level Trends website.
So, why is this? Why are sea-level change rates so variable? The primary reason is the difference between what scientists call “eustatic” and “relative” sea level.
Eustatic sea level is global sea level, and is determined by the mass of water in the oceans, the temperature of that water, and the volume of the ocean basins to hold that water. The latter generally only changes over long (millions of years) timescales. But, due largely to human-induced global warming, the volume of water in the world’s oceans is increasing today because there is more of it due to melting of land-based ice sheets (Antarctica, Greenland) and mountain glaciers (Alps, Alaska, etc.) as well as the shifting of water from groundwater reservoirs (followed by use in everything from agriculture to your morning shower) to the oceans; and because the water in the oceans is getting warmer as it absorbs heat from the atmosphere, and is thus expanding.
Relative sea level, on the other hand, is the sea level that we experience: how high is the water outside your window? It is partially controlled by eustatic sea level, but also by local tectonics (for example, sea level fell instantaneously in some areas of New Zealand by 6 feet in response to the earthquake in November), winds, ocean currents, land subsidence (due to water and oil withdrawal for example), and myriad other factors. One of those factors is called isostacy.
Isostacy is the “floating” of Earth’s tectonic plates on the semi-liquid upper part of the mantle. In the same way a block of wood or a cube of ice will float in tank of water, whole continents float on the mantle. If you add a second block of wood – or a second ice cube – on top of the first, the first will sink lower into the water. Likewise, if you add a large mass on top of one of the continents – say like the 1(+)-mile-thick sheets of ice that covered much of North America and northern Europe during the last ice age, the continent underneath will sink as it is pushed down by the mass of that ice above it. When that ice melts, as happened ~15,000–20,000 years ago, the continent will “bounce” back up. This is called isostatic rebound.
Isostatic rebound occurs very slowly. So slowly, in fact, that some landmasses near the center of ice sheets are still rebounding today. The ice sheet around Hudson Bay, Canada, for example, only fully melted ~5000 years ago, and that land is still rebounding at ~0.5 inch/year. The result? Even though global eustatic sea level is rising 0.12 inch/year, the local apparent (relative) sea level around Hudson Bay is actually FALLING. How much? Well, take the global eustatic rise (0.12 in/yr) and subtract it from the local rate of uplift (0.5 in/yr), and you get 0.38 in/yr, or ~9.5 mm/yr, exactly what we see in local tide gauge records from Churchill, Canada:
Of course, as I noted before, if sea level is falling in one location due to isostacy, it must be rising somewhere else. In this case, that “somewhere else” is the US East Coast. And it is rising faster there for the same reason it is falling in Hudson Bay: isostacy.
Picture North America as a large I-beam suspended in the air. If you place a literal ton of bricks on the middle of that beam (Hudson Bay in this case), then the middle will bend down and the ends will bow up ever so slightly. Remove the weight and the middle bounces back up and the ends bend back down so the beam is again straight.
That is how North America works: the continent (or “continental plate”) is the I-beam and the ice sheet is the bricks. So, as the continent near Hudson Bay continues to rebound, the land far away (the US East coast for example), sinks (or “subsides”) faster. In Virginia, where I live, that rate is ~0.05 in/yr (see the figure below, from Engelhart et al., 2009 for details).
Add that 0.06 in/yr on top of the eustatic 0.12 in/yr, and you find that the rate of relative sea-level rise in Virginia is higher than the global average: at the mouth of the Chesapeake Bay, it is ~0.23 in/yr:
Note that this is higher than the 0.17 in/yr calculated from just rebound subsidence plus eustatic sea-level rise. That is because coastal Virginia is also experiencing subsidence for reasons other than glacial isostacy, such as groundwater withdrawal and slipping along deep fractures in the crust related to a 35-million-year-old meteor impact.
As you can see, sea-level rise is not simple, it is not happening at the same rate everywhere (in fact, sea level is falling in some places!), and it therefore will not affect everyone the same way. Isostacy is a key player in these different rates, and will continue to be for hundreds to thousands of years to come.
Dr. Chris Hein
Dr. Chris Hein is a professor of Marine Geology and Coastal Geology at the College of William and Mary, and the Virginia Institute of Marine Science. Click here to learn more about Chris and his work.