A CLOSER LOOK AT SEA LEVEL(S)
Global mean sea level (MSL) isn't a physical constant so much as a concept. For one thing, cold, salty waters occupy less volume than warmer, fresher seas, so MSL tends to be higher near the equator and lower toward the poles. However, trends in temperature and salinity, so-called steric processes, aren't unfolding equally across the globe, because some areas are warming and/or freshening more than others.
Since 1950, the steric contribution to sea level has added as much as 10 cm (4 in) to parts of the northwest Atlantic and Gulf of Mexico, while lowering parts of the northern and tropical Pacific by 5 cm (1.5 in) or more. For the globe as a whole, steric changes made up about half of the rise in sea level from 1993 to 2003, with the rest coming from the melting of ice sheets and mountain glaciers. Data since 2003 hint that the global steric rise may have plateaued, at least temporarily (see below).
Although Earth’s mean sea level rose by about 7–10 cm (3–4 in) from 1950 to 2003, some parts of the globe actually saw a drop in MSL, as shown in this reconstruction based on tidal gauge data and an ocean model. (Image courtesy William Llovel and the journal Climate of the Past (2009), Llovel et al., Figure 2(b).)
Wind-driven circulations also help elevate the sea in some areas. Persistent trade winds push water from east to west across the tropical Pacific, which spans a third of the globe. That accumulation of water, which warms during its long equatorial trek west, helps make the MSL in the Philippines as much as 60 cm (2 ft) higher than on the south coast of Panama.
Off the east coast of North America, there's a more complex twist to ocean circulation that makes the Atlantic shore especially vulnerable to climate change. Just as a jet stream in the atmosphere separates zones of higher and lower air pressure, the fast-moving Gulf Stream separates areas of differing MSL as it runs up the Atlantic coast. The average MSL to the east of the Gulf Stream is considerably higher than it is between the current and the shoreline. For example, sea level typically runs about 60 cm (2 ft) higher in Bermuda than in New York. "I don't think most people appreciate this fact," says Michael Schlesinger, a professor at the University of Illinois at Urbana-Champaign (UIUC) who has studied sea-level rise since the 1980s.
If the Gulf Stream were to weaken, the difference in sea level on its east and west sides would slacken, thus bringing higher sea levels to the U.S. and Canadian coasts. This could happen through changes in the powerful conveyor belt known as the meridional overturning circulation (MOC), which includes the Gulf Stream. The MOC pulls warm surface water from the South to North Atlantic, where it descends in the far north to form cold, deep water, then completes the loop by flowing southward at depth.
Climate change is expected to slow the MOC by roughly 20-40% by 2100, according to the Intergovernmental Panel on Climate Change (IPCC). While this wouldn't be a catastrophic, cinematic collapse, Ă la
The Day after Tomorrow, even a milder slowdown would lead to a weaker Gulf Stream, with major implications for sea level from North Carolina northward.
These implications hadn't been studied in depth until Jianjun Yin examined them, collaborating with previous advisers Schlesinger and Ronald Stouffer (NOAA Geophysical Fluid Dynamics Laboratory). Yin took a closer look at the modeling carried out in support of the IPCC's 2007 climate assessment, especially the runs performed by GFDL's CM2.1 ocean-atmosphere climate model.
Yin found that the Atlantic's weakening MOC in the GFDL model not only allowed more water to flow toward the U.S. East Coast, but also reduced the formation of cold bottom water, thus warming and expanding the North Atlantic. Together, these dynamic and steric effects could raise New York's sea level by more than 20 cm (8 in) by 2100 under the IPCC's business-as-usual scenario (A2). This finding was robust across other IPCC models as well, according to Yin, who is now a climate modeler at Florida State University. His study was among those appearing in the July issue of
Nature Geoscience.