ONE should applaud the leaders of North Carolina's coastal community for bringing a little light relief to these dismal days. Aghast at a state commission's scientific findings about the local sea level rising 39 inches (or one metre, as it is known to the rest of the world) by 2100, coastal business leaders and property developers pressured the state's legislators into banning all sea-level projections based on climate-change data. As a result, House Bill 819 would require future projections to use only historical data. “N.C. considers making sea-level rise illegal,” joked Stephen Colbert, a popular television comedian.



Like King Canute's sycophantic courtiers, who suggested their monarch was so powerful he could command the rising tide to retreat, the bill's backers would annul the laws of nature. By ignoring climate change, they reckon the increase will be eight inches at most. A sea-level rise of 39 inches “could quite frankly kill development on the coast”, a business leader admitted. A rise that high would threaten 2,000 square miles of desirable coastline, sending property values plummeting and insurance rates sky high. Ocean views can account for as much as 60% of the value of beach-front property.



The News & Observer of Raleigh labelled House Bill 819 “a burst of legislative folly”. Having become the laughing-stock of the country, North Carolina's legislature is now reworking the bill, inserting the “need for additional studies” that will take three to four years to complete. That should get embarrassed law-makers off the hook for the time being. Nevertheless, a moratorium is to be slapped on the 39-inch figure.



Whether North Carolina's officials approve or not, though, there can be no denying that sea levels around the world are rising rapidly, especially along the Atlantic coast of America. In the past century, the global climate has warmed by about 1ºC. And as the Earth has become warmer, the sea level has risen by one or two millimetres a year.



Lately, however, this rise has put on a spurt. Satellite observations over the past two decades record an increase of three to four millimetres a year. Like it or not, the only explanation for this acceleration is the amount of greenhouse gases being emitted by human activity.



Two phenomena are propelling the process. One is thermal expansion: as the oceans get warmer, the volume of the water stored in them increases. The other is the melting of land-based ice—mainly mountain glaciers and island ice caps. Large masses of floating ice, such as icebergs and even the Arctic sea ice itself, do not count. They already displace liquid water equal to their mass (Archimedes principle), so their melting would add little to the sea-level's height. Even the vast East Antarctic sheet, which accounts for 80% of the Earth's ice, does not count. Its temperature is so low that it will remain in cold storage for many thousands of years to come, no matter how much global warming ensues.



Though small by comparison, the two sources of most concern are the Greenland and the West Antarctic ice sheets. Were both to melt completely, they would raise the global sea level by 13 metres or more. This may sound an awful lot. But after the last glacial epoch subsided 20,000 years ago, the sea subsequently rose 120 metres to reach roughly today's level. That represented an average of six millimetres a year—twice today's rate.



It is not hard to see why homeowners and developers along the Atlantic coast are concerned about the encroaching ocean. In a 600-mile swath from North Carolina to Massachusetts, sea levels have been rising lately at an annual rate three to four times faster than the global average. The projection made by North Carolina's scientific panel cannot be faulted. It is in line with broader studies by the United States Geological Survey.



The USGS notes that, while the global sea level has risen on average by two inches since 1990, it has increased by as much as 4.8 inches in Norfolk, Virginia; by 3.7 inches in Philadelphia; and by 2.8 inches in New York. By the end of the century, the global sea level is expected to have increased by around 40 inches, with the Atlantic coast of America having seen a rise of 50 inches or so.

The reason the eastern seaboard will do so badly is the Gulf Stream, a warm current that runs through the North Atlantic. Global warming is reducing temperature differences across the globe. That, in turn, is causing ocean currents to slow down as the natural heat engines which drive them are forced to work between smaller temperature differences, and thus become less effective.



As the Gulf Stream and its northern extension, the North Atlantic Drift, reduce their speed, the surface of the surrounding water changes its slope, to keep the hydrodynamic forces in balance. Like a see-saw, the flatter ocean surrounding the slowing Gulf Stream piles up water elsewhere—especially along the east coast of America.

This is bad news for such a populated area. Storm surges can now be expected to happen more often, and to do more damage. Rising sea levels will exacerbate coastal erosion, threatening not only homes and holiday settlements, but also valuable agricultural land, fragile wetlands and coastal lagoons. Worst hit will be places where natural barriers such as sand dunes have been bulldozed for development—as is too often the case.



The other side of the country does not have to deal with such hydrodynamic difficulties, but there is still no reason for complacency. The bluffs overlooking the Pacific where your correspondent hikes each morning have been crumbling steadily. Remains of the original cliff-top road, built in the 1940s, lie scattered below. Barricades now block much of the road in front of the homes left perched above the ocean—as it, too, begins to fall off the edge. The multi-million-dollar houses facing the road cannot be far behind.



In a report published on June 22nd 2012, the National Research Council of Washington, DC, said it expects the sea level along the Californian coast to rise by about 40 inches over the rest of the century—broadly in line with the global average. But Oregon and Washington state should fare better. That is because the Juan de Fuca tectonic plate, inching steadily east across the Pacific floor, collides with the old North American plate along the 680-mile Cascadia subduction zone, forcing the land to rise as it grinds its way beneath. As the land rises, the sea level relative to benchmarks onshore falls.



Would, though, that it were that simple. For the clash between the two plates means the Cascadia coastline is home to some of the most destructive earthquakes on the planet. Megaquakes of magnitude nine or more have occurred there every 200 to 600 years for at least the last 10,000 years. The most recent happened 312 years ago (see “The really big one”, March 18th 2011). And as the National Research Council points out, a big earthquake in the area would not merely create damage through violent shaking and a tsunami that would dwarf anything seen lately in Japan. It might also cause parts of the coastline to collapse spontaneously, raising the local sea level by a further 40 inches or more.



On the southern Californian coast, your correspondent is comforted by the thought that his glass-and-stucco eyrie is 720 feet above the ocean. Long after he has gone, his descendants may see the Pacific rise three feet or more. That would be enough to wipe out Venice Beach, the Santa Monica seafront and much of Malibu. But as Canute—who called the sycophants' bluff by having his throne carried onto a beach to conduct the experiment they suggested—knew all too well, you can't cheat nature. Even if you are the duly elected legislature of North Carolina.