The threat of sea-level rise

My old high school, Ysgol Aberconwy, is located next to a sandy spit at the mouth of the River Conwy in North Wales. Since I graduated in 1981, the sandy spit, known locally as the Morfa, has expanded. A stretch of muddy beach outside the school was just that when I was a pupil. Now it has been reclaimed and hosts the school’s playing fields.

I’m not sure what physical processes are responsible for the Morfa’s expansion. Silt from the river is one possibility, so too are falling sea levels. Although Earth’s mean sea level is currently rising at a rate of 3.3 ± 0.4 mm/y, sea levels are falling in North Wales, Canada, and other parts of the world that were covered with vast, thick ice sheets 100 000 to 12 000 years ago. Earth’s crust, free of its weighty burden of ice, continues to rebound to the elevation it had before the most recent glacial period began.

I first wrote about the relationship between changing sea levels and changing coasts for Physics Today‘s February 2004 issue. The subject of my news story was a paper by Keqi Zhang, Bruce Douglas, and Stephen Leatherman in the journal Climatic Change.

In “Global warming and coastal erosion ,” the three authors set out to test whether a simple, idealized formula that related sea-level rise to coastal retreat applied to the US East Coast. Because the ice sheet that covered North America extended as far as the Chesapeake Bay, coasts north and south of the sheet’s terminus have experienced different levels of sea-level rise.

For the barrier islands of Long Island, Delaware, and North Carolina, the simple formula, which had been devised in 1962 by coastal engineer Per Bruun, seemed to hold. And it implied that for every 3 cm of sea-level rise—the global mean rise in the past decade—those sandy coasts would retreat by 1.5 to 3 meters.

Out of curiosity, I asked the authors if they had updated their decade-old study. In principle, coastal retreat of 1.5–3 m should be visible by comparing satellite images from 2004 and earlier with those from 2014. But, wrote Zhang in an email, “a 10 yr period is too short to figure out the long-term coastal change because large short-term shoreline changes (5-100 m) caused by episodic nor’easters and hurricanes and other factors mask the relatively small long-term change.”

Although the rate at which climate change is shrinking coasts is hard to determine, sea-level rise is a slow, steady process whose rate can be determined from satellite altimetry. The threat of sea-level rise is real and present.

My 2004 story was accompanied by an image of beach house in North Carolina whose foundations had been washed away by the sea. Given its location and size, the house was likely expensive to buy and insure. In the aftermath of Hurricane Sandy, which struck a wide swath of coast two years ago this week, a coastal engineer was asked if he would ever build a house on a vulnerable coast. “Yes,” he said. “But only if I could afford to lose it.”

London generates 22% of the UK’s GDP. Only the more populous cities of Tokyo, New York, Los Angeles, and Seoul have a larger urban GVA (gross value added, the regional equivalent of GDP). When it became clear in the 1950s that rising sea levels were increasing the frequency and severity of floods that beset London, the British government decided to build an adjustable bulwark, the Thames Barrier, to protect the capital. In its first decade of operation, 1982–91, the Thames Barrier was raised 10 times. In its most recent decade, 2004–2013, it was raised 36 times.

North Carolina has taken a different approach to sea-level rise. In June 2012 the state’s senate approved legislation that would have mandated using only linear projections of future sea-level rise, regardless of whether such projections are scientifically justified. The modified law that passed two months later removed the linear provision, but still banned state agencies from weighing accelerated sea-level rise in their decision-making for four years.