Some North Carolina lawmakers have accused scientists of using "made up" estimates of sea-level rise. But a top researcher says some of the world's best evidence for climbing oceans comes from the ground beneath their feet.
Stefan Rahmstorf, a German climatologist whose research led scientists to reconsider accelerated sea-level rise, said an embattled report by North Carolina experts, recommending that the state prepare for a 39-inch rise by 2100, is a reasonable policy when building homes and infrastructure.
That level of rise, although a projection, is possible as warmer temperatures expand ocean water and begin to melt the world's supply of ice, which presents the unlikely chance of adding 60 meters (197 feet) to the seven seas if it all turned to water.
That would take thousands of years and considerable warming. The 1-meter estimate under attack in North Carolina, however, is based on "simple physical logic" discerned from the state's ancient salt marshes, not a nightmare scenario, Rahmstorf said.
The 1-meter estimate is being used for future planning in some of Europe's coastal cities and in U.S. states, where creeping saltwater and bigger storm surges threaten to flood homes and inundate lowlands.
"If you want [new buildings] to be there in more than 30 years' time, then you better take this factor into account," Rahmstorf said of accelerating sea-level rise in an interview.
North Carolina launched political debates around climbing oceans last week when the state Senate easily passed a bill prohibiting state officials from planning for a rise of 39 inches, or 1 meter, over the next 90 years.
The legislation rebukes a recommendation by the state Coastal Resources Commission, which stirred economic development concerns and skepticism about climate change in 2010 with a report warning of future threats along the state's coastline. It recommended using a benchmark of 39 inches of rising seas when planning new developments.
The legislation instead requires state officials to consider only historical data when calculating sea-level rise, dismissing the future relationship between climbing temperatures and rising seas -- which was verified in part through traces of the state's salt marshes dating back to 2,000 years ago.
Warming and the seas -- both on the rise
Those ancient samples of sediment from 10 coastal wetlands in North Carolina provide some of the best evidence that sea-level rise closely follows warmer temperatures, Rahmstorf says.
Between 100 B.C. and A.D. 950, sea level was stable. Then sometime between A.D. 850 and 1080, it slowly began to rise by about 0.6 millimeter a year for four centuries. It stalled again around 1400, or perhaps even fell slightly, because of the cooling effect of the Little Ice Age. That period lasted until about 1900, when the fastest rise in sea levels began in the time since 100 B.C., according to a study of the marsh sediment by Andrew Kemp, a scientist at the University of Pennsylvania.
The rise of 2.1 millimeters per year during that period corresponds to rising temperatures following the beginning of the Industrial Revolution in the mid-1700s.
"This historical rate of rise was greater than any other persistent, century-scale trend during the past 2,100" years, the study says, noting that "modern warming is unprecedented in the past two millennia."
The debate in North Carolina has created tension between scientists and lawmakers. One member of the state House, Republican Rep. Bill Cook, said of the benchmark standard in an interview last week, "Good God, that would have cost this state a fortune for some made-up numbers, as far as I can tell."
The commission abandoned the benchmark earlier this year, but a coastal development group whose chairman rejects human-related climate change pressed for legislation to inhibit similar standards in the future. The group, NC-20, is named for the state's 20 coastal counties.
Cheaper to plan now?
The bill is now in the state House of Representatives, which is showing signs that it may be less enthusiastic about the controversial measure. That has led some observers to suspect that the chamber might be thinking about avoiding a vote on the bill before the Legislature adjourns later this month.
The House majority whip, Rep. Ruth Samuelson (R), didn't dispute that scenario in an email late last week. "We want to talk about it before taking action," she said.
Opponents are concerned about the potential costs of restrictions related to sea-level rise, like lost land to wetland status and degraded tax revenue. But other communities say the cost could be bigger if future threats aren't planned for.
Copenhagen, Denmark, has proposed a suite of recommendations to lift buildings and infrastructure higher, strengthen them against water damage and avoid areas prone to flood. It assumes the sea will rise 1 meter over the next century but acknowledges that its policies need to be flexible -- and revisited often -- to account for future uncertainty.
But in the end, the city's climate adaptation plan says, it's "cheapest by far to establish the protection over a longer period, where sections can be protected in connection with other construction projects."
For his part, Rahmstorf says he and other scientists can continue to talk publicly about the methods by which they estimate future sea-level rise, in the hopes that public officials will become more comfortable with their findings.
He begins by highlighting the evidence of North Carolina's salt marshes because they are a storyline from the past -- a period that state lawmakers seem to be emphasizing over future predictions developed by computer models.
"This is not the result of some horrendously complex model," Rahmstorf says of sea-level rise's connection to warming temperatures. "This is experience based on the past."