Numbers in sea-level debate not so simple

knordstrom@newsobserver.comJune 18, 2012 

— The debate in the legislature over the expected sea-level rise along the North Carolina coast over the next century has been presented as a choice between two numbers: eight and 39.

The number would guide local governments as they decide where to permit development in coming decades. If the number is too small, more coastal property could be damaged due to erosion and storm surges. If it’s too large, economic development could be unnecessarily stifled, and insurance rates might soar.

But like so much having to do with climate change, the two numbers aren’t as simple as they seem.

Let’s start with the eight. The bill passed by the Senate and headed for a joint conference committee would mandate that local governments plan for a rise in sea level of as little as 8 inches by 2100.

The 8-inch number is based on historical data from the southern North Carolina coast. Since the ocean rose 8 inches during the last century, legislators would require that governments assume a rise of 8 inches during the next century, at least in that part of the state. The future will look like the past.

But the ocean rose much more along the northern North Carolina coast over the last century, as much as 17 inches at Duck. The bill has been amended to reflect that variability, acknowledging that there “shall not be one rate for the entire coast.”

In other words, planners could anticipate a rise of as much as 17 inches in some areas, if the bill becomes law.

That’s still a far cry from what a panel of scientists appointed by the state have predicted. The Coastal Resources Commission Science Panel came up with the 39-inch figure in a report in March 2010, setting off the alarm that led to the bill in the legislature.

The panel based its forecast on the widely held idea that a warming atmosphere will accelerate the rise in sea level over the next century.

The science panel’s report recommended using one number for the whole coast as a starting point, concluding that “local differences are likely to be overwhelmed by global effects.” It chose the 17-inch figure from the Duck gauge. From the panel’s perspective, while the gauge had the highest rate of rise of the eight gauges along the coast, it was also the best source of data, because it was the least influenced by human activity such as widening of shipping channels.

Sea-level rise has three contributing factors:

• The land itself can sink, though this is somewhat random and hard to predict.

• Glaciers melt due to rising temperatures, increasing the amount of ocean water.

• Ocean water expands as it is heated, just like the wedding ring that’s a bit too tight.

Scientists say temperature also influences the rate of sea-level rise. A much-cited 2007 Science paper by German oceanographer Stefan Rahmstorf demonstrates this effect using historical data. So if the global temperature is increasing, as climate scientists generally believe, then sea level rise must be accelerating, going up at a faster rate every year.

The science panel took this concept and used the range of 2100 temperatures predicted by climate scientists. This gave best- and worse-case scenarios of 20 and 55 inches of rise.

But panel member Stan Riggs, a geology professor from East Carolina University, said the state requested a single number, so the panel somewhat begrudgingly chose 39 inches, or 1 meter, as a compromise between the best- and worst-case scenarios.

“We only stand by the range reported,” Riggs said. “It’s science; there are uncertainties, and they get larger as you forecast further into the future. But the state can’t deal with uncertainties and wanted a specific number.”

The panel further cautioned in its report that its number was not a prediction but a plausible number for planning purposes.

After critiques of the report emerged, attacking the choice of location and the validity of the model, the panel provided an addendum to its original report in April.

In the addendum, the panel stood by its report as the best estimate based on scientific knowledge at the time but also cautioned that as models and data improve, so will the projections. Panel members say the projections should be reassessed at least every five years to take into account the latest science.

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