The evidence for sea-level rises in North Carolina

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In one US state, it is now illegal for sea level rise to speed up. Although climate models predict that sea level rise will accelerate over the coming decades, North Carolina's state senate has passed a bill saying that its Division of Coastal Management cannot "include scenarios of accelerated rates of sea level rise".

The lobby group supporting the bill, NC-20, has released a number of statements arguing that "the point of the bill is to conform rulemaking to actual scientific evidence". It says there is no indication that sea level rise has accelerated over the last few decades.

New Scientist looks at the evidence.

Are sea levels rising? ?
This is an easy one: yes. Levels have been rising since at least the middle of the 19th century, and over the 20th century they rose on average 1.7 millimetres per year. The rise was first reported in 1941 by oceanographer Beno Gutenberg.

Two sources of data confirm this: tide gauges and satellites. The data from tide gauges goes back more than 100 years, though it becomes increasingly patchy the further back you look. Satellites with altimeters only came online around 20 years ago, but unlike tide gauges, which are limited to coastlines, they cover much of the ocean.

Much of the rise is caused by water expanding as it warms up. Melting glaciers and crumbling ice sheets are also contributing, as is humanity's overuse of groundwater.

Is sea level rise accelerating?
This is a harder question to answer, because older records are incomplete. "It's a hot debate," says Simon Holgate of the National Oceanography Centre in Liverpool, UK. Some papers claim that the rise has accelerated over the 20th century, while others (often using the same data) say it hasn't.

"The overall sense of the community is that there's a small acceleration, but there is a lot of noise in the signal," Holgate says.

The best dataset is thought to be one compiled by John Church of CSIRO Marine and Atmospheric Research in Australia, and colleagues. According to a widely-cited 2006 paper, sea level rise has been accelerating at 0.013 mm/y² since 1870 (Geophysical Research Letters, DOI: 10.1029/2005GL024826). A more recent paper, from 2011, found a smaller acceleration of 0.009 mm/y² using data that goes back to 1860 (Surveys in Geophysics, DOI: 10.1007/s10712-011-9119-1).

By the end of the 20th century, sea level rise had accelerated to 3mm/y, compared to earlier rates around 1mm/y. But Holgate has found that some tide gauges recorded similar high rates much earlier in the century. That means it's hard to be sure that the current rates are truly out of the ordinary (Geophysical Research Letters, DOI: 10.1029/2006GL028492).

Another confounding factor is that the satellite data shows that sea level rise has slowed in the last decade. That's because strong El Ni?o events can temporarily slow down sea level rise, Holgate says. A powerful El Ni?o increases rainfall, effectively moving water from the ocean to the land. The water then takes a couple of years to get back into the ocean. Sea level rise visibly slowed in 2006 and 2010, in the aftermath of El Ni?o events. Such slowdowns are only temporary, and don't affect the long-term trend.

What is the evidence that sea level rise isn't accelerating?
Some studies have found no evidence of acceleration, and NC-20 cites these in its literature.

One was published last year by Robert Dean of the University of Florida, Gainesville. Dean looked at 57 US tide gauges whose records go back at least 60 years, and re-analysed other datasets. His conclusion: sea level rise has slowed down since 1930 (Journal of Coastal Research, DOI: 10.2112/JCOASTRES-D-10-00157.1).

Why did Dean get such different results? According to Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, it's because he started his analysis at 1930, rather than using the full dataset going back to 1870. Global temperatures cooled slightly around 1940, so sea level rise stopped accelerating. As a result, starting the analysis in 1930 gives a misleading answer.

"It's a cherry-picked date," Rahmstorf says. "If you use any other date, you see an acceleration."

Why does the acceleration matter?
If sea levels continued rising at the 20th-century rate of 1.7 mm/y, the entire 21st century would see a rise of just 17 centimetres. Such a small rise isn't without its problems, but humanity could cope.

But most predictions show much larger rises. The 2006 report of the Intergovernmental Panel on Climate Change (IPCC) predicted a rise of 19 to 59 centimetres by 2100, but this was known to be a lower end estimate because it didn't include the effect of melting ice caps. Ice physics were poorly modelled at the time.

We understand ice better now, but the uncertainties are still massive. Holgate says the rise could be as much as 1.5 metres by 2100, but he says that is "an extreme upper end". The linear rise mandated by North Carolina is the lowest possible, he says.

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