Ben Horton: Sea level rise faster now than in 2,000 years

Using microscopic fossils from core samples obtained in a coastal salt marsh in North Carolina, researchers have determined the sea level has risen and fallen over the past 2,000 years and that – at the end of the 19th century, going through the 20th century – a rapid rise in sea level began. They say sea level is rising faster today than at any time over the past 2,000 years.

The timing of a rapid rise in sea level in our time was “very, very abrupt,” according to these researchers, whose paper was published in June 2011 in the Proceedings of the National Academy of Sciences.

EarthSky spoke with paper co-author and University of Pennsylvania geologist Ben Horton. He said his team didn’t use computer models. Instead, they gathered direct physical evidence in North Carolina, on the U.S. Atlantic coast.

Salt marsh in North Carolina. Image Credit: Nature’s Images

Ben Horton told EarthSky:

You have the observational record that we have in the 20th century, from tide gauge records, or from satellites in space, they record that sea level is rising. What you want to do is go back though time, and to see whether this sea level rise is out of the ordinary.

To determine how much the sea level in North Carolina has fluctuated over the past two millenia – and precisely when it fluctuated – Horton analyzed core samples from a coastal salt marsh. The samples contained fossils of a microscopic organism sensitive to levels of salt in its environment. These fossils helped the team determine that sea level has fluctuated over the past 2,000 years – and that its rise and fall has tended to be gradual. But, Horton said:

At the latter part of the 19th century, going through the 20th century … we have a pronounced increase in the rate of sea level rise to two millimeters per year. And the timing of that change was very, very abrupt.

It is another piece of very, very strong evidence to support that the climate or the environment in which we have been living in the 20th century are so very different from what we have been living in prior to the industrial revolution.

A foraminiferan, a type of organism that is sensitive to salt. Image Credit: Safay

Horton added that this work may help experts understand better how sea level rise – believed by virtually all climate scientists to stem from an overall warming of the planet – will vary from coastline to coastline. It’s something he’s working to understand better, right now.

Horton talked a little more about the process he used to record historical sea level rise and fall on North Carolina’s coast:

If you go out on a salt marsh, they would notice this sort of pattern of vegetation, you’d find different plant communities. And what they’re responding to are changes in salinity. And therefore what you can think is that if the sea level changed, then the salt marsh species would change. We didn’t look at the plant species themselves. We looked at microscopic organisms known as foraminifera. They live in the salt marsh sediments.

Each species of foraminifera has a very specific level of salinity that it likes to live in, and an area it doesn’t like to live in. So for example you can get a certain species that likes to be inundated by the tides 10% of the time, and a certain species that may want to be inundated by the tides 50% of the time. So you can take a core of the salt marsh, and if you see that in one part of the core that [Ed. Note: there are different species of microbes in different layers], you can see that there is obviously a sea level response, there.

Horton said that the team also used fossilized pollen, in part, to date sea level rise and fall, because different species of plants were introduced to the area at different periods over the past two millenia. He credited lead author Andrew Kemp with this extremely creative method of creating timeline “horizons” for the salt marsh cores.

He described the team’s major findings. They sounded a little like a historical novel. The last 2,000 years are great to study, he said, because the Earth’s major systems – the currents, the ice sheets, the storm patterns – are pretty similar to what they are today. Yet, things were still in flux.

The first thing we found was that sea level was variable. Second of all, we could take this 2,000 years and divide it into four stages. What we found was that 0 A.D., the Roman period going through about 1000 A.D., things were pretty stable. It didn’t really do anything. Then it’s around 1000 A.D., the Medieval Warming Period, where temperatures increased. Not as warm as they are today, but they certainly increased. Sea level also rises, at rates of less than one millimeter per year. Very small rates, but they certainly were noticeable. And that period lasted for about 300 years.

Then at the 14th century, sea level stabilized, and perhaps fell. And that’s known to have occurred during what’s called Earth’s Little Ice Age period when temperatures have been known to have stabilized or fallen, and we get a sea level response. And the fourth stage is at the latter part of the 19th century, going through the 20th century when we have a pronounced increase in the rates of sea level rise to two millimeters per year … the fastest rates of rise in the past 2,000 years. And the timing was very, very abrupt. The other timings were very gradual.

He added that one scientific advantage of creating a continuous physical and observational record of sea level rise is that there’s less margin of error, in terms of results. That translates not only into better understanding the past, but also more accurately modeling the future.

Listen to the 90-second EarthSky interview with Ben Horton on abrupt sea level rise in our time – a faster rise now than in the past 2,000 years – at the top of this page.

Beautiful barrier islands seen from space, as sea level rises

Harold Wanless on sea level rise in South Florida

August 15, 2011

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