Salt marshes predict sea level rise

PENN STATE (US) — Scientists are using a 2,000-year history to refine models used to gauge climate-change induced sea level rise, which, during the past century, rose at a faster rate than ever before.

“One of the largest uncertainties in projecting the impacts of climate change involves predicting the amount and rate of future sea level rise,” says Michael E. Mann, professor of meteorology at Penn State.

“The societal ramifications are as great as any climate change impact, but, because the uncertainties are particularly large due to limitations in the representations of some key processes, such as ice sheet collapse in existing models, we still do not know how sea level will rise.”

To create the sea level timeline, researchers examined sediment cores from salt marshes in North Carolina to create an unbroken record of sea level through time using remains of foraminifera, tiny plankton-like creatures that live in the oceans.

Because different species of foraminifera live at different depths in the oceans, a survey of the types of remains can show how deep the ocean was in that particular spot at the time the sediment layer was laid down. Dating of the layers provides a timeline of sea level changes.

The findings are reported online in the journal Proceedings of the National Academy of Sciences.

Combining the sea level changes through time with the already established temperature record for the past 1,000 years, researchers created a model, partly based on observations, that match what happened historically and can be used to predict future changes in sea level.

One problem with current model estimates of sea level rise is that they don’t take into account a variety of potentially important processes.

The simulations cited by the Intergovernmental Panel on Climate Change for its 2007 report, for example, don’t include effects of ice melting from glaciers in Greenland and Antarctica.

“Prior to the past few decades there was no obvious contribution from melting ice sheets,” says Mann. “It is only over the past five years or so that we have clear evidence that the ice sheets are losing mass. Prior to that they appear to have been stable as far back as the end of the last ice age.”

Because the sea level model in the current study is based on observations, it includes, in principal, all relevant processes, including contribution from melting ice sheets, mountain glaciers, and the expansion of seawater with increased temperatures.

Researchers chose North Carolina salt marshes for their sediment samples because the area is relatively free of impacts related to the slow rebounding of the Earth’s surface from the weight of the ice sheet that covered parts of North America during the last ice age, minimizing the necessary adjustments for the rebound.

“The temperature and sea level reconstructions were determined independently from each other, and yet each shows what we would expect based on the other,” says Mann. “Higher temperatures correspond with higher rates of sea level change and vice versa.”

From 100 B.C. to A.D. 950, the sea level in North Carolina was stable. From 950 to 1400, sea level rose at a rate of a bit over 0.02 inch per year due to the relative warmth during the Medieval Period. From 1400 until about, sea level was again stable due to the effects of the Little Ice Age. The rate of rise from 1880 through 1920 in North Carolina was 0.08 inch a year, according to the researchers.

“This historical rate of rise was greater than any other persistent, century-scale trend during the past 2,100 years.”

Researchers from the University of Pennsylvania, Aalto University School of Engineering, Finland, and Potsdam Institute for Climate Impact Research, Germany contributed to the study that was funded by the National Science Foundation, National Oceanic and Atmospheric Administration, U. S. Geological Survey, Academy of Finland Project, and the European Cooperation in Science and Technology.

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