The rise in sea levels is perhaps the most dangerous consequence of climate change. A majority of the world's population lives in coastal regions, and especially in poorer nations, hundreds of millions of people are threatened with an ever-increasing number of floods.
And the forecasts for sea levels are only pointing in one direction: upwards. The United Nations Intergovernmental Panel on Climate Change predicted in its status report from 2007 that, on average, global sea levels could rise by 59 centimeters by the year 2100. In a UN report released last week, the estimated rise in sea levels could be between 90 centimeters and 1.60 meters over the course of this century.
Now researchers have reconstructed how sea levels have risen historically, and have found that oceans are now swelling at a faster pace than ever before in the past 2,000 years.
Until now, the relationship between air temperature and sea levels has only been proven for the last 130 years, says Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK). His team studied fossilized calcareous shells of microbes that came from the sediment in salt marshes in the state of North Carolina on the US Atlantic coast. Because these microbes to a certain degree depend on the ebb and flow of the sea waters, they show the scope of the rise in sea levels.
Mapping Out 2,000 Years in Sea Levels
Rahmstorf and colleagues published their findings in the current issue of the Proceedings of the National Academy of Sciences. They have been able to map out four phases in the rise in sea levels. From 200 B.C. until 1,000 A.D., the water levels were relatively stable. Starting in the 11th century, sea levels rose about five centimeters per century for 400 years, which could be due to the warm periods of the Middle Ages. After that, there was another stable period with cooler temperatures that lasted until late in the 19th century, according to their report.
With industrialization came a dramatic upward trend: In a little more than 100 years, sea levels have risen by about 20 centimeters, or several times that which occurred per century over the past 2000 years.
Researchers trace that back to two factors: When water gets warmer it expands and sea levels rise; then melt water from the glaciers and from the large ice masses in Greenland and the Antarctic raise levels higher.
"Man continues to heat up the climate with the release of greenhouse gases, and as a result the glaciers are melting even faster and the sea levels are rising quicker than ever," Rahmstorf says. "The new study confirms our model of the rise in sea levels, and the data from the past sharpens our view of the future."
Critics Point to Limited Data
But other experts are doubtful. They see the new study's limited data as its primary drawback, and question whether something based on findings from the North Carolina coast alone can be applicable for the whole world. "This study is, therefore, not at all suited for making predictions," says Jens Schröter of the Alfred Wegener Insitute for Polar and Marine Research in Bremerhaven, Germany.
Rahmstorf and his colleagues concede that local sea level fluctuations can differ from global trends. Nevertheless, the scientists conclude that their data, by and large, reflects the changes in global sea levels.
Schröter argues that over a time period of more than 2,000 years, the effect of things like continental drift and the so-called isostatic adjustment would be noticeable.
Isostatic adjustment, one of the results of the last ice age, occurred after the glaciers melted and the land masses, freed of such heavy weight on top of them, started to slowly rebound, a process which continues today. In Scotland, some areas have seen a sea level rise of 60 centimeters over the last century, while in parts of southern England and the French Channel coast levels have fallen by about the same amount.
'One of the Best' Regions for a Study
Rahmstorf and his colleagues also collected data from other parts of the world for their study, but it deviates considerably from the findings from North America. "Only the data from North Carolina works, to some degree, in reconstructing sea level trends," says Schröter. He is critical of the fact that the PIK researchers have tried with their data to validate their already existing model. "If one tried, on the basis of the data alone to develop a curve, it would have probably been difficult," he says.
Michal Kucera, of the University of Tübingen in southern Germany, also asks how representative the data from North America is. At the least, though, the region is "one of the best" for such a study, he says, noting that in other locations the conditions would have been even worse.
Mojib Latif of the Leibniz Institute of Marine Sciences at Kiel University (IFM-Geomar) describes the observation period of 2,000 years as one of the "strengths of the study." But the long-term natural fluctuations in sea levels are poorly understood, he says. "What happened in time periods of 300 and 400 years is highly contentious," he says.
Satellite images offer proof, though, that sea levels have risen significantly in recent decades. And this development occurred exactly at the same time as industrialization and the rise in air temperature. It would be "hard to argue" that that could be accidental, Latif says, thus siding with Rahmstorf.
But when it comes to the predictions for the future, he has the similar doubts as Schröter. No one really knows how much ice will be lost in the Arctic and Antarctic in the coming decades and centuries. "One must be honest about that," Latif says.