Rising Sea Level, Not Storm Intensity, Is Culprit in Erosion

For East Coast residents in the US, the debate over man-made climate change is irrelevant. They are already experiencing the increased destructiveness from severe storms that prophets of global warming envision for the next century. They have to act now to adapt to increased coastal erosion even though there's little evidence that, this century, coastal climate has changed.

That's the implication of a new study of East Coast storminess over the past seven decades. Scientists at the laboratory for coastal research at the University of Maryland in College Park have, for the first time, constructed a history of storm frequency and intensity from tide gauge data. The researchers - Keqi Zhang, Bruce Douglas, and Stephen Leatherman - published their findings recently in the geophysical weekly Eos.

Dr. Zhang explains that the culprit is the slow rise in sea level that has been going on for many decades. The destructiveness of severe storms - mostly northeasters and hurricanes - comes from the waves that ride the surge of water that the storms drive ashore. The higher the water level, the higher up a vulnerable cliff face the waves can pound, the farther inland the waves can reach.

Three factors determine water level at any given site. Average sea level for the site is the base. The state of the local tide then adds to (or subtracts from) that level. Finally, the extra water forced in by a storm raises the local water level even higher. That's why the worst destruction often comes when a storm surge coincides with high tide.

The tides have not changed appreciably in this century. Now the Maryland study indicates that storm intensity hasn't changed either. What has changed is the average sea level. It has risen enough so that storm surges that were relatively benign a century ago now reach high enough for the waves riding them to cause massive coastal erosion.

To get this result, the Maryland scientists had to distinguish between the added height of a storm surge itself and the underlying water level due to the combination of average sea level and the local tide. They used hourly water level data released by the National Oceanic and Atmospheric Administration. Zhang notes: "Nobody checked those data before. So we checked it to see if we could dig out some information."

Using data from 1911 to 1993 for Atlantic City, N.J., and from 1922 to 1993 for Charleston, S.C., the researchers were able to pick out surges for individual severe storms and get a measure of overall storm intensity. They found "no significant long-term trends" in that intensity.

The scientists now are studying records from 10 other East Coast sites in the US stretching from Maine to Florida. Zhang says that, so far, "the results are almost the same" as they are for Atlantic City and Charleston.

Global climate-change forecasts envision an increase in storm intensity as climate warms. There is no sign of anything like that having happened along the East Coast. Yet storm-driven erosion has increased. The only possible climate-change effect would be an average sea-level rise. Sea-level researcher Allan Clarke at Florida State University in Tallahassee says that global average sea level has been rising at a rate of 1 to 2 millimeters a year. This will vary according to local conditions. The cause of that rise is debatable, but may be due to global warming, Dr. Clarke says.

What this means for East Coast residents is clear. Any 21st-century climate change that made storms more intense would only exacerbate a situation that already is dangerous. Sens. John Kerry (D) of Massachusetts and Ernest Hollings (D) of South Carolina have filed legislation to form a presidential commission to study ocean and coastal policy. It also would establish a federal interagency task force to review what's happening to the oceans and coastal regions.