Sea level rising, Delaware sinking

Are coastal storms becoming more intense? State Climatologist Dan Leathers says no. But as the sea level continues to rise and Delaware continues to sink, he said, flooding is getting worse and could cause more damage in the future.

Leathers was the featured speaker at the Lewes Mitigation Planning Team's presentation on climate change and sea-level rise Aug. 24 at the Lewes Public Library. Leathers, a professor in the Department of Geography at the University of Delaware, used the time to break down the trends found in data collected over the last century.

Leathers said Delaware's sea level is rising at a rate of 1-2 mm per year. At the same time, Delaware is sinking at a rate of about 1.7 mm per year. Combined, he said, the sea level is rising about 3.4 mm per year. Over the next century, he said, the sea level will likely rise 13 inches.

“A foot of sea-level rise is pretty darn important when you have many roads in Delaware that flood every time there's a full moon and a high tide,” he said. “You add a foot onto that, and it becomes a big deal.”

But why is the sea rising and Delaware sinking? Leathers said land-based glaciers and ice sheets are melting and that water is going into the ocean. As the Laurentide ice sheet has receded since the last Ice Age, he said, the weight on the Earth's crust has decreased. Earth's crust is pliable; because the ice sheet did not reach Delaware, he said, the Mid-Atlantic land surface bulged up due to the weight on the crust nearby. The crust is now rebounding, and Delaware is sinking back.

As sea level rises, the Cape Region will continue to see more flooding. Leathers said noncatastrophic nuisance is already on the rise. In terms of moderate and severe flooding, he said, data shows Delaware has had 36 moderate flooding events and six major flooding events since 1957.

Frequency and severity of coastal storms remains static, though. Delaware averages 32 storms per year, and has had as many as 50 and as few as 15.

“When you do get an intense coastal storm, and the sea level has risen by a foot, that causes bigger problems than it did a century ago as far as flooding is concerned,” he said.

This all has an effect on Lewes and the other coastal towns as municipal officials continue to monitor and change their flood-plain ordinances, which dictate what can be built and how buildings are built in flood areas. They are also one of the factors in a town's flood insurance discount, administered through the federal government's Community Rating System.

Leathers also discussed Delaware's climate. While precipitation levels remain variable, he said, the mean annual temperature continues to rise. Based on data dating back 1895, Leathers said, the state's annual temperature has risen .2 degrees Fahrenheit per decade.

“If you think that's a very small temperature change, you're absolutely right,” he said. “But now we have 12 decades and you multiply that by 12, and it's a 2.5 degree change from what we would've expected back in the 1890s.”

The National Oceanic and Atmospheric Administration recently released a report that supports Leathers' data. NOAA reports that July was the 15th consecutive month that global temperatures over land and ocean surfaces surpassed the previous high dating back to the beginning of record keeping in 1880.

As far as what to expect in the future, Leathers said, models show a continued gradual upward trend in temperature for at least the next century.