Q&A on the News

Q: There are people who claim the oceans are rising. Others claim they are not. How are ocean levels measured?

—Harris Gottlieb, Dunwoody

A: Ocean levels are measured in two ways. The more traditional method is the global network of tide gauges in several countries around the world. They date to the mid-19th century and provide a history of sea levels, Stephen Gill, senior scientist at the Center for Operational Oceanographic Products and Services at the National Oceanic and Atmospheric Administration told Q&A on the News in an email. The drawback to this method is that most of the gauges are in the Northern Hemisphere and "data must be corrected for vertical land movement before being applied to global sea level understanding," he wrote.

The other, and more modern method, is a series of ongoing international satellite altimeter readings. Gill said these provide “true global coverage,” but date to only 1993. Both methods show the sea levels generally are rising, but “there is lots of variability along our coasts, mainly due to variations in vertical land motion.” Levels are rising faster in some areas – like Louisiana and Chesapeake Bay – “where there is lots of land subsidence.”

The altimeter measurements suggest the global sea level has been rising at a faster rate (3 millimeters a year) than the long-term trend (1.8 millimeters) that has been determined from the tide gauges. The Center for Operational Oceanographic Products and Services has been measuring ocean levels for more than 150 years.

Andy Johnston wrote this column. Do you have a question about the news? We’ll try to get the answer. Call 404-222-2002 or email q&a@ajc.com (include name, phone and city).