Report of Rising Ocean Prompts Fears of Flooding

Southern California’s ocean water has warmed slightly during the past four decades, causing a minuscule rise in sea level each year--a trend that concerns some scientists who say it could one day lead to coastal flooding, according to a report in a scientific journal released today.

Local ocean surface waters have gradually warmed by about 1 degree Celsius (1.5 degrees Fahrenheit) since 1950, which has caused the seawater to slowly expand and rise 1.5 inches, said Dean Roemmich, author of the study in Science.

“The ocean is indeed changing here,” said Roemmich, an associate professor of oceanography at the Scripps Institution of Oceanography. “This translates to some degree of concern as to what happens in the next 20 years.”

The total sea level rise, however, is such a small increase that it poses no immediate threat to beaches, Roemmich said. It is not yet known what impact--if any--the warming or the sea level rise will have on marine ecology.

The global sea level has been rising at a rate of about 2 millimeters each year, which scientists believe could be caused by several factors, including the greenhouse effect, melting glaciers, and changes in the shape and volume of the ocean basins.

Global warming, some experts say, is caused by the emission of carbon dioxide and other gases from cars and industry. These gases collect in the atmosphere and act like a greenhouse, trapping heat from the sun.

But scientists have not yet determined how much of the ocean’s warming is due to the greenhouse effect or to other culprits. Nor does today’s study indicate that the warming of local waters is caused by the greenhouse effect, Roemmich and others say.

“To make the leap that it’s proof of global warming is too big a leap,” said Bruce Parker, a principal investigator for the National Oceanic and Atmospheric Administration in Maryland. “It’s very difficult to separate out the potential causes.”

Parker points out that other factors could be involved in the apparent rise of sea level, including earthquake activity that could cause the land to shift: “If the land is sinking, it looks like the sea level is rising.”

But others say Roemmich’s findings are cause for concern, particularly if the warming trend quickens.

“If it does accelerate, we are in serious trouble. Even a 10-centimeter rise is nothing to laugh at,” and could trigger coastal flooding and other environmental problems, said Robert Watts, a professor of engineering at Tulane University who studies climate change.

For his study, Roemmich examined water samples collected along the coast for the last 42 years from Del Mar to Point Conception, extending near the shore to 310 miles offshore. The target area covers 77,000 square miles of the Pacific Ocean.

Hundreds of thousands of ocean measurements were taken from research ships participating in the California Cooperative Oceanic Fisheries Investigation, a program launched by Scripps, the National Marine Fisheries Service, and the state.

After analyzing the samples, Roemmich found a steady increase in the warming of the top layer, or first 325 feet, of the ocean. Beyond that level, temperatures began to drop down to 1,000 feet.

“I can’t conclude (from the data) whether this is part of the natural climate or it’s induced by human activity. If it’s human activity, then I suspect the rate of rise and warming could accelerate in the next 20 years,” Roemmich said. “If the sea level increases, you worry about coastal flooding.”

During an El Nino condition, the sea surface temperature along the Equator and tropics increases by as much as three degrees Celsius. In his study, Roemmich found that the local waters rose during El Ninos in 1957-58 and 1982-83 and never dropped back to previous levels.

Experts do not know how the marine ecology is affected by the gradually increasing water temperatures. While one degree warmer may not seem significant to most swimmers, the temperature can have an impact on the plants and fish, said Tom Hayward, an associate research oceanographer with Scripps.

Water circulation, ocean currents and winds are “major determinants of the biological properties of the ocean,” Hayward said. “The step we don’t know very well is how this change of one degree will affect the physical structure.”

Hayward points out that an El Nino can have a dramatic impact on marine ecology, affecting both the temperature and the ocean’s circulation.

Microscopic plants and animals, or plankton, serve as food for larger animals and play a crucial role in the ocean’s cycle of nutrients and chemicals. When the ocean’s surface warms, these populations can be depleted. But it’s unknown how much damage is wrought by small temperature increases.