Rising Tide

Released: 11/21/2008 1:00 PM EST
Source Newsroom: University of California, San Diego, Scripps Institution of Oceanography
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Newswise — In "Dover Beach," the 19th Century poet Matthew Arnold describes waves that "begin, and cease, and then again begin"¦and bring"¨the eternal note of sadness in."

But in the warming world of the 21st Century, waves could be riding oceans that will rise anywhere from 0.5 meters (19 inches) to 1.4 meters (55 inches), and researchers believe there's a good chance they will stir stronger feelings than melancholia.

Several scientists from Scripps Institution of Oceanography at UC San Diego are finding that sea level rise will have different consequences in different places but that they will be profound on virtually all coastlines. Land in some areas of the Atlantic and Gulf coasts of the United States will simply be underwater.

On the West Coast, with its different topography and different climate regimes, problems will likely play out differently. The scientists' most recent conclusions, even when conservative scenarios are involved, suggest that coastal development, popular beaches, vital estuaries, and even California's supply of fresh water could be severely impacted by a combination of natural and human-made forces.

Scripps climate scientists often consider changes in average conditions over many years but, in this case, it's the extremes that have them worried. A global sea level rise that makes gentle summer surf lap at a beachgoer's knees rather than his or her ankles is one thing. But when coupled with energetic winter El Niño-fueled storms and high tides, elevated water levels would have dramatic consequences.

The result could transform the appearance of the beaches at the heart of California's allure.

"As sea level goes up, some beaches are going to shrink," said Scripps oceanographer Peter Bromirski. "Some will probably disappear."

Sea level has been trending upward for millennia. For the last 6,000 years, it is estimated that global sea levels have rising an average of five centimeters (2 inches) per century. Before that, between 18,000 and 6,000 years ago, the seas rose a full 120 meters (400 feet). Step by step, they bit into rocky coastlines like California's by smashing cliffs, creating beaches with the debris, rising a bit more, and repeating the process over and over again.

Humans are speeding up the pace of that assault. The United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC) reported that sea level rose, on average, 1.7 millimeters (0.07 inches) per year over the entire 20th Century. But recent estimates from satellite observations find a marked increase, at 3.1 millimeters (0.12 inches) per year since 1993.

The oceans are rising because the warming ocean water increases in volume and because water is being added from melting glaciers and land-based ice sheets. The complex difficult-to-predict contribution of the latter is such a matter of controversy that the recent IPCC Fourth Assessment report didn't factor glacial melt into its sea level rise estimates. Today there is quite broad-based opinion that the IPCC estimates are considerably lower than the higher range of possible sea level rise. Some individuals, pointing to the quantity of water frozen in Greenland and Antarctica and to ancient sea level evidence, have suggested that sea level rise could reach several meters by the end of the 21st Century. However, an August paper in the journal Science co-authored by former Scripps postdoctoral researcher Shad O'Neel suggests that some of the more exaggerated claims that water could rise upwards of 10 meters (33 feet) by century's end are not in the realm of possibility. O'Neel and co-authors indicate that the realities of physics impose a cap of 2 meters (6.6 feet) for possible sea level rise by 2100.

"That's fine," said Scripps climate researcher Dan Cayan, who is leading an analysis of climate change scenarios for the state of California, "but two meters is still enough to do a lot of damage."

Recent news footage of overtopped levees makes it easy to envision what two meters' difference means to low-lying cities like New Orleans, especially when extreme events like hurricanes are factored in. Any flooding would be proportionately higher than it is now. Additionally Bromirski recently showed that sea level rise will amplify the power and frequency of hurricane-generated waves that reach shore, even if the storms themselves don't make landfall.

In contrast to the beaches of the East Coast, many of which are covered with vast expanses of sand, California's coastline is predominantly bedrock covered by a relatively thin veneer of sand. That sand can shift or disappear during storms. Thus, preserving the precious supply that keeps the tourists coming has for decades been a priority for state officials. Resource management, however, has required them to make trade-offs. They have constructed seawalls to protect houses built on ocean cliffs. They have dammed rivers to create supplies of water for drinking and to prevent floods and debris from damaging downstream developments.

In so doing, nature's two primary sources of beach replenishment have been muted in a process known as passive erosion. Managers have compensated through artificial beach replenishment projects but at a costs that approach $10 per cubic yard. Since usually millions of cubic yards of sand need to be moved, there are monetary limits to what they can reasonably accomplish.

Reinhard Flick, who received his doctorate in oceanography from Scripps in 1978, needs only to look out his office window to watch the losing battle of beaches unfold. During his student days, he used to play volleyball on stretches of sand that are now underwater except during low tide. Rocks buried under several feet of sand four decades ago are now exposed for large parts of the year.

The staff oceanographer for the California Department of Boating and Waterways, Flick said that seawalls causing passive erosion will likely combine with sea level rise to doom some Southern California beaches. The change will become most apparent during El Niño events, when a pool of warm Pacific Ocean water settles off the coast for a year or two. El Niño has a dual effect on the West Coast. It not only feeds more intense storms but the warm ocean water itself causes a temporary spike in sea level that is above and beyond the rise that climate change is causing. During the 1997-98 El Niño, for instance, tide gauges off San Francisco recorded that sea level was 20 centimeters (8 inches) above normal for more than a year, including the winter storm season. That temporary rise is about equal to the rise observed for the entire 20th Century.

If sea levels rise substantially, when a large storm coincides with a high tide during an El Niño event, there could be widespread inundation along the California coast. Effects could range from a submersion of areas of San Diego's Mission Beach to an inundation of the Sacramento-San Joaquin Delta. There, an overtopping of the delta's levees by brackish water could paralyze the main component of the state's water delivery system. Cayan noted that repairs to the system could take months.

The threat resonates with state officials, who have tasked Scripps and other institutions with creating and updating sea level rise scenarios.

"There's no clear path forward with sea level rise," said Tony Brunello, deputy secretary for climate change and energy at the California Resources Agency, a key Scripps partner in developing the state's response to manifestations of global warming. "You typically want to work with one number (but) what we want people to do is work with the whole range of estimates."

Cayan and other Scripps researchers who are collaborating to study sea level rise emphasize that there remains a great deal of uncertainty in the creation of estimates for the coming century. The range of rise estimated by Cayan is based on scenarios of global air temperatures over the next 100 years, which range from about 2° C (3.6° F) to about 6° C (10° F). By 2100, global sea level rise reaching a half-meter seems likely, and if the higher rates of potential warming occur it could rise by more than one meter. The potential cost of any government project or policy change puts a high premium on narrowing this range. As O'Neel and his co-authors observed in their paper, the cost of raising Central Valley levees only 15 centimeters (6 inches) to prepare for higher sea levels has been estimated at more than $1 billion.

"These are very broad-brush preliminary kinds of studies right now, but you have to start somewhere," said Scripps coastal oceanographer Bob Guza.

Flick said it will be essential for scientists to be able to study the effects of the next El Niño so they can begin to understand not just where damage will happen on the California coast but to what extent. He only had surveyor's equipment and aerial photos available to him to measure beach changes after the 1982-83 El Niño, but Guza and his collaborators now have light detection and ranging (LIDAR) and GPS technologies to make precise surveys of beach and cliff damage. Guza and Flick hope that Scripps can not only enhance its use of such technology but to deploy it within hours of a major storm event.

"We need to be geared up to quantify what beach changes are," said Flick. "We have to do an even better job of studying wave forces and wave climate."

If there's any good news for Southern California, Scripps climate scientist Nick Graham has estimated that ocean warming trends will drive storm tracks farther north, perhaps sparing the state's lower half from the full brunt of buffeting El Niño waves the 21st Century will generate. Graham compared winds produced in three different simulations of climate change with those generated in the late 20th Century. The models showed that Southern California can expect a moderate decrease in wave size of about 0.25 meters (10 inches). But even there, Graham sees a problem.

"I'm a surfer. I think that's horrible," he said.