

Bob Marshall, The Lens By Bob Marshall, The Lens
on December 13, 2008 at 8:36 PM, updated December 25, 2009 at 12:25 PM



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In 50 to 100 years, the numbers tell them, rising seas caused by global warming, combined with the steady subsidence of Louisiana's coast, will lift the Gulf of Mexico two to six feet higher in many areas surrounding New Orleans.


Read related story: Sea levels have been rising globally for ages.

"The delta of the Mississippi River is the most vulnerable location in the nation to global warming, because it is sinking at the same time sea level is rising, " said Virginia Burkett, a senior researcher at the National Wetlands Research Center in Lafayette and one of the nation's foremost experts on climate change. "And it's only going to get worse.

"This area is facing big trouble from climate change. I think there's consensus on that point."

A lot at stake

As the scientific forecasts of global warming gain popular acceptance, many Americans now ponder how their lives might change.

Changing coastline: Download file

Longer, hotter summers. Shorter, warmer winters. Less rain, or more. Lifestyle adjustments ranging from different light bulbs to hybrid-powered cars.

But climate scientists now say residents in low-lying, fast-sinking southeast Louisiana will have a more serious concern: survival.

"People who live here have a lot more at stake in what happens in the Antarctic and Greenland than any people in this country, " Tulane researcher Torbjorn Tornqvist said. "We know we're sinking, and we know sea level is rising. . . . If either gets much worse, we'll be among the first to experience disaster."

These predictions come on top of already dire warnings that the traditional forces of coastal erosion -- sediment deprivation and canal dredging -- have left the state with less than a decade to fix that problem or face permanent land loss.

We are not alone: Download file

The additional threat from global warming not only reinforces the need to speed coastal restoration efforts, scientists say, it also raises critical questions about many vital hurricane protection and coastal restoration projects in the planning stages or already under way.

Are the projects being designed to meet the increased threats from sea-level rise, including higher storm surges and expanding areas of open water? Can the planned structures be adapted to meet increased threats, as the Gulf rises and the land sinks? Will pumping stations that battle rainwater flooding have enough power to lift water two to four feet higher? Are the causeways and bridges that link the region's communities high enough to survive the gradually rising tides -- not to mention stronger storm surges?

Evidence behind sea rise: Download file

Are the state and city even planning for the changes the world's scientific community says are heading our way?

Not if, but when

Scientists involved in global warming research speak with confidence about the threats to coastal Louisiana, because they are based on three factors that generate little debate:

-- Subsidence in Louisiana, documented for decades, will continue at alarming rates for the foreseeable future.

-- Sea-level rise is one of the most widely accepted, easily measurable effects of the warming climate.

-- Even if the world moves aggressively to reduce suspected causes of global warming, sea levels would continue to rise for centuries as the oceans slowly respond to temperatures that have been rising since the 1800s.

"The debate within the scientific community is no longer 'if' this will happen. It's now 'when and how quickly, ' " Burkett said.

University of New Orleans researcher Shea Penland, in one of his last interviews before he died this year, summed up the scenario: "Without some really huge and immediate steps to meet this new challenge, we're just S.O.L."

Yet scientists are concerned that a threat growing by only fractions of an inch each year will be underestimated by decision-makers. In contrast to an instant and overwhelming disaster like Katrina, sea-level rise will proceed slowly, almost imperceptibly -- until it's too late to address.

"We're like that frog in the pot of water on the stove -- if we wait until it starts boiling, we won't be able to jump out, " said Burkett, echoing a sentiment common in the scientific community.

Scientific agreement

Worldwide, the scientific community speaks through the Intergovernmental Panel on Climate Change , established in 1988 by the World Meteorological Organization and the United Nations. Consisting of scientists and government agencies from dozens of nations including the United States, the panel sought to determine whether the planet was warming, the causes and potential impacts, and how governments might adapt.

In 1990, the panel began releasing a series of reports confirming global warming, and outlining the primary cause: Greenhouse gases, the carbon-based pollutants released in the burning of fossil fuels, are trapping heat inside the atmosphere. The panel has recommended immediate dramatic reductions in these pollutants to begin curbing the problem. However, the panel admits the process is so advanced that many changes already under way will continue through this century even in the face of an aggressive cleanup.

One of those changes is accelerating sea-level rise.

Because the forecasts from the climate change panel rely on complicated computer modeling, researchers can't predict impacts with certainty. But as the panel moves toward its third decade, members' confidence has increased. More sophisticated models have been supported by real-world events, such as the recent rapid melting of glaciers and polar ice fields. Predictions that once were termed "possible" are now made with "high confidence."

Although serious scientific debate remains about some of those projections, the predictions for sea-level rise, which could slowly drown coastal communities worldwide, has drawn wider agreement.

That consensus rests on two indisputable events that will occur as oceans continue to warm:

-- Sea-water will expand as it warms, encroaching into land masses worldwide.

-- The runoff from melting glaciers and ice fields will increase the total volume of the oceans.

The latter impact has recently become a grave concern because ice fields and glaciers have started melting more rapidly than the climate panel's models predicted just two years ago. Climate scientists, alarmed by the increase, are struggling to understand the causes.

"If these rates continue in Greenland and the Antarctic, then all bets are off, " Burkett said. "Then we're not talking about two to four to six feet of rise, we're talking about something much greater and even more rapid."

Worldwide, the midrange estimate predicts oceans will rise 18 inches by 2100.

Louisiana faces a far more alarming forecast. Here, those same models predict that, relative to land, water levels will rise 2 to 6 feet, with the highest rates in the southeastern coast surrounding New Orleans.

The subsidence problem

The difference owes to subsidence.

Louisiana falls victim to what scientists term "relative" sea-level rise: the net result when water rises at the same time land sinks. And the southeast portion of the state's coast, the vast delta of the Mississippi River, is subsiding at one of the fastest rates in the world.

Healthy coastal wetlands could probably handle a rise of 18 inches over 100 years, scientists say, because they have a natural ability to gain elevation through the regular arrival of new building material from three sources: sediment from spring river floods, storm surges that carry offshore sediments onshore, and the steady deposit of new soil created from decaying plants in healthy wetlands. This is the process called "accretion."

Wetlands can also adjust to rising sea level by migrating northward in their basins and colonizing higher ground.

"And, in fact, there are wetlands in this region that have been doing quite well against current levels of relative sea-level rise, " said Denise Reed, a wetlands researcher at UNO. "So, by itself, the projections of sea-level rise we're seeing published are no reason to think healthy wetland ecosystems can't keep pace."

An example of such a healthy wetland is the delta of the Atchafalaya River on the central Louisiana coast. Unrestrained by levees, the Atchafalaya has built more than 27 square miles of new land in the past 40 years.

But the health of southeast Louisiana's wetlands began to fail in the early 1900s when federal and state levees shut off river sediment from flowing into the wetlands. Erosion accelerated in the mid-1900s with extensive canal dredging for oil, gas, shipping and housing development, cutting through healthy wetlands and ultimately creating vast expanses of open water.

Those problems alone make it difficult for much of the region to keep pace with the 18-inch rise in sea level expected by the end of the century just from rising surface temperatures. When subsidence is added to the equation, natural adaptation becomes impossible, coastal experts think.

Fate of the region

The numbers are grim. Southeast Louisiana is expected to sink between two and five feet by the end of the century -- one of the fastest subsidence rates on the planet. Those estimates are supported by real-life measurements that show sea level has been rising one inch every 30 months in some sections of the southeast coast. That rate would result in a 16-inch rise by 2050.

The Intergovernmental Panel on Climate Change asked its computers how high the Gulf of Mexico would rise along the Louisiana coast. Researchers calculated varying scenarios of subsidence and rates of sea-level rise, determined by how quickly the world moved to reduce greenhouse gases.

The best-case scenario, which includes a rapid atmospheric cleanup and slower subsidence, shows rises of 12 inches in 50 years and 24 inches in 100 years.

The worst-case scenario, using little change in greenhouse gas build-up, shows a 38-inch rise in 50 years and more than 6 feet in 100 years, a rate that could drown many areas surrounding New Orleans and make the city all but an island.

And studies completed since those 2007 projections trend away from any best-case endings, indicating greenhouse gasses are accumulating much faster than predicted just 12 months ago. This latest research, Burkett said, showed the amount of carbon in the atmosphere by the end of this century could be double the pre-industrial levels of the late 1800s.

"The elephant in the room remains the rate of ice sheet declines, either in Greenland or western Antarctica, " Burkett said. "If they were to disintegrate, we could see a sea level rise" of 16 to 19 feet.

Researchers familiar with southeast Louisiana's rapidly deteriorating coastal wetlands agree that even the best-case scenarios threaten to inundate all areas outside of hurricane levees during the next century -- unless rapid and aggressive coastal restoration starts within a few years.

"Most of that area (outside the Atchafalaya) is struggling to stay even with the old rates of sea-level rise, so I don't think they stand much chance of surviving what the models are forecasting, " said Don Cahoon, a U.S. Geological Survey researcher who wrote some of the most detailed studies of accretion in Louisiana marshes. "They just can't gain enough elevation under the present conditions to adjust."

UNO's Penland laughed off hopes that healthy marshes in the region could survive even the low-range sea-level rise predictions.

"When you add subsidence to rates of sea-level rise we know are coming due to global warming, " he said, "the scenario goes from threatening to disastrous."

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Bob Marshall can be reached at or 504.826.3539.