Wednesday, March 21, 2012

New research lowers past estimates of sea-level rise

From: Science Codex

The seas are creeping higher as the planet warms. But how high could they go?

Projections for the year 2100 range from inches to several feet, or even more.

The sub-tropical islands of Bermuda and the Bahamas are two seemingly unlikely places scientists have gone looking for answers.

The cliffs and ancient reefs on Bermuda and the Bahamas have lured fossil-hunters for decades. The land on the Bahamas, for example, has a foundation of fossil coral; the stone is derived from the disintegration of age-old coral reefs and seashells.

These areas are now attracting scientists investigating global sea level rise.

By pinpointing where the shorelines stood on cliffs and coral reefs in the Bahamas and Bermuda during an extremely warm period 400,000 years ago, researchers hope to narrow the range of global sea-level projections for the future.

After correcting for what they say was sinking of these islands at that time, scientists estimate that the seas rose 20 feet to 43 feet higher than today--up to a third less than previous estimates, though still a drastic change.

Scientists have found a new explanation for why beach deposits in the Bahamas are 70 feet above sea level. (Photo Credit: Paul Hearty)

The study infers that the Greenland and West Antarctica ice sheets collapsed during the ancient warm period, but that ice loss from the vast East Antarctic Ice Sheet was negligible.

The results are reported in this week's issue of the journal Nature.

"Our research provides a simple explanation for high beach deposits [such as fossils in the Bahamas]," said the paper's lead author Maureen Raymo, a scientist at Columbia University's Lamont-Doherty Earth Observatory.

Average global sea-level rose eight inches since the 1880s, and is currently rising an inch per decade, driven by thermal expansion of seawater and melting of glaciers and ice sheets, including the still mostly intact ice in Greenland and West Antarctica.

In its most recent report, the Intergovernmental Panel on Climate Change estimated that the seas could rise up to two feet by 2100.

That number could go higher depending on the amount of ice melt and the quantity of greenhouse gas emissions.

Will the fate of the Bahamas in a time of sea-level rise mirror that of other coastal locales? (Photo Credit: Government of the Bahamas)

The United Nations estimates a five feet sea-level rise would be enough to swamp 17 million people in low-lying Bangladesh alone.

The new study factors in the loading and unloading of ice from North America during the ice ages preceding the long-ago sea-level rise.

As the ice sheets grew, their weight pushed down the land beneath them while causing land at the edges--Bermuda and the Bahamas--to bulge upward, says Raymo.

When the ice pulled back, the continent rebounded, and the islands sank.

"We're re-thinking many of our estimates of past sea-level rise now that we're more aware of the effects of unloading of ice," said Bil Haq, program director in the National Science Foundation's (NSF) Division of Ocean Sciences, which funded the research. "We now have a meaningful way of calculating the rebound.

"This study is a good example of collaboration between paleoceanography and geophysics to resolve an important issue: the question of future sea-level rise."

Today, both Greenland and West Antarctica are losing mass in a warming world, but signals from East Antarctica are less clear.

Raymo said the research helps show that "catastrophic collapse" of the East Antarctic ice is probably not a threat today.

"However, we do need to worry about Greenland and West Antarctica."

The islands of the Bahamas and their fossil cliffs contain clues to sea-level rise. (Photo Credit: NASA)

Source: National Science Foundation

Sunday, March 18, 2012

Study: Rising Seas Threaten Millions of US Homes

From: Voice of America, by: Tom Banse 

A bicyclist makes his way past a stranded taxi on a flooded New York City Street as Tropical Storm Irene passes through the city, August 28, 2011.

Photo: AP

A bicyclist makes his way past a stranded taxi on a flooded New York City Street as Tropical Storm Irene passes through the city, August 28, 2011. A global warming-fueled sea level rise over the next century could flood millions in the US, according to a new study.

Small island nations, such as the Maldives and Kiribati, raised the alarm first. Now larger and more populated places are assessing their vulnerability to sea level rise caused by climate change. A new study suggests millions of American homes could be inundated over the next century. Recognizing the change is one thing, though, and taking action and deciding who pays is another matter.
The latest projections of the impact sea level rise will have on coastal communities appear in the journal Environmental Research Letters.  The study authors - from the non-profit Climate Central and University of Arizona - calculated how many people in the United States live less than one meter above the high tide line. And how many is that? Close to 2 million homes sheltering 3.7 million Americans could be flooded by rising seas. The biggest concentrations of vulnerable homes are in Florida, followed by Louisiana, coastal California, and then New York and New Jersey.
Nate Mantua is co-director of the Climate Impacts Group at the University of Washington. He was not involved in the Climate Central study. He said the study’s authors used a novel combination of high resolution elevation, population, and tidal datasets - but didn’t factor in that the rate of sea level rise can differ from place to place.
"They didn't actually consider any specific future sea level rise scenario. Instead, they are using a one-meter increase as a reference and asking how many people would be vulnerable to a sea level rise of one meter. They didn't actually say that's our scenario for 2100."
Other studies have calculated how high the seas could rise regionally due to melting glaciers, disappearing ice caps and the simple expansion in volume of water as it gets warmer. Right now, the world's oceans are rising at an average rate of about 3 millimeters per year. One meter is at the higher end of the range for the end of this century. But Mantua points out there is a great deal of local variation.  For starters, the land you live on may be sinking or rising ever so slightly depending on underlying geology.
"You can have this different relative sea level just because the land is not staying still. It's moving. Different parts of the coast are moving up or down at different rates," said Mantua.
There are numerous examples of coastal cities and provinces that have completed vulnerability assessments to rising sea levels. San Diego, New York City and British Columbia, Canada, are three that have done so. Examples of places that have taken some concrete action in response, however, are much harder to find. In the port city of Aberdeen, Washington, public works director Larry Bledsoe thought up an affordable defense. He's recommending the city council progressively raise the minimum elevation of ground floors in new construction.
"It seems prudent to make small adjustments now incrementally before the flood is upon us," said Bledsoe.
Another example from western Washington State: A couple of years ago, Nisqually Wildlife Refuge managers considered sea level rise when designing new dikes to protect freshwater wetlands. The new dikes have an extra wide base so they can more easily be made higher in coming decades. 
Margaret Davidson, coast services director for the National Oceanic and Atmospheric Administration, said coping with rising waters requires difficult choices. She looks to the Dutch for confirmation, "because after all, they're been holding back the sea for nearly a thousand years."
"Even the Dutch in their Building with Nature initiative recognize you cannot engineer your way out of everything. We can only save those places that are economically or politically significant. Then we'll have to look at managed retreat and other options for the places that are less so important," said Davidson.
"It's a bit like the options open to the military during a war. We can defend or we can retreat. Both are not very palatable options," she said.
John Clague, an expert on sea level change at Simon Fraser University in Vancouver, Canada, said holding back the waters or relocating inland are both "very, very costly" options.
"Where the money is going to come from, I don't think anyone knows because it's a large amount of money. You know, it's outside the ability, I think, of most communities to deal with this problem, most urban communities in coastal areas," said Clague.
Clague worries about signs that indicate sea level rise is accelerating. His home province of British Columbia is telling its local jurisdictions to prepare for an average rise of about 1 meter over the next century.