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New York Times
ARE THE OCEANS RISING?
Analysis by Cristen Conger
Wed Jul 7, 2010 09:47 PM ET
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Recently, the head of the United Nations Intergovernmental Panel on Climate
Change (IPCC) made public statements in defense of the agency's research
associated with rising sea level estimates.
Earlier this year, the scientific body came under fire for certain sea level projections,
such as a 3-foot rise that would flood 17 percent of Bangladesh by 2050. Its
calculation of a global sea level rise of roughly 30 to 75 inches (75 to 190
centimeters) by 2100 has also been highly scrutinized.
These back and forth arguments within the scientific community have undoubtedly
left many in the public scratching their heads at the original question of whether the
oceans are rising at all.
“Sea levels have been rising for the past 20,000 years,” said Benjamin Horton, who
heads the Sea Level Research Laboratory at the University of Pennsylvania. “I
think the more important question is whether it’s due to anthropogenic (human-made)
activity.”
As Horton points out, rising sea levels aren’t a new natural phenomenon.
Around 20,000 years ago as the last ice age relinquished its grip on the planet,
enormous sheets of land ice began melting. Over several thousand years, the melt
waters filled the oceans, raising sea level around 120 meters (394 feet) to near
present-day levels. The rise leveled off about 3,000 years ago, and since the rate of
sea level rise has hovered near 1 millimeter (.04 inches) per year.
In the 20th century, the rise began to accelerate. Today, the
IPCC plots
annual sea
level increase at around 3 millimeters (0.12 inches) per year, likely due to thermal
expansion of water and melting land ice as a result of global climate change.
Where will things go from here? Hard to say. One of the main problems with
estimating future sea levels is that scientists don’t entirely understand how quickly
past sea level changes occurred, which would give them a model to work from.
WATCH VIDEO: Can we reduce sea level? Does our planet's future look like the movie "Waterworld?" Find
out here.
Until researchers have more precise data at their fingertips, Horton thinks it’s
impossible to precisely gauge sea level rise.
But what about the IPCC’s global sea level projections of an added 30 to 75 inches
(75 to 190 centimeters) by 2100?
“Global sea level rise is commonly misinterpreted as the sea level change where
(one lives),” Horton said.
In other words, a global average is just that -- an average.
Some locations will bear a greater brunt of sea level change, which Horton says
depends on three main factors: tectonic activity, glacial isostatic adjustment and
land subsidence.
All three deal with natural land movements that either increase or decrease the sea
level at any given location.
In addition to land shifts caused by tectonic plate movement, land masses that sat
beneath huge sheets of ice have also uplifted in a process called glacial isostatic
adjustment, or post-glacial rebound.
After the last ice age around 20,000 years ago as the climate warmed and melted
ice sheets, unburdened land has slowly rebounded, actually lowering sea levels in
some places even as the seas swelled, as in Canada’s Hudson Bay region.
In the Mississippi River delta and other similar environments around the world, the
land is literally sinking from years of compaction and/or oil extraction. This
phenomenon, known as subsidence, can greatly increase the rate at which water
encroaches on regional coastlines.
As a result, location-specific sea level rise will vary from place to place.
“It’s a complex subject to try to get people to understand,” Horton said. “And what
people really want is a figure; they want an (exact number for projected sea level
rise).”
Yet, even though the data at hand can’t predict exact sea level rise for any given
region, it’s enough to highlight some major trouble spots.
“The northeastern U.S. coast is very likely to experience the fastest and largest sea
level rise this century due to the change in the North Atlantic Ocean circulation,”
said Jianjun Yin, a research scientist at Florida State University.
Yin has studied the potential impact of changing ocean currents on sea level, which
until fairly recently, had gone unnoticed as a contributor to the water rise.
In a nutshell, as the global temperatures rise and warm the waters, the northern
current that heads toward the chilly North Atlantic might slow down and initiate a
literal ripple effect to the south, exacerbating the sea level rise along the eastern
U.S. coastline.
“Ocean circulation is projected to cause 20 centimeters (7.8 inches) extra sea level
rise on top of the global mean sea level rise along the northeastern U.S. coast
during this century,” Yin said.
But that only accounts for one small land area. The full measure of sea level rise
depends on a host of factors in addition to changing ocean currents and land ice
melt. Prevailing winds on the oceans, thermal expansion of warmer waters and
melting glaciers also play roles.
For that reason, scientists are modeling those different aspects of sea level rise to
piece together the entire picture.
“The effect of ocean currents and prevailing winds on seal level is realistically
represented in models,” Yin said. “However, models are still unable to predict the
land ice contribution to sea level rise through the dynamical process. This results in
a wide range of future sea level rise predictions.”
Horton at the University of Pennsylvania emphasizes that people should take global
sea level rise estimates with a grain of salt at this point.
“Sea level rise isn’t simple,” Horton said. “It’s region-specific; it’s spatially variable;
it’s land-specific and ocean-specific.”
Image: go_greener_oz on Flickr
Tags: Climate Change, Geology, Glaciers, Global Warming, Oceanography