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.
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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.”
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