Falling sea levels

After more than 250 years of glacial retreat in Alaska, the race continues between a rebounding landscape and the rising sea of a warmer world.

And the winner is?

Land, according to SeaLevelRise.org, a non-governmental organization tracking the global rise of sea levels.

At this time, the group puts Alaska’s land ahead of the sea.

“Since 1950, sea levels off Alaska’s coast have declined as much as 32 inches,” the organization reports. “Scientists know this because the sea level is measured every six minutes using equipment like satellites, floating buoys off the coast, and tidal gauges to accurately measure the local sea level as it accelerates and changes.”

Along the shores of the state’s largest cities, sea levels have almost universally fallen since the 1970s, according to the U.S. Army Corps of Engineers, and future predictions as to whether they will go up or down vary hugely.

The state’s largest city now has a sea level more than an inch lower than in 1972, but that is expected to change. By how much is the big question mark.

The Corps’ best guess for the waters off Anchorage call for a rise of about 16 inches by the end of the century, but that is within a range that stretches from a minimum of less than four inches by 2100 to a maximum of more than four and a half feet by that time.

The maximum rise could cause some flooding in areas such as Ship Creek, but Anchorage is generally well sited to deal with such climate-caused change. Downtown Anchorage is about 118 feet above sea level. 

Even most low-lying areas of the city are above a four and a half foot rise. Campbell Lake is 10 feet above sea level. Westchester Lagoo is 13 feet above sea level. 

Unrebounding Anchorage

Sea level rise along Anchorage’s waterfront is projected to be significantly greater than for other Alaska cities because the land beneath the state’s largest city isn’t rising as fast as that in other parts of the state.

The reasons are simple. The glaciers that squatted atop Anchorage long ago are long gone, and the geologic uplift caused by the collision of tectonic plates in the North Pacific ocean is tiny compared to that in other parts of the state.

Tectonic uplift from the Pacific Plate sliding under the Yakutat Block in the St. Elias Mountains some 350 miles south of Anchorage has been estimated to be causing the land to rise steadily upward at the rate of approximately four-tenths of an inch per year.

In Yakutat – where sea level fell by almost half an inch per year from 1979 to 2007 – the Corps’ range for the change by the end of the century stretches from a drop of another three and a half feet to a rise of about nine inches.

Yakutat sits near the middle of the St. Elias range which boasts the tallest, coastal mountains in the world, and the area’s slowly retreating glacier are not far from the community of about 600. The local chamber of commerce bills the city as “the gateway to the Hubbard Glacier” only 30 miles to the north. 

The Corps’ best guess for the Yakutat sea level in 2100 is a drop of another two and a half feet.

Big decreases are also projected for other Alaska Panhandle communities even as the global waters rise. The Corps’ best-guess estimates call for drops of nearly three feet in the Capital City of Juneau and more than four feet in the historic gold-port city of Skagway to the north.

But water could come up about three and three-quarter inches by 2100 in Sitka, the old Russian port on the outer coast of Southeast Alaska, and rainy Ketchikan at the southern end of the Panhandle could see a sea-level rise of about a foot.

Regionally, sea levels in Kodiak, Seldovia and Nikiski are projected to stay the same or drop slightly by 2100 while those of Seward and Valdez would rise by somewhere in the neighborhood of 10 inches.

It’s complicated

SeaLevelRise.org – which says its goal is “to enlighten and enable elected officials to implement widespread solutions to sea-level rise” – was forced to ponder “Why is Sea Level Rise Complicated in Alaska?”

Most of the answer is in the state’s melting glaciers which add a smidgeon to global sea-level rise while at the same time adding a gob to regional land rise.

“The active tectonic deformation of the region is…considered as a possible source of the uplift,” a group of University of Alaska Fairbanks scientists observed in the definitive study of the subject. “However, this effect is a minor contribution relative to viscoelastic rebound,” or what one might simply call glacial rebound.

After studying the area surrounding Glacier Bay National Park and calculating the weight of the ice that pushed the area down before the glaciers melted and retreated, their peer-reviewed study published in Geophysical Journal International concluded that “that glacial isostatic rebound associated with post-Little Ice Age melting can entirely account for the rapid uplift of southern Alaska over the last approximately 250 years.”

The Glacier Bay area is one of the fastest rising parts of Alaska. The researchers found one area that appeared to have risen almost 19 feet above sea level since 1750.

Though Alaskans living in small coastal villages built on barrier islands or along meandering 49th state rivers are facing increasing risks from climate-related flooding linked to rising sea levels or increased river flows, the SeaLevelRise website makes the problems in Alaska sound relatively minor compared to those of some other states.

“The sea level around Louisiana is up to 24 inches higher than it was in 1950,” the organization says. “This increase is mostly due to sinking land, and it’s causing major issues. New Orleans is the largest population center at risk from sea level rise in the country and is now experiencing one of the highest rates of sea-level rise in the world.”

The New Orleans Metropolitan Area is home to almost 1.3 million people.  That’s only about 200,000 people shy of twice the population of the entire state of Alaska. 

Coastal Louisiana has been known to be sinking for a long time due to soil compaction, human lowering of the water table, plate tectonics and more, but Louisiana scientists reporting in the journal of the Geological Society of America in 2017 identified the biggest problem as human-caused changes in the Mississippi River drainage affecting the wetlands surrounding Louisiana’s uplands.

“While a variety of factors have contributed to Louisiana’s wetland loss problem, the fundamental culprit is the isolation of the sediment-delivery system (the Mississippi River) from its delta plain and the adjacent coastal zone due to the construction of flood-protection levees. As a result, the majority of the sediment carried by this system is funneled into the deep waters of the Gulf of Mexico, rather than offsetting the naturally occurring high subsidence rates,” they wrote. 

And they don’t expect the situation to get any better in the future.

“A landmark study (Blum and Roberts, 2009) has shown that this problem is likely to worsen in the future due to limited sediment loads and accelerated sea-level rise,” they wrote.

Louisiana is sort of the opposite of Alaska. While significant parts of the 49th state are rising, large parts of the 18th state are sinking. While Alaska has coastal mountains rising to heights of thousands of feet within miles of the ocean, Louisiana’s highest point – Driskill Mountain – is but 535-feet above sea level.

Meanwhile, significant parts of New Orleans are below sea level – up to 13 feet below sea level – and the highest neighborhoods are less than 30 feet above sea level, according to a map generated by the Greater New Orleans Community Data Center.

Most of the city is built on land less than 10 feet above the sea.