Sea-level rise likely to swallow many coastal mangrove forests

This is a re-post from Yale Climate Connections by new regular contributor Jeff Masters

Coastal mangrove forests aren’t adapting rapidly enough to escape rising sea levels, and many could disappear by 2050 in much of the tropics, according to recent research published in Science.

Authors of a study reported June 5 used sediment cores from 78 sites on five continents to determine when mangroves first appeared over the past 10,000 years, as sea-level rise had slowed once Earth fully emerged from the Ice Age. They found that mangrove ecosystems did not develop unless relative sea-level rise was less than 6 to 7 millimeters* per year. (The term “relative” is used because the rate of sea-level rise is determined by the increase in water volume of the oceans plus subsidence or uplift of coastal land).

Sea-level rise is accelerating

The global rate of sea-level rise has doubled from 1.8 millimeters per year over the 20th century to approximately 3.4 millimeters per year in recent years. In many coastal areas, the rate of relative sea-level rise is much higher as a result of subsidence resulting from human causes, such as groundwater pumping and fossil fuel extraction.

For example, the Mekong Delta of Vietnam is subsiding at a rate of 6 to 20 mm/year and the Ganges-Brahmaputra Delta by 1 to 7 mm/year. At the same time, sediment supply to the coast has declined as a result of damming of rivers and mining and export of sediment, further increasing the vulnerability of mangroves to sea-level rise.

Coastal wetlands act as natural levees against storms as a result of their ability to reduce water velocity and wave turbulence. Moreover, wetlands accumulate sediments that provide protection against rising sea levels and local subsidence. In the U.S., per square kilometer, wetlands save $1.8 million per year in storm damages.

A March 17, 2020, study in PNAS, Coastal wetlands reduce property damage during tropical cyclones, showed just how valuable wetlands are in reducing storm damage. The researchers analyzed property damage caused by 54 tropical storms and 34 hurricanes hitting the U.S. between 1996 and 2016. They found that counties with more wetland coverage experienced significantly less property damage: a 1% loss of coastal wetlands was associated with a 0.6% increase in property damage. (Side note: a 1% increase in wind increased damages by 7%, and counties on the storm path’s right side experienced 140% more damage than those on the left.)

The expected economic value of the protective effects of wetlands varied widely, averaging that $1.8 million per square kilometer. Wetlands conferred relatively more protection against weaker storms and in states with weaker building codes. Wetland losses of 2.8% in Florida between 1996 and 2016 are estimated to have increased property damage from the 2017 Hurricane Irma by $430 million.

Figure 1Figure 1. Annual county-level marginal value (the value gained from either consuming or producing one additional unit of a good or service) of coastal wetlands for storm protection in (A) northeastern coastal counties, (B and C) eastern and southeastern coastal counties, and (D) coastal counties from Texas to Florida. The highest-value wetlands were located in the Boston, Massachusetts area. Credit: Sun et al., Coastal wetlands reduce property damage during tropical cyclones, Proceedings of the National Academy of Sciences Mar 2020, 117 (11) 5719-5725; DOI: 10.1073/pnas.1915169117.

Mangroves in Louisiana and Texas in trouble

In the U.S., mangroves grow along much of the Florida coast and along large portions of the coasts of Louisiana and Texas. Most of the mangroves in those two states are experiencing rates of relative sea level that will threaten their existence. The NOAA Tides & Currents website indicates that six out of seven coastal tide gauges between Rockport, Texas, and New Orleans, Louisiana, over the past 40 to 50 years had rates of relative sea-level rise exceeding 5.8 mm/year:

Southeast coast mangrovesFigure 2. Distribution of mangrove forests in the U.S. Credit: Osland, M.J. N. Enwright, R.H. Day, and T.W. Doyle. 2013. Winter climate change and coastal wetland foundation species: salt marshes vs. mangrove forests in the southeastern United States. Global Change Biology 19:1482-1494

9.6 mm/yr: Eugene Island, Louisiana
9.1 mm/yr: Grand Island, Louisiana
6.6 mm/yr: Galveston, Texas
6.0 mm/yr: Sabine Pass, Texas
5.8 mm/yr: New Canal, New Orleans, Louisiana
5.8 mm/yr: Rockport, Texas

2017 analysis of wetlands change at 185 sites across the Mississippi Delta found even higher rates of sea-level rise since 2006: 13 ± 9 mm  per year. About 65% of these sites were able to keep up with this rate of sea-level rise over that relatively short period of time. The situation is more encouraging in Florida: The highest rate of relative sea-level rise at NOAA tide gauges along Florida’s mangrove forests is 3.8 mm per year, in the Florida Keys.

The results of the June study support the findings of a May 22, 2020, paper in Science Advances, Tipping points of Mississippi Delta marshes due to accelerated sea-level rise. That research presented an 8,500-year-long sediment record from 355 boreholes in the Mississippi Delta marshes of Louisiana. It showed that at rates of relative sea-level rise of 6-9 mm per year, marsh conversion into open water occurs in about 50 years. Even at slower rates of relative sea-level rise of 3 mm per year, the researchers found that marshes in 80% of cases transitioned to open ocean in a few centuries, and they concluded that drowning of the Mississippi Delta marshes is inevitable.

An excellent analysis of this paper at details provisions in Louisiana’s coastal master plan – a $50 billion effort to defend the coast against rising sea levels – to help deal with the threat.

Commentary: Think Scooby-Doo ‘Ruh-Roh’

Mangrove forests provide a U.S. coastal population of more than 200 million people with services like protection from intense storms and waves, reduction of coastal flooding, sequestering of carbon, improved water quality, and preservation of biodiversity and fisheries.

Some might say it’s something of a Scooby-Doo “Ruh-roh” moment to see science predicting the loss of much of the world’s mangroves in just 30 years. Human development and sea-level rise have already led to the loss of one-fifth of the world’s mangroves between 1980 and 2010; Tampa Bay, Florida, has lost almost half of its mangroves in the past century.

Nonetheless, the Trump administration has torpedoed a major Obama administration regulation protecting wetlands: On June 22, new EPA regulations substantially reducing the number of water bodies and wetlands protected by the Clean Water Act are to take effect.

Orrin Pilkey: ‘… we can walk away methodically, or we can flee in panic.’

Sea-level rise is expected to cause massive upheavals to civilization in coming decades:

– forcing millions of people to abandon the coast as rising seas inundate populated areas and major cities;
– opening the way for climate change-amplified hurricanes to drive higher storm surges farther inland;
– knocking out transportation systems and sewage treatment plants;
– swallowing prime agricultural land and barrier islands; and
– infiltrating aquifers with salt water.

But the impacts of sea-level rise are not limited to future decades—they’re happening right now. Hurricane Sandy’s storm surge in New York City in 2012 caused an extra $2 billion in damage as a result of higher water levels the city experienced from 20th century sea-level rise. “Nuisance” flooding has become a growing problem in places like Miami Beach, Norfolk, and San Francisco. And in Maryland, for instance, both Annapolis and Baltimore now get more than nine times the number of flood days they experienced in the 1960s. (I review many more examples in a December 2017 review of Jeff Goodell’s must-read book on sea-level rise, “The Water Will Come: Rising Seas, Sinking Cities, and the Remaking of the Civilized World.”)

In response to news that a long-predicted acceleration of sea level is already underway, one does well to heed the words of Duke University sea-level rise expert Dr. Orrin Pilkey and co-authors in their 2016 book, “Retreat From a Rising Sea: Hard Choices in an Age of Climate Change”:

Like it or not, we will retreat from most of the world’s non-urban shorelines in the not very distant future. Our retreat options can be characterized as either difficult or catastrophic. We can plan now and retreat in a strategic and calculated fashion, or we can worry about it later and retreat in tactical disarray in response to devastating storms. In other words, we can walk away methodically, or we can flee in panic.

Posted by Guest Author on Thursday, 11 June, 2020

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