Rapid sea level rise could drown protective mangrove forests by 2100

Mangrove forests can only
take so much. The famously resilient, salt-tolerant and twisty trees have so
far managed to keep pace with rising sea levels, providing a valuable buffer to
coastal communities against pounding storm surges. Now, researchers have found
the forests’ limit.  Mangroves cannot survive in seas rising faster than about 7 millimeters per year, the scientists
report in the June 5 Science

Sea levels are rising globally at an average rate of about 3.4 millimeters per year,
according to the Intergovernmental Panel on Climate Change (SN: 9/25/19).
But over the next few decades, that rate is projected to accelerate to between
5 millimeters per year and 10 millimeters per year by 2100, scientists say.

That could drown the
forests, which act as a buffer protecting many coastlines around the globe by
reducing erosion from tides and dampening the energy of storm waves sweeping
ashore. And mangroves come with additional boons, says Neil Saintilan, a
biogeographer at Macquarie University in Sydney. They provide a safe nursery
habitat for tropical fish and help reduce atmospheric levels of the
climate-warming gas carbon dioxide.

Mangroves are carbon-sequestering
engines, drawing carbon dioxide out of the atmosphere and swiftly burying it in
soils. From about 8,600 to 6,000 years ago — a period of particularly rapid
expansion for the mangroves — this coastal ocean–based
“blue-carbon” storage by the mangrove forests amounted to about 85 pentagrams
of carbon, enough to lower atmospheric carbon dioxide levels at the time by
about 5 parts per million, Saintilan and colleagues estimate. Currently, the
average concentration of carbon dioxide in Earth’s atmosphere is about 417 ppm.

These valuable forests are typically
resilient to changes in sea level, holding their ground by building up sediment
amongst their tangled roots. Scientists have observed this in the modern era,
Saintilan says, by recording how quickly sediment accumulates and the land
surface elevation within the forests rises.

But those data span only a
few years to perhaps a decade or two, he says. As a result, there have been two
big unknowns: How long mangrove forests might be able to keep up this balancing
act; and at what point the seas might simply rise too quickly for the trees,
drowning the forests.

New mangroves form from slender seedlings called propagules, which drop from the trees into the shallow water and float until they take root in a new location. Coastal mangrove forests can buffer nearby communities from storm surges and accumulate new sediment around their roots, building up new land and sequestering carbon.N. Saintilan

How quickly the seas rise
over the next century will depend on the rate of global warming, which causes seawater
to expand and ice sheets to melt — and that, in turn, depends on rates of
greenhouse gas emissions.

To understand how mangroves may
respond to faster-rising seas, Saintilan and colleagues turned to the past. The
peak of the most recent ice age was between about 26,000 and 20,000 years ago.
After that, the ice sheets began to retreat as the world warmed, and sea levels
began quickly rising, at rates faster than 12 millimeters per year.

Saintilan and colleagues
focused on a time period between 10,000 and 7,000 years ago, as sea level rise
began to slow and mangrove forests began to appear. The researchers examined previously
published data on 78 organic carbon-rich sediment cores collected from coastal
sites around the planet, and compared those with computer simulations of sea
level rise rates for each site, to assess when the waters rose slowly enough for
mangrove forests to grow.  

The forests did not grow
until the sea level rise had slowed to an average global rate of 6.1
millimeters per year, the team found. Today, under a scenario of high
greenhouse gas emissions, sea level rise will accelerate to about 6 or 7
millimeters per year within the next 30 years. Even under mid-range scenarios
that include cuts in greenhouse gases, the rate of rise will exceed that
threshold by the end of the century, the researchers note. At that point, the
mangrove forests sheltering many coastal communities will be unable to keep up,
the researchers say.

“The future
of the world’s mangroves is in our hands,” Saintilan says.

Establishing
a threshold for mangrove survival is key to future coastal management, writes
Catherine Lovelock, an ecologist at the University of Queensland in Australia,
in a commentary
in the same issue of Science
. The threshold itself may vary by mangrove species or by the
frequency and intensity of the storms that strike a particular coastal setting,
she says.

The
findings also underscore the need for the world to “quickly and aggressively”
act to mitigate greenhouse gas emissions, says ecologist Holly Jones of
Northern Illinois University in DeKalb, who was not involved in the new study.

In
a study published May 29 in PLOS ONE, Jones and colleagues estimated
that mangrove
forests currently help protect
about 5.3 million people around the world from storm surges and
other effects of sea level rise. “It is especially painful to think that
through our actions, we could be causing mangroves that provide critical
protection to people … to drown,” she says.

Future
studies, Jones says, can incorporate the newly reported threshold to anticipate
mangroves’ response. From that, scientists may be able to determine which existing
forests will survive, and which may need help migrating inland, such as by
reshaping coastal landscapes to help the trees propagate.

“We need to get started yesterday to ensure these important ecosystems are around to protect us into the future,” Jones says.

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