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The Drowning Sinks · The Climate Translation

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If you've ever walked along a marshy area at low tide, you know these places seem to
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have a rhythm of their own.
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You can smell the salty mud.
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You can see the tangled roots rising out of the water like a maze.
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The tide rolls in and it covers everything.
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Only to retreat a few hours later and reveal a landscape once again.
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Years ago as part of my own research, I spent time studying estuaries in the northeast
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United States and it taught me something important.
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These coastal environments aren't just places where land and water meet.
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There also places where enormous amounts of carbon can be stored, buried and locked
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away for centuries.
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For years, scientists have viewed tropical mangrove forests as one of nature's most effective
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tools for pulling carbon dioxide out of the atmosphere and locking it away in coastal
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soils.
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But what happens when the ocean itself begins to rise faster than those forests can adapt?
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Today we're going to explain why mangroves are such powerful carbon reservoirs, what
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new research is revealing about their future and why rising seas may be creating an unexpected
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challenge for some of the planet's most valuable coastal ecosystems.
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Welcome back to the podcast, I'm Dr. Mac and this is The Climate Translation.
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Like all plants, mangroves pull carbon dioxide out of the atmosphere through photosynthesis.
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They use that carbon to produce leaves, branches, trunks and roots.
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At first glance, it doesn't sound very different than any other forest, but mangroves have
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a unique advantage.
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They live with their feet and salt water.
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Years ago, while conducting research in the Niponset estuary in Boston, I spent a lot of
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time thinking about how coastal wetlands function.
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One of the things that makes these environments so fascinating is the fact they don't just
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store carbon in the plants themselves.
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They store enormous amounts of carbon beneath the surface.
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And that's where the story gets really interesting.
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Imagine a leaf falling from a tree in a typical inland forest.
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It lands in the ground.
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Bacteria and fungi begin to break it down.
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There, oxygen is plentiful, so over time much of the carbon is released back into the
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atmosphere as carbon dioxide.
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Now, imagine that same leaf falling into a mangrove forest along the coast.
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Instead of landing on dry ground, it settles into water-logged mud.
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And in that mud, oxygen is extremely limited.
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I often explain this to my students using a food preservation analogy.
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You can think of this sediment beneath those coastal mangrove forests like a giant natural
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vacuum-sealed bag.
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Without much oxygen available, the microbes, responsible for decomposition, can't work
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nearly as well, and thus the breakdown process slows dramatically.
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Instead of quickly returning to the atmosphere, leaves, roots, and other organic material
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become buried in the sediment.
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And so year after year and century after century, layer after layer, they build.
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This is what scientists call blue carbon.
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It refers to the watery ocean environment where this sequestration takes place.
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And as you can tell, it involves a process many people don't realize.
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The carbon isn't stored in the trees.
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It's stored in the soil beneath them.
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Over hundreds, even thousands of years, these coastal ecosystems can build enormous underground
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carbon reservoirs.
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In some cases, mangrove wetlands can store several times more carbon per acre than many
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terrestrial forests.
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That's one reason conservation groups and climate scientists have become so interested
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in protecting and restoring them.
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The idea seems straightforward.
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If you protect the mangroves, you protect the carbon vault.
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But that vault depends on a delicate balance of biology, geography, and sea level.
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The mangroves aren't fully aquatic plants.
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They still need access to oxygen.
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Many species have specialized roots that protrude above the mudline like snorkels, allowing
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them to breathe when the tide is low.
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As long as the forest can keep pace with changing conditions, the system works well.
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But if rising seas begin to outpace the forest ability to adapt, the balance can start
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to break down.
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And that is where new research begins.
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Now at this point, it will be easy to assume that mangroves are simply victims waiting for
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rising seas to overwhelm them.
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But it's not really the case.
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In fact, mangroves are remarkably resilient ecosystems.
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They have survived changing coastlines, shifting sediments, storms, and natural sea level fluctuations
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for thousands of years.
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And the reason they have survived so long is that they possess two natural ways of adapting
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to rising water.
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The first strategy is simple.
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They just build upward.
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Remember those tangled root systems we talked about earlier?
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The roots don't just anchor the trees.
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They act like giant sediment traps.
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As tides move in and out, they carry tiny particles of mud, sand, and organic material.
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And the roots slow down that water and capture some of that sediment.
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Over time, the sediment accumulates, and the floor of the mangrove forest slowly rises.
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I sometimes tell students that you can imagine standing on an escalator that's moving downward.
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As long as you continue to climb upward at the same speed, you stay at the same place.
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For thousands of years, many mangrove forests have effectively been climbing that escalator.
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As the sea level rose slowly, forests built themselves upward.
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But mangroves have a second strategy as well.
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They can move.
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Many mangrove species produce floating propagules, essentially young seedlings that can drift
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with the tides before taking root elsewhere.
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If conditions become unfavorable near the shoreline, new mangroves can establish themselves
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farther inland, where elevations are slightly higher.
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In other words, mangrove forests don't stay in one place.
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Over long periods of time, entire forests can slowly migrate landward as coastlines shift.
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And historically, these two strategies worked pretty well.
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They built upward, they moved inland. Problem solved.
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But new research suggests human activity is creating what we might think of as a double stress
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on the system.
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The first stress involves the speed of sea level rise itself.
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As glaciers melt and ocean water expands due to warming temperatures, global sea levels
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are rising faster than they did through much of the recent past.
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That's where that escalator analogy becomes useful again.
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Imagine the escalator suddenly moving faster, you can still climb, but eventually there
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comes a point where you may not be able to climb quickly enough to keep up.
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That's the concern scientists are investigating.
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Not whether sea levels are rising, but whether sea levels are rising faster than some mangrove
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ecosystems can build the new sediment beneath themselves.
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If the water deepens too quickly, those specialized root structures we discussed earlier
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remain submerged for longer periods of time.
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The trees become stressed and growth slows.
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And in some cases, portions of the forest may begin to die back.
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But let's assume the forest tries that second strategy.
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They try to migrate inward, historically that worked.
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Today, it often doesn't.
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This is what scientists call the coastal squeeze.
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Over the last century, humans have transformed coastlines around the world.
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We've built roads and seawalls, hotels and ports, shrimp farms and neighborhoods, and
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entire cities.
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When sea level rises and mangroves attempt to move inland, they increasingly encounter
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something that previous generations of mangroves never faced... us.
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Instead of finding open land they encounter infrastructure.
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Their migration pathway has disappeared.
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The forest becomes trapped between the rising ocean on one side and human development on
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the other.
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That's the squeeze.
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And this is where the problem becomes particularly interesting from a systems perspective.
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The same coastlines we're trying to protect from rising seas are often the very coastlines
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preventing mangroves from adapting to those rising seas.
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The natural escape route is blocked.
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And when an ecosystem loses both of its adaptation strategies, the ability to move upward and
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inward, the risk begins to change.
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Now we're not talking about how mangrove's store carbon, we're starting to ask what happens
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when that storage system starts to fail.
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That brings us to a central question that's raised by a recent research study at the University
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of Exeter.
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To answer that question, we need to return to the sediment beneath the mangrove forest.
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For centuries, those waterlogged soils have acted as an enormous storage reservoir.
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Layer after layer of organic material accumulated in an environment where decomposition was slowed
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by the lack of oxygen.
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But that storage system depends on the forest remaining intact.
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The trees aren't just storing carbon.
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They're protecting the very sediments that contain it.
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Their roots stabilize shorelines.
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They slow water movement.
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They trap sediment.
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And they maintain the physical structure of the ecosystem.
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When portions of a mangrove forest die, some of that protection begins to disappear.
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Roots can decay and shorelines become more vulnerable to erosion.
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The stored sediments become disturbed, redistributed, or exposed to new environmental conditions.
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And this is where scientists are becoming concerned.
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Remember that vacuum-sealed bag analogy from earlier?
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The reason carbon remains stored for so long is that oxygen is limited.
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But if erosion exposes those buried sediments to more oxygen, microbes can then begin to
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break down all of that stored organic material much more quickly.
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Over time, part of that carbon may return to the atmosphere as carbon dioxide.
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In some situations, even methane can be involved, depending on the conditions within the sediment.
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Now it's important to stop at this point because this is where the headlines can sometimes
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get ahead of the science.
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That Exeter research is not suggesting that all mangrove forests are going to suddenly
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become carbon sources.
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Nor the scientists suggesting that centuries of stored carbon is going to be released overnight.
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But what they are identifying is a risk.
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It's a risk that some mangrove ecosystems may lose part of their ability to store carbon
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if sea level rise outpaces their ability to adapt.
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And the evidence is suggesting the risk is not distributed evenly.
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Some mangrove forests continue to perform remarkably well.
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In regions with abundant sediments supplied by rivers, they can build elevation fast enough
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to keep pace with rising water.
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In those with areas with room to migrate inland, they can remain resilient for decades.
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But other locations face greater challenges.
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That's particularly true for those experiencing rapid sea level rise, reduced sediment supply,
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and/or extensive coastal development.
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This is one reason scientists are increasingly viewing mangroves not as isolated forest,
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but as a dynamic system that's connected to our rivers, our coastlines, the tides,
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and our human infrastructure.
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It also introduces a feedback loop.
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That should sound familiar if you've listened to previous episodes.
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The more greenhouse gas we add to the atmosphere, the warmer the climate becomes.
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A warmer climate contributes to rising sea levels.
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Seeing sea levels place additional stress on coastal ecosystems.
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And if some of those ecosystems become less effective at storing carbon, a portion of
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the natural buffering capacity we depend on begins to weaken.
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You notice what's happening there?
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The concern isn't mangroves are suddenly becoming the villains in the climate story.
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The concern is that one of nature's most effective carbon storage systems may become
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less effective under increasing stress.
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It's a reminder that climate change is not simply changing temperatures, it's changing
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the entire system and how it functions.
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So where does this leave our understanding of conservation?
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It's an interesting thing to think about.
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For a long time, conservation was largely built around a simple idea.
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You find something valuable, you draw a boundary around it, and you protect it.
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For many environmental challenges, the approach works pretty well.
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If you want to protect a forest, you establish a preserve.
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If you want to protect a wetland, you limit development.
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The goal is to keep the ecosystem exactly where it is.
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But the mangrove story highlights a new challenge.
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What happens when the ecosystem has to move?
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Remember mangroves have never been completely static.
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Throughout history, they've responded to changing coastlines by building upward and gradually
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migrating inland.
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Movement has always been part of their survival strategy.
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The difference today is that human infrastructure increasingly limits that movement.
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And that's forcing scientists and conservation planners to rethink what success looks
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like.
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Instead of protecting a single location, we may need to protect the entire pathway.
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Instead of asking how do we protect this forest where it is today, we may have to start
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to think how do we preserve the ecosystem's ability to adapt tomorrow.
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That shift in thinking is already influencing how some coastal regions approach their restoration
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and management efforts.
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Scientists are increasingly looking beyond the boundaries of existing mangrove forest,
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and evaluating the surrounding landscape.
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Is there room for migration?
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Is sediment still reaching the coast from nearby rivers?
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Are sea walls, roads, or development projects creating barriers that could eventually trap
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the ecosystem?
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In many cases, the long-term survival of a mangrove forest may depend on as much the land
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behind it as the water in front of it.
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And that brings us back to a theme that's appeared many times in this podcast.
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Natural systems don't operate in isolation.
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Those mangroves are connected to the rivers that supply sediment, and they're connected
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to the tides that shape the shoreline.
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They're connected to the atmosphere, through the carbon they store, and increasingly,
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they're connected to the choices we make about how and where we build.
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The new research isn't telling us that mangroves have stopped being valuable carbon sinks,
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far from it.
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If anything, it reinforces how valuable they really are.
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What it tells us is that protecting a carbon vault requires more than protecting the vault
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itself.
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It requires protecting the conditions that allow the vault to exist in the first place.
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For a long time, we tended to think of our forest and wetlands as carbon sinks and
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permanent fixtures on the landscape.
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And to a large extent, they still are.
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But what this emerging research is reminding us is that these ecosystems are not static.
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They're living systems constantly responding to changes in water, sediment, temperature,
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and sea level.
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The story of the mangroves isn't about the trees, it's about the balance between land
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and ocean, sediment and erosion, and adaptation and change.
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For thousands of years, mangrove forests have maintained the balance well.
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They've built upward and inland and adjusted to changing coastlines.
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The question scientists are now asking is whether the pace of change is beginning to outstrip
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the ability of some of the systems to keep up.
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Protecting a carbon sink isn't simply about protecting the trees, we see it the surface.
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It's about protecting the conditions that allow the ecosystem to survive, adapt, and continue
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doing the work we've come to depend on.
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And as we've discussed many times on this podcast, the climate system is not a collection
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of separate pieces operating independently.
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The atmosphere, the oceans, the land, and the living ecosystem are all connected.
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And when one part of the system changes, the effect ripples outward in ways that are
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sometimes surprising.
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I'm Dr. Mac, and this has been the Climate Translation.
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If you have a question about the climate you've been to afraid to ask, or if you
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have a differing opinion, I want to hear from you.
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I can use your viewpoints in future episodes.
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You can reach me at theclimatetranslation at gmail.com.
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I'll see you next time.