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The Domino Line · The Climate Translation

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  If you've ever lined up a row of dominoes on a table, you know something interesting
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  about how systems behave.
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  You can tap the first one gently, barely enough to notice.
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  But as long as it falls, the rest will fall hard.
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  The force you applied may have been small, but the effect was not.
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  If someone that doesn't study or research climate change, they might think it behaves in
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  a straight line.
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  If you add a little warming, you get a little melting.
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  If you melt a little ice, you get a little sea level rise.
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  But as we have talked about on this podcast, Earth's systems aren't isolated.
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  They lean on each other.
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  Right now, scientists are closely watching such a chain that stretches from Greenland to
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  the North Atlantic, right down through the great ocean circulation that moves heat around
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  the planet, and all the way down to the ice shelves of Antarctica.
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  Each of these systems is like a domino lined up, and what's raising concerns among scientists
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  is how tightly they might be connected.
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  Welcome back to the podcast.
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  I'm Dr. Mac, and this is the Climate Translation.
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  We're going to start in Greenland.
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  Greenland holds the second largest body of ice on Earth.
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  In places that ice is nearly two miles thick.
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  You can stand at the very top of it, and you'd be standing on what's essentially a frozen
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  mountain range.
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  At a high elevation, the air is cold.
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  That's one reason the ice has been able to survive for thousands of years.
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  But here's where the physics gets interesting.
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  As the planet warms, and the surface of that ice begins to melt, the height of that ice
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  sheet slowly drops, and when the surface drops, it moves into slightly warmer air.
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  Imagine Greenland sitting on an invisible elevator.
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  As the melting lowers the surface, the elevator moves down into warmer air.
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  Warmer air causes more melting.
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  More melting lowers the surface further.
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  The elevator keeps descending.
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  Scientists call this melt elevation feedback.
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  It does not mean ice disappears overnight.
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  But it does mean that once enough lowering happens, the melting can reinforce itself.
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  That's what we mean by a threshold.
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  There isn't an instant collapse.
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  But there is a point where the system becomes harder to stabilize.
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  But Greenland isn't the first domino because it's melting.
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  It's the first domino because of where that melt water goes.
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  Every year, hundreds of billions of tons of freshwater flow off Greenland and into the
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  North Atlantic.
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  And freshwater behaves differently than saltwater.
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  Saltwater is denser.
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  It sinks more easily.
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  Freshwater is lighter.
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  It tends to stick near surface.
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  If you remember in a previous episode, we discussed that there's a giant conveyor belt
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  of ocean currents that help to move heat and nutrients around the world's oceans.
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  Cold, salty water sinks at the poles.
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  It moves deep, dragging much-needed dissolved gases like oxygen with it.
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  The waters then move towards the equator where they warm and rise.
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  Then the waters move north again to complete the process.
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  Around Greenland, we're talking about the Atlantic Meridional Overturning Circulation.
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  The AMOC.
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  By dumping all the fresh water into the sea, Greenland is changing the density balance that
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  keeps this motor running.
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  Observations over the past decade suggests the AMOC has weakened compared to its mid-twentieth
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  century strength.
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  There's active debate about how close it may be to certain thresholds, with some newer
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  models suggesting that the margin may be smaller than what we once thought.
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  And when the ocean's engine slows down in the North, the energy in the system has to
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  go somewhere else.
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  And that's when the next domino, thousands and miles away in the Amazon, starts to
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  feel the shove.
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  Around the equator, the energy of the sun warms the oceans.
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  The AMOC carries this heat from the tropics northward.
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  But if deep water formation in the North Atlantic weakens, the entire overturning circulation
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  slows.
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  A slower AMOC transports less heat northward.
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  Thus, the balance of heat between the Earth's hemispheres shifts.
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  The North Atlantic can cool relative to the global average, where more heat remains in
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  the southern hemisphere.
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  And when that balance changes, the atmosphere responds.
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  One of the most important features it adjusts is the tropical rain belt.
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  It's a band of heavy rainfall near the equator that scientists call the Inter-Tropical
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  Conversion Zone, or the ITCZ.
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  That rain belt tends to drift towards whichever hemisphere is warmer.
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  If more heat remains in the southern hemisphere, that rain belt can shift slightly southward.
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  And that shift matters for places like the Amazon.
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  The Amazon just isn't a forest, it's its own moisture recycling system.
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  Trees pull moisture from the soil, releasing it to the air.
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  That moisture forms clouds.
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  The clouds form rain.
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  Which then goes back to the surface.
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  In many parts of the basin, a significant fraction of rainfall is recycled locally.
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  But that recycling system needs an initial supply of moisture blowing in from the Atlantic.
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  If the Atlantic temperature patterns change, and if the tropical rain belt shift, parts
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  of the Amazon can receive less initial rainfall.
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  Over time, that can stress trees and increase the risk of large-scale forest loss.
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  Now, it's important to point out the Amazon is not controlled by ocean circulations and
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  rain belts alone.
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  El Niño, deforestation, rising temperatures, they all play a role.
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  But when you stack stresses together, the system can become more fragile.
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  Scientists call the risk of large-scale forest loss, Amazon die back.
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  Again, it's not going to happen overnight.
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  But over time, this gradual loss affects the resilience of the system.
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  If enough forest is lost, the region could shift from being a net carbon sink, absorbing
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  carbon dioxide, to a net carbon source, and that shift would then amplify warming.
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  So what began as melting in the North Atlantic hasn't stayed there.
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  Through ocean circulation disruption and atmosphere patterns, it can alter rainfall thousands
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  of miles away, nudging another major earth system, closer to different thresholds.
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  And when additional carbon enters the atmosphere, that extra heat doesn't stay confined to the
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  tropics.
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  It spreads, which brings us to the Southern Ocean, and the ice shelves of Antarctica.
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  When the balance of the heat between the hemisphere shifts, the atmosphere and oceans adjust.
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  A weaker ocean circulation can contribute to that shift, but it's only one piece of
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  a much larger warming system.
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  The southern ocean is already absorbing a tremendous amount of excess heat from global
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  warming.
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  In fact, it's one of the primary heat sinks for the entire planet.
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  And some of that heat does something really specific.
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  It circulates beneath floating ice shelves.
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  The thick extensions of glaciers that spread out over the ocean.
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  One of the most studied examples is the Thwaites Glacier.
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  You may have heard it in the news called the Doomsday Glacier.
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  It's a headline name, but let me translate what's actually happening.
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  The media often call the Thwaites Glacier a cork and a bottle.
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  It's not very accurate, though.
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  I like to think of the glacier more like a structural brace, like you would see on the
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  sides of major bridges.
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  The floating ice shelves attached to glaciers like the Thwaites act as buttresses.
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  They slow the flow of the massive ice sheet resting behind them.
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  And warm ocean water can slip beneath those shelves and melt them from below.
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  As they thin, they lose strength.
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  And when that buttressing weakens, the glacier behind them can then move forward faster
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  into the sea.
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  Scientists are closely monitoring what's called a grounding line, the point at which the
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  glacier lifts off the sea floor and begins to float.
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  In some regions of West Antarctica, that grounding line is retreating.
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  And here's where the risk comes in.
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  Much of West Antarctica sits on bedrock that slopes downward as you move away from the
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  coast.
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  That geometry makes it vulnerable to what's known as marine ice sheet instability.
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  It's a very fancy way of saying that ice can move quickly towards the sea if given
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  the right geometry and conditions.
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  If the grounding line retreats past a certain point, the process becomes harder to reverse.
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  Not because of a pop of a cork, but because gravity takes over, the slope of the land begins
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  to work together and the movement of the ice towards the water accelerates.
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  Again, this would not happen overnight.
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  But over decades to centuries, sustained melting of glaciers and retreat in parts of
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  the West Antarctica ice sheet could contribute significantly to sea level rise, measured
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  not in just millimeters or inches, but meters or feet over long-term timescales.
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  This is why Antarctica is part of this domino line.
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  It isn't triggered by just one factor alone.
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  When warming, ocean circulation shifts, and carbon feedbacks all interact, the dominoes
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  begin to lean heavily on one another.
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  One push can affect the thresholds around the globe.
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  Earth's systems redistribute energy, and sometimes that redistribution lands at the base
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  of an ice shelf thousands of miles away.
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  When we step back and look at this domino line analogy, the lesson isn't that everything
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  collapses at once.
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  Earth is too complex for that simple explanation.
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  It's that some systems don't respond in straight lines.
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  We're so used to thinking in gradual terms, a little warming is a little melting, a
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  little melting is a little sea level rise.
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  But in the real world, certain systems can accelerate and change quickly, whwn thresholds
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  are crossed.
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  That doesn't mean there's no middle ground.
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  The Amazon doesn't go from rainforest to grassland overnight.
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  Ice sheets don't vanish in a single season.
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  Ocean circulation patterns don't switch off like a light.
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  Be really careful when you see these terms in the news.
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  But what's important are the tipping points.
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  They describe the shifts in stability.
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  Up to a certain point, a system can resist change.
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  It can absorb stress.
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  It can adjust.
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  But beyond that point, the same amount of stress will cause the domino to fall, producing
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  a much larger response.
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  Think less about a tower suddenly falling, more about a slope getting steeper.
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  As the slope steepens, gravity does more work.
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  Scientists are looking for warning signs of that steepening.
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  They monitor changes in ocean circulation.
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  They track the thinning ice shelves.
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  They measure rainfall variability in the Amazon.
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  The concern isn't that the collapse has already happened.
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  It's the concern that the margins of safety are shrinking.
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  And that's important, because if change accelerates beyond thresholds, it becomes harder
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  to slow down, not impossible, but harder.
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  And that's where my commonly used term speed comes back into the conversation.
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  The faster greenhouse gases accumulate, the more pressure systems feel at the same time.
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  That domino metaphor I've talked about here isn't inevitable.
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  But it's a discussion about connection.
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  I often teach in my classroom that we have to think of the Earth as one large system.
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  Don't focus on just one part.
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  What begins in one region of the planet can and does influence another.
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  Through energy changes, water flow, and carbon movement, understanding that interconnectedness
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  is the first step to understanding what critical thresholds are.
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  Because when systems are linked, well, the timing matters.
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  The longer stress builds, the more likely that one shift is going to happen.
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  And that's why speed matters.
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  It's not because everything collapses tomorrow, but because slowing the rate of change keeps
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  more systems on the stable side of that slope.
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  And that gives us options.
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  The Earth is not a collection of isolated parts.
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  It's a connected system, ice, ocean, forest and atmosphere, each one influencing the others.
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  What we've explored today isn't a story of instant collapse.
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  It's a story of stability and how that stability can shift as pressure builds.
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  From Greenland to the Amazon to Antarctica, the message isn't panic, it's connection.
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  And understanding those connections is the first step towards making better decisions
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  about the speed at which we change the system.
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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 too afraid to ask, or if you have
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  a differing opinion, please let me know.
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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.