Latest / Talking Science / Glaciers and Ice Sheets – Rivers of Ice

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Glaciers and Ice Sheets – Rivers of Ice · Talking Science

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I want you to just take a second and picture
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standing on a vast, silent expanse of ice. Like,
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it just stretches all the way to the horizon
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in literally every direction. And in some places,
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it's actually thick enough to bury an entire
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mountain range. Right, exactly. And when you're
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standing there, I mean, it feels utterly solid,
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right? It feels completely motionless underfoot.
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But here's the thing, that stillness is a total
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illusion. That entire mass of ice is actually
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moving. It really is, yeah, one of the most brilliant
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illusions in nature. You're standing on a glacier
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or, you know, if we zoom out, an entire ice sheet.
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And while it might look like the most static,
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unchanging landscape imaginable, it's actively
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flowing downhill. Just under its own weight.
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Right, under its own weight. And it's violently
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carving the Earth beneath it and quietly preserving
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a record of the planet's past all at the same
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time. Which is just wild to me. Diving into our
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sources for today's deep dive, that is what fundamentally
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shifted my perspective. We think of ice as this
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rigid, shattering thing, but on a macro scale,
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it's essentially a slow motion river. Yeah, it
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really is. But I want to make sure we do this
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justice. Our mission today is to trace that incredible
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journey. Like, how does ordinary falling snow
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turn into a landscape carving glacier? We're
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going to decode the climate secrets trapped inside.
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And, you know, separate fact from media fiction
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regarding the future of these ice sheets. And
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I think it's important to say right up front,
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we are going to take our time unpacking this.
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You don't need any science background to appreciate
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the marvel of glaciology. We're just going to
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break it down. Perfect. So just to get a glacier
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going, the math of it is kind of crazy. It's
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not just like a really big snowfall. It has to
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be a localized surplus over decades. Right. Exactly.
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It requires a specific climate. Basically, more
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snow has to fall each winter than melts away
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over the following summer. So it's just year
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after year of buildup. Right. Year after year,
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those layers accumulate. And if you look at fresh
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snow, I mean, it's mostly empty space. Oh, right.
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Because of all the air. Yeah. It's just a loose,
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fluffy arrangement of delicate ice crystals with
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huge amounts of air trapped between them. So
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a thick blanket of fresh snow can look incredibly
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deep, but it actually contains very little water.
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So if it doesn't melt, the weight of, say, Year
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two compresses year one. I was trying to visualize
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this, and it's kind of like packing a snowball
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in your bare hands, forcing all the air out,
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but scaled up to the weight of a literal mountain.
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That's a great way to think about it. Under that
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sustained pressure from above, the delicate snow
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crystals gradually round off and they start to
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interlock. Oh, so they change shape. They do.
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In fact, some ice crystals literally grow at
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the expense of others. And this creates an intermediate
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material. The technical term for it is fern.
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Fern. Okay. Fern. It's granular and partly compacted.
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So it's denser than fresh snow, but it's not
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yet true. Solid ice. And to officially be classified
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as fern, it typically has to survive at least
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one full year buried under fresh snow. Wow. Okay.
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And then it just keeps getting squeezed as more
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decades pass. Like how long does it take? Yeah.
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It takes a very long time. Eventually, the sheer
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weight of all those accumulating layers above
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just squeezes out virtually all of the remaining
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air. And that's when it becomes genuine, dense
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glacial ice. So where does the air go? Does it
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all escape? Well, most of it is squeezed out,
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but the only air left is trapped in these isolated,
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completely sealed off bubbles. And over the full
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depth of an ice sheet. This transformation from
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fluffy snow to dense ice can take many hundreds
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of years. Hundreds of years. So like a single
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snowflake falls on the interior of an ice sheet
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today. It's delicate, it's recognizable, but
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it might not finish its journey into solid glacial
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ice until after you and I are long gone. That's
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exactly right. And once it does, that single
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water molecule could be locked in the flowing
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ice for thousands of years before it ever snitches
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the ocean. It really requires a completely different
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perspective on time. It totally is, which makes
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the physics of it so strange to me. So we have
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this dense, massive, solid block of glacial ice
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packed over centuries. If I drop an ice cube
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on the floor, it shatters. I just don't fully
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grasp how a solid mass can flow downhill without
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just... cracking apart into a million pieces.
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Right. It seems counterintuitive, but it comes
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down to what is happening at the molecular level.
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Under the immense sustained stress of hundreds
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or even thousands of vertical meters of ice piled
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up above it, the ice deep within the glacier
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actually becomes plastic. Okay, but plastic?
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Because I think of, you know, a synthetic water
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bottle when I hear plastic. Yeah, that's a common
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confusion. In physics, calling a material plastic
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just means it's malleable. It permanently deforms
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and changes shape over time without cracking.
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Okay, so it's not breaking, it's just bending.
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Yes, and we call this internal deformation. The
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water molecules in the ice's crystal structure
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are under so much pressure that individual layers
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can slowly slide past their neighbors without
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breaking their bonds entirely. That is wild.
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It is. And this internal sliding is what allows
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the glacier as a whole to gradually change shape.
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It flows from the high elevations where snow
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is piling up down toward lower, warmer elevations.
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OK, I get the internal sliding part. I can visualize
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that. But it still seems like, I mean, you've
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got millions of tons of ice scraping against
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solid bedrock at the bottom. Wouldn't that cause
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so much friction it would just get stuck? Or
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wait, does that friction actually help it move?
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You hit the nail on the head. The friction is
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actually the catalyst for the second mechanism,
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which is called basal sliding. Basal sliding.
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So sliding at the base. Exactly. The intense
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pressure from all the ice above, combined with
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the friction of grinding against the underlying
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rock, actually raises the temperature right at
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the base of the glacier. Even if the air outside
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is freezing. Yeah. Even if the air far above
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is well below freezing, it causes localized melting
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right there at the bottom. Oh, I see. So it creates
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this thin layer of liquid melt water between
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the ice. It's kind of like a car hydroplaning
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on a wet road, right? That is a perfect analogy.
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acts as a lubricant it allows the entire mass
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of ice to slide over the bedrock much much faster
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than internal deformation alone would allow because
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of this glaciers that have a warm base and access
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to melt water they flow noticeably faster than
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glaciers that are you know just frozen solidly
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to the rock beneath them millions of tons of
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ice sliding on a microscopic film of water That's
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terrifying and amazing. But as it moves, it has
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to be tearing the landscape apart. Because, like,
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a river cuts a V -shape, right? Because flowing
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water seeks the lowest point of a valley. But
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a glacier is filling the entire valley floor
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to a massive depth. Right, it's nature's bulldozer.
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It erodes the landscape on multiple fronts simultaneously,
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and it does this through two main processes.
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The first one is called plucking. Plucking, like
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plucking a guitar string. More like plucking
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a weed out of the ground. Melt water from the
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glacier seeps into cracks in the underlying bedrock.
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Then it refreezes and expands, and it effectively
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grips onto fragments of the rock. Wow. Yeah,
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and as the glacier inches forward, it just tears
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those rock fragments right away from the ground.
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So it's basically stealing the bedrock. And once
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those rocks are plucked out and embedded in the
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bottom of the moving ice, they must act exactly
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like grit on sandpaper. Exactly. And that second
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process is called abrasion. Those embedded rocks
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scour and grind the bedrock, basically smoothing
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and deepening it. So it just files the mountains
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down. Over thousands of years, yeah. This massive
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icy sandpaper carves out broad, deep U -shaped
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valleys instead of the V -shape of a river. Right,
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the U -shape. And when those U -shaped valleys
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are carved right down to the coast and the ice
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eventually melts, the ocean floods in. That's
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how we get fjords, right? Yeah. Like those incredible
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cliff -lined inlets in Norway or Chile. They're
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literally just drowned glacial valleys. You've
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got it. And geologists read these features as
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clear signatures of past ice. It's not just fjords
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and U -shaped valleys, though. Glaciers also
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leave behind moraines. Moraines. What are those?
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A moraine is basically a ridge of jumbled, unsorted
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rock debris. It's everything from fine grit to
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massive boulders. The ice bulldozes this material
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forward or drags it along its sides and then
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simply drops it all in a mound when it melts.
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Just leaves its trash behind. And the source
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has also mentioned erratics. Which are those
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giant boulders sitting out in the middle of nowhere
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that don't match the surrounding bedrock at all.
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They just hitched a ride inside the flowing ice
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from somewhere completely different and were
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dropped exactly where the ice happened to melt.
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Right. They're entirely out of place. And geologists
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lean heavily on this combination of features.
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Together, they form an unambiguous fingerprint.
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When you see them, you know ice was the primary
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sculptor of that landscape. Okay, so this brings
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up a huge question for me. If these U -shaped
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valleys and moraines are evidence of past ice,
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and we know glaciers are always flowing forward
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downhill under their own weight, It feels contradictory
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to constantly hear about glaciers retreating.
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How can something be moving forward and retreating
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at the exact same time? It sounds like a paradox,
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doesn't it? But the paradox disappears when you
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look at mass balance. You can think of mass balance
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as the running ledger of ice gained versus ice
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lost over the course of a single year. Ah, okay.
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Think of it like a bank account for you listening.
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Your paycheck is the fresh snow depositing at
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the top of the mountain, and your expenses are
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the melting happening at the bottom. That works
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perfectly. High up on the glacier, at colder
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elevations, snow falls and accumulates. We call
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this the accumulation zone. The paycheck zone.
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Right. And further down the valley, at lower,
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warmer elevations, is the ablation zone. This
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is where the glacier loses mass through melting.
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Or, you know, if the glacier reaches the ocean,
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it loses mass through calving. Calving is when
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the large chunks break away from the leading
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edge as icebergs, right? Yeah, exactly. So you
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have accumulation at the top and ablation, melting,
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or calving at the bottom. And somewhere in the
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middle is the break -even point. Right. The boundary
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where accumulation and ablation perfectly balance
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out over an average year is called the equilibrium
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line. If a glacier gains more mass in its accumulation
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zone than it loses in its ablation zone, the
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mass balance is positive. The leading edge will
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actually advance further down the valley. But
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if your expenses outpace your paycheck... your
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account balance shrinks. So the glacier is still
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flowing forward like the cash is still moving,
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but it's melting at the front faster than the
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flowing ice from behind can replace it. Precisely.
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The mass balance is negative, causing the leading
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edge to retreat back up the valley. And the overwhelming
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majority of monitored glaciers around the world
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today are in a state of negative mass balance.
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And this isn't just a guess, right? No, not at
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all. This is a well -documented trend using standardized
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measurement techniques all over the globe. They
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are retreating at a pace that is highly unusual
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compared to the historical record. Okay, so if
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glaciers are in this widespread state of negative
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mass balance, how do we actually know if this
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is an anomaly or just, you know, Earth's natural
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rhythm? Does the glacier somehow keep a record
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of what the atmosphere looks like in the past?
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It does, actually, in a remarkably literal way.
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We talked about how snow compresses into fern
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and then solid ice, right? Yeah, the snowball
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getting squeezed. Well, when that happens, those
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pockets of surface air get sealed inside. They
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are trapped and preserved as tiny bubbles, layer
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upon layer, essentially completely undisturbed.
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Wait, so scientists just drill down into the
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ice and pull off a literal sample of old air?
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They do. They extract a continuous cylindrical
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sample called an ice core. By bringing these
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cores to the surface, they can extract those
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ancient air bubbles and directly analyze the
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composition of the atmosphere from the specific
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year that snow fell. That is mind -blowing. It
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is. They can measure greenhouse gases like carbon
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dioxide from tens or even hundreds of thousands
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of years ago. So we aren't looking at a model
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or an estimate here. It's a direct physical sample
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of ancient air. It's essentially an atmospheric
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time machine. But, um... How do they date the
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layers? Like, how do you know which bubble is
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from which year? It's fascinating. In favorable
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conditions, you can count the layers in an ice
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core almost exactly like counting the growth
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rings of a tree. Oh, like tree rings, really?
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Yeah. seasonal differences in snowfall or, you
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know, the amount of dust blowing around during
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different times of the year, they leave visible
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or chemically detectable annual bands in the
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ice. OK, so the sources noted they also use the
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ice itself to reconstruct past temperatures using
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something called an isotope proxy. I was really
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trying to wrap my head around that mechanism.
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It's a bit technical, but essentially water contains
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slightly different atomic variations or isotopes
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of oxygen. And the heavier isotopes fall out
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of the clouds at different rates depending on
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the temperature of the air at that time. Okay,
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so the temperature controls which type of oxygen
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makes it to the ground. Exactly. So the ratio
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of those isotopes in the ice acts as a highly
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calibrated stand -in, or proxy. By measuring
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that specific ratio, scientists can reconstruct
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the past temperatures at the exact location the
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core was drilled. So it can measure the exact
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temperature at the time the snow fell. That's
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incredible. How far back does that record go?
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Well, the longest continuous ice cores retrieved
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so far come from the deep ice of East Antarctica.
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They stretched back roughly 800 ,000 years. 800
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,000 years preserved one snowfall at a time.
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That's just wow. And what about the ice cores
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from Greenland? Are they the same? They're a
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bit different. They don't reach quite as far
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back because snow falls much more heavily in
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Greenland. So the older, deeper record gets compressed
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into thinner, harder to resolve layers at the
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bottom. I see. But that heavy snowfall is actually
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a massive advantage for recent history. Because
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the annual layers are so thick, Greenland ice
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cores can resolve individual years and sometimes
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even individual seasons with extraordinary detail.
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So it's like zooming in on the timeline. Right.
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They are absolutely perfect for studying abrupt,
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short -term climate events in our more recent
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past. Okay, so looking at those two modern giants
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holding most of the planet's freshwater, Greenland
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and Antarctica, they seem to be reacting to the
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current climate in very different ways. They
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operate very differently, yeah. The Greenland
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ice sheet is losing mass predominantly through
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processes happening right at its surface. Warmer
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air temperatures cause increased summer melting,
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and much of that meltwater simply runs off directly
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into the ocean. It's a very straightforward mechanism
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to observe. You can just see it melting. But
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Antarctica is vastly larger and way more complex,
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right? It's essentially two separate systems.
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Exactly. East Antarctica sits mostly on highland,
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well above sea level, and it seems relatively
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stable. But West Antarctica is the one triggering
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all the alarming headlines about vulnerability.
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Right, because a substantial portion of West
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Antarctica rests on bedrock that is actually
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below sea level. Yes. Its ice sheet is in direct
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contact with the ocean through enormous floating
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extensions called ice shelves. And these shelves
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act like structural braces. They literally hold
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back the ice sheet on land, slowing down its
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natural tendency to flow into the sea. So if
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warmer ocean water reaches underneath those floating
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shelves and melts them, they thin out and lose
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that bracing effect, like knocking out a support
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beam. And then the ice behind it... flows into
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the ocean considerably faster. That's exactly
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what's happening. But the really concerning part
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in our sources is something called marine ice
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sheet instability. Can we walk through the actual
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mechanics of that? Because it sounds pretty intense.
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It is intense. The mechanism hinges entirely
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on the shape of the bedrock beneath West Antarctica.
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Instead of sloping upward as you move inland
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from the coast, the bedrock actually slopes downward.
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Wait, so the further inland you go, the deeper
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the bedrock gets below sea level. Exactly. And
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the point where the ice sheet finally lifts off
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the bedrock and starts floating is called the
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grounding line. If the protective ice shelf thins
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and that grounding line is forced to retreat
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inland, it moves into deeper and deeper water.
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Because of that downward slope. Right. And because
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the ice is thicker in that deeper water, it exerts
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more outward pressure, which accelerates the
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flow. And more of the ice face is exposed to
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the warm ocean water. Which accelerates the melting
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even more. It creates the self -reinforcing feedback
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loop. The further it retreats, the deeper the
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water gets, the faster it flows, and the faster
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it melts. This is marine ice sheet instability.
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And the Swates Glacier in West Antarctica is
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a prime example of this mechanism in action.
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OK, I want to push back on this just a bit because
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we see Swates referred to constantly in the media
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as the doomsday glacier, which, you know, those
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are very heavy panic inducing terms. For anyone
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trying to filter through the noise here, how
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much of this instability is happening right now
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and how much is just theoretical modeling? That
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is a very fair question. It is vital to separate
16:50
what we observe from what we model. We know with
16:53
absolute certainty that mass loss is actively
16:56
happening right now. Because we can measure it.
16:59
Right. We have decades of satellite measurements
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of ice elevation and gravity, direct field measurements,
17:04
repeat photography. The thinning of the ice shelves
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and the retreat of the grounding lines in West
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Antarctica are indisputable facts. The instability
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is active. So what is the uncertain part? The
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genuine scientific uncertainty lies in the pace.
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Modeling exactly how quickly a massive fractured
17:21
block of ice will respond to complex ocean currents
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and atmospheric changes over the coming decades
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is incredibly difficult. So we know what's happening.
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We just don't know exactly how fast it's going
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to happen. Exactly. Translating current mass
17:34
loss into a precise, confident projection for
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sea level rise, say 50 or 100 years from now,
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it requires navigating a wide range of probabilities.
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Reputable scientific body are always up front
17:45
about this margin of error. It's a balance. We
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can't dismiss the reality of the mass loss, obviously,
17:50
but the extreme worst -case timelines push -in
17:53
headlines are often the lowest probability models.
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Right. The physics of marine ice sheet instability
17:58
are sound, and the retreat is documented. But
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the Earth's system has many interacting variables,
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and forecasting the exact speed of collapse is
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an ongoing scientific challenge. We really need
18:09
to avoid both unwarranted alarmism and unwarranted
18:12
dismissal. That makes a lot of sense. Man, this
18:15
completely changes how we'll look at ice. I mean,
18:18
from a single fluffy snowflake to a continent
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-crushing river carving fjords, the stillness
18:23
of a glacier is really just a trick of our limited
18:25
human lifespan. It really is. It is a constantly
18:28
moving landscape and just an undeniably honest
18:31
archive of the planet's history. But I want to
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leave you with one final thought to mull over.
18:36
We talked about how those deep ice cores perfectly
18:38
capture the atmosphere from hundreds of thousands
18:40
of years ago in those tiny air bubbles. The atmospheric
18:43
time machine. Right. Think about the snow falling
18:46
on the high, quiet interior of Antarctica right
18:50
this very second. It is trapping today's air
18:52
right now. What exactly will the bubbles we are
18:55
laying down today say about us to whoever or
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whatever might drill an ice core 10 ,000 years
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in the future? That is quite a thought to end
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on. It really is. Thank you so much for joining
19:05
us on this deep dive. We will catch you next
19:07
time.