- This is 15 meters or about 50 feet of sea level rise.

That's the amount of rise the world's top climate scientists have said is possible.

By the year 2300, that amount of sea level rise would completely destroy many of the world's largest cities.

- I do not want to be in the case where I say, look, we were right.

This is happening because we didn't take action.

- If sea level just kept rising at today's pace, which is about 4.5 centimeters per decade, we might realistically be able to adapt, build higher, defend more, and retreat slowly.

But the latest science shows a very different future.

We're already dangerously close to tipping points that could push our ice sheets into long-term, irreversible retreat resulting in sea levels rising faster than any coastal city on earth can cope with.

- We're walking very slowly towards the edge of a cliff before we know it, we'll drop off and then it's too late.

- So in this episode, we're asking three big questions.

What happens when sea level rise crosses the point of no return?

Where exactly is that point?

And most importantly, do our coastal cities stand a chance?

About 230 million people live at or below one meter of sea level, and around a billion people live below 10 meters.

And cities are just not preparing for the type of sea level rise we may see in the coming centuries.

Without adaptation, conservative estimates suggest that just 20 centimeters, which is about eight inches of sea level rise by 2050, would lead to average global flood losses of $1 trillion or more per year.

Sea level rise has been easy to ignore and delay planning for because until recently our oceans had been rising slowly, but that's changing fast.

The rate of sea level rise has been creeping up from 2.1 millimeters per year to 2.9 to now 4.5.

- Four and a half millimeters seems small, right?

But the rate of acceleration is what's important.

That four and a half millimeters very quickly becomes six and a half millimeters, and that six and a half if the current rate of acceleration continues, becomes 10 millimeters by the end of century - 10 millimeters per year may not sound dramatic until you realize that adds up to one meter per century.

That's an unimaginable rate of rise, and the science shows that this will continue beyond 2100.

- And I think that number starts to scare people or it should do.

- And remember, these are global averages.

Places like the US Gulf Coast and the East Coast will face far higher local rates of rise thanks to sinking land, changing ocean currents and gravity effects from melting ice sheets.

And while cities do have plans to deal with the rising seas, almost all of them only look a few decades ahead, partly because sea level rise unfolds over centuries far beyond political cycles, budget timelines, or the way we normally make decisions.

And also because the sheer magnitude of the problem is almost impossible to fully grasp, let alone plan for.

But before we talk about what that means for adaptation, we need to understand why sea level rise is speeding up in the first place.

There are two main drivers.

The first is pretty simple, something called thermal expansion.

When water's warm, it expands.

And since we're warming the planet, that's been the dominant driver of sea level rise, at least until recently.

But the second source is the one that scientists are really worried about: ice sheet melt.

Around 2005, melt from Greenland and Antarctica overtook thermal expansion as the dominant cause of global sea level rise.

- Their contribution has quadrupled over the last few decades, and that's really worried us because we used to think that they would take a long time to melt, they'd be very slow and sluggish.

- Scientists have known about ice sheet melt for a while, but it wasn't until recently that we started to understand the true urgency of this threat.

In 2024, for the first time in human history, the planet spent an entire calendar year above 1.5 Celsius of warming, and were on track to roughly double that by the end of the century.

But a new paper from Chris and his colleague, Andrea Dutton, suggests that even 1.5 degrees may already be too much.

Potentially enough to lock in catastrophic long-term sea level rise.

- We may have already committed ourselves to multiple meters of sea level rise just based on the amount of warming we have already done, much less the future warming that is most certainly underway.

- And if that's true, it completely changes how we think about global warming.

It means hundreds of millions of people may have to learn how to live below sea level or move away entirely.

So how did they reach such a dire conclusion and how dire are we talking?

Exactly.

- One of the things that we did in this paper was simply ask the question, what happened to the ice sheets the last time?

Global mean surface temperatures were around a one to 1.5 degree value.

- The last time the planet was this warm was about 125,000 years ago.

During the last interglacial period.

- There's widespread evidence that sea level was higher than present, even though the global average temperature during the last interglacial was pretty similar to today.

One of the very big questions that scientists are trying to answer is, did the West Antarctic ice sheet collapse during this last interglacial period?

- Scientists have been piecing this together from ice cores, marine sediment and other geologic clues.

And also octopus DNA?

- They were looking at this octopus that lives now around the edges of Antarctica and looking at the DNA to see how related these different populations of octopus are to each other.

And they realized at some point in the relatively recent past, in a geologic perspective, these must have been in direct contact with each other.

And the only way to do that would be to collapse the West Antarctic ice sheet.

- And it wasn't just Antarctica.

Scientists have also found that Greenland lost a significant amount of ice during that same period.

- Around some of the margins in different parts of Greenland.

The ice sheet did retreat and and so it did melt and it probably contributed to sea level rise on the order of about two meters.

And when you add in Antarctica's contribution, we have estimated that sea level may have been as much as six to nine meters higher than today.

That's 20 to 30 feet.

That's a lot for a time when global average temperature was about the same as it is today.

- And when scientists look back even further to Pliocene around 3 million years ago, they see an even starker warning.

CO2 levels were similar to today, and sea levels were dramatically higher.

- We have fewer data points, but the data points that are out there point to sea levels rising on the order of 10 to 20 meters.

- All this gives us a preview of what could happen to us unless we not only slam the brakes on fossil fuel emissions, but somehow put the system into reverse, which is why the IPCC said that 15 meters of sea level rise by 2300 is a possibility.

But remember, while the total amount of sea level rise matters, it's the rate of rise that most determines what we can or can't adapt to.

- We can get hung up on the kind of absolute numbers.

You know, is it gonna be four meters?

Is it gonna be five meters?

But what's really important is the rate of sea level rise.

It's generally viewed that once you get above about seven to 10 millimeters per year.

So that's something that will happen in the second half of this century.

That's when it becomes really very, very challenging to adapt to.

It's very hard to basically build coastal defenses that can keep a pace with a one meter added to sea level every century.

- Sea walls, levees and pumps can't keep up with that kind of moving target.

- Coastal planners have got some serious decisions to make.

Are you gonna try and spend billions of dollars on trying to defend yourself against one, two meters of sea level rise?

Or is your adaptation simply to move people in land and rehouse them?

- And that doesn't just affect coastal communities.

When communities and entire cities are relocated, sea level rise becomes a global issue.

The paleo climate record gives us a window into how sea level rise responded to warming in earth's past.

But in order to understand why Greenland and Antarctica are so vulnerable today, we need to understand the mechanisms that are driving the accelerating melt.

And it all comes down to feedback loops.

- There are mechanisms out there that cause ice sheets to melt 10 times more rapidly than they are at present.

- One reason ice can melt that fast is something called surface elevation and melt feedback.

As an ice sheet starts to shrink, its surface lowers into warmer air, which makes it melt even faster.

Creating a runaway feedback loop.

- There's one or two papers now suggesting that the self-reinforcing melt feedback is starting to occur in Greenland.

And if that kicks in, we could start to see quite rapid jumps in sea level rise from melting of the greenland ice sheet.

- The next feedback loop is called marine ice sheet instability.

- When you start to retreat the ice sheet, can you imagine warm waters approaching the floating parts of the ice sheet?

And if they can get underneath that and they can start eroding and melting the ice sheet, that ice sheet starts retreating deeper and deeper onto the sea bed as you move up ice.

And that becomes a runaway process.

We are very worried about that surface melt elevation feedback in Greenland, and we're really worried about the marine ice sheet instability in Western Antarctica.

- These feedback loops are bad news because once they're underway, they're pretty much impossible to stop.

- So these instabilities, the reason that we're worried about them is that these are mechanisms through which we could rapidly accelerate sea level rise and hit those rates of 10 millimeters.

15, 20 millimeters per year that at the moment seem unimaginable.

- Here's the bottom line.

The Paris Agreement set a global goal to limit warming to 1.5 degrees Celsius.

But even if we somehow managed to hit that target, which now looks nearly impossible, it still wouldn't be enough to stop catastrophic sea level rise.

- Part of the reason we wrote this paper was because there is a Paris agreement out there that's been negotiated saying that we need to limit warm to two degrees, ideally 1.5.

And I would speak to policy makers and they would say, well, so if we can make 1.5, everything's gonna be fine, right?

And I would say, no, no, no, no, no, that that's not true.

1.5 will be great.

We must do all we can to limit warming to 1.5, but don't for a minute think that this means that sealable rise is gonna go away.

1.5 degrees would almost certainly see us step off the edge of the cliff.

- And regardless of if we've already stepped off that cliff or not, one thing is clear, we don't have any time to wait around to take action.

- It's like we're stumbling around in a dark room and we know there's a monster in there, and we don't know exactly when we're gonna encounter it, but the longer we stay in the room, you can be sure we're gonna encounter it.

- We've covered many climate crises here on weathered, but sea level rise is one that worries me the most because it's slow and it's easy to ignore today.

But over the long run, it'll be so extreme that it'll completely reshape our coastlines, our cities, and the lives of hundreds of millions of people.

What's especially scary is that Chris and Andrea estimate that even about 1.2 degrees of global warming may already be too much for large parts of Greenland and Antarctica's ice sheets.

And to truly avoid catastrophic sea level rise, we need to bring temperatures down to about one degree Celsius, which means not only stopping carbon emissions, but also removing CO2 from the atmosphere.

But that's a topic for another episode.

So does Chris and Andrea's paper mean all is lost, or is there still hope?

- The simple message is every 10th of a degree matters.

Doesn't matter whether we're talking about sea rise, the Amazon rainforest, the permafrost melting in the Arctic, every fraction of the degree, the consequences get worse.

So we must do all we can to reduce emissions so that we stabilize global temperature.

- Our actions today will control what we're committing ourselves to in this long term.

We are the people who are going to impact what's going to happen in the future.

- And right now is obviously the time to lower our emissions because there are clearly two worlds awaiting.

One that we can adapt to and one that we probably can't in the next decade will determine which one of those becomes our reality.