The truth about sea level rise
Perhaps you’ve seen read some of the scary headlines.
Writers say that we have 10 years to solve global warming. Some say that civilization will collapse by 2050. One medium article informs us that patients are talking to their therapists about global warming.
An article from Sustainability Action uses this image to show the future of sea level rise:
How bad will the problems really be? How fast will the changes come?
If all the ice in the world melted, sea level would rise about 200 feet (60 m):
That’s up to the statue of liberty’s waist, not her shoulders. The clickbait title image isn’t quite right, the ocean will never rise that high. But New York would still be devastated by 200 feet of sea level rise:
Florida would be gone, so would New Orleans. Coastal cities would be flooded around the world:
But, far from the headlines talking about 12 years, or 50 years, getting there would take a very long time, well over a thousand years.
Scientists are only forecasting 1–3 feet of sea level rise by 2100.
Sea level rise is slow:
The main surprise I have, when I read papers about sea level rise, is how slow the process is.
At the moment, the sea is only rising 3 millimeters a year. At that rate, the water would only be 7 inches (24 cm) higher, by 2100. But climate forecasters think that it will accelerate a bit.
Raising the temperature a few degrees can actually be enough to melt an entire continent. You only need to get above the melting point, and the entire glacier will go. But, it won’t happen in a year, it will take centuries.
Glaciers in Greenland and Antarctica are unfathomably large, with ice that’s 2 miles thick:
Melting all of Greenland would cause 20 feet of sea level rise. Melting Antarctica would cause the other 180 feet.
At the moment, it looks like the planet is warm enough that Greenland is melting, but Antarctica is not. More specifically, west Antarctica is melting a bit, East Antarctica is getting more snow, it seems to roughly balance out.
These glaciers formed slowly. During ice ages, it snowed a little bit more than it melted, every year. The snow got packed down until it became ice. The ice built up over thousands of years.
Likewise, it takes a long time for a glacier to melt.
If you put an ice cube in a hot room, it doesn’t melt right away. Maybe it takes 30 minutes. Imagine that the ice cube is 2 miles thick, and the room is only a few degrees above the melting point. Now it takes hundreds or thousands of years.
If we put a certain amount of carbon into the atmosphere, the planet heats up. That commits us to a certain amount of melting. But, the melting is a slow process.
If we all quit burning fossil fuels tomorrow, we’re already committed to some sea level rise, maybe as much as 10 feet. But it will take a few hundred years to see all of it.
We won’t stop burning tomorrow, though. Suppose it takes us 100 years to stop, and our use averages about the current level for that time. If we do that, we’ll be on a path for 60 feet of sea level rise:
60 feet would be a really big deal, wrecking cities all around the world.
But that rise is forecast to happen over a few thousand years:
Even 10 feet of rise would be enough to flood streets across Manhattan, a disaster for the city:
The long term disaster could be mitigated. Over a few hundred years, we could pull CO2 out of the air to cool the planet again. Or we can prepare for sea level rise. We could build levees or move to higher ground.
But, again, none of that will happen soon. In the next 100 years, we’re only expecting 1–3 feet of rise. For complicated reasons, New York will see less rise than the rest of the world will.
Greenland will flood the world, but it won’t flood northern countries:
Imagine that the glacier sitting on Greenland melts entirely. That puts 20 feet of extra water into the world’s oceans.
Now imagine you have a house, right on the coast of Greenland. You’d think that your house is now destroyed, sitting 20 feet under water.
Surprisingly, that house wouldn’t actually flood at all. For counterintuitive reasons, the water near your house will actually drop. Most of the newly melted water will end up in more distant parts of the ocean.
First, understand that continents aren’t fixed in place. The continents have moved over time. The Earth’s crust is lower density rock, floating on top of the higher density mantle.
When you put a heavy weight on top of one continent, the crust sinks a little bit deeper into the mantle. A two mile thick shelf of ice weighs down the land significantly.
As the ice melts, the land slowly rebounds.
Canada was heavily glaciated in the last ice age, as were some northern states in the US. You can still see the scars on the land — Michigan is the land of a thousand lakes, Minnesota claims ten thousand, Manitoba is nothing but small lakes. The ice has all melted, but the ground is still slowly rebounding, today, about a centimeter each year (faster than sea level rise).
So, as Greenland starts to melt, the ice will weigh down the island less, and the ground will rise.
If you live on the shore, the ground will slowly rise out of the ocean, faster than the water will rise.
The water level of the ocean also adjusts to gravity. The Earth isn’t a perfect sphere, the ocean’s surface adapts to where the most mass is. When one continent has more mass, water flows towards it. When it loses mass, water flows away.
These effects spread out gradually over thousands of miles. So, Greenland won’t flood nearby countries like Ireland or Norway at all. It will have only small effects on New York or the Netherlands.
Worldwide, the average water level will be 20 feet higher, if Greenland melts. But the water level next to Greenland will actually drop. New York might only see 5 feet of rise. Asia and South America will see more than 20 feet of rise.
So, if we get 3 feet of rise by 2100, North America and Europe will have less than that. But the threats will be reversed, if Antarctica eventually starts to melt.
Pacific Islands won’t sink into the ocean
Even 3 feet of sea level rise sounds scary if you’re living on an island that’s barely above sea level. Will flooded islanders have to move, in the next 100 years?
This is also counterintuitive, but these islands won’t sink under the rising Pacific ocean. Islands we’ve studied in the last 50 years have actually been growing, not shrinking, as sea level rose.
What’s going on, here?
First, we have to remember what creates a coral atoll island. Why do we find these small rings of sand out in the middle of the ocean?
Pacific atolls are ancient volcanoes that have sunken under the waves.
Volcanic islands grow when they’re active and then slowly subside, from erosion, and from the weight pushing down on the Earth’s crust below. As the volcano gets near the water line, though, the coral reefs around it keep building the land back up.
Since the island is now made of sand (broken down coral or rock), the height can adjust a bit. As the water slowly rises, high tide waves deposit more sand on the shore. Wind and erosion can also move the sand, to keep islands from building up. They naturally end up only a bit above sea level.
This is surprising, but it makes sense, if you think about it more. Sea level has changed in the past. 10,000 years ago, during the last ice age, sea level was 200 feet lower. Atolls were not 200 feet above water at that time, they were probably still right at the waterline.
As the ice age ended, glaciers melted and the sea rose. The islands didn’t get submerged, the waves gradually moved the sand so that they stayed above water. The same process will continue, with global warming.
We can see all the stages of an island’s life, looking at Hawaii:
The Hawaiian volcanic hot spot stayed in one place while the Pacific plate slowly moved over it, leaving an island chain behind:
The big island of Hawaii is the newest volcano, and the tallest one. Each older island is a bit shorter. Maui is a million years old, Kauai is 5 million. The sand on the beaches is finer from having longer to grind it down.
By the time you get out to Midway island, 27 million years old, there’s just a coral atoll remaining:
There are a few caveats here— a low rocky island, with no coral, would still get submerged. Coral only grows in tropical seas. Someone living on a low, northern island would still get flooded.
And the process that builds up sandy islands only happens so fast. If sea level rose very quickly, for some reason, it could flood pacific islands. Things aren’t happening that quickly, right now.
So, finally, who’s at risk?
In the next 100 years or so, people in low lying areas might see floods more often. The effects will probably be worst in Asia. Bangladesh will be in trouble, with 15 million people living just above sea level:
North America shouldn’t be that bad, but some places like Miami Beach will have some problems.
If we let the process get out of hand, then major coastal cities all around the world will eventually start to flood, over the next few centuries. That, or we’ll just have to build a lot of levees and sea walls. Or, millions of people will simply move.
With some work, we’ll have cheap green energy long before things get that bad.
Lastly, people aren’t the only ones affected by all of this.
Antarctica is a covered by a very thick glacier. The north pole is different, with only a thin layer of sea ice floating on water. That sea ice isn’t 2 miles thick, it’s only a few feet thick. Polar bears live on that thin ice and hunt the seals swimming beneath it.
It won’t take as much time or warming to melt the thin, floating sea ice.
So far we’ve raised global temperature around 1 degree Celsius.
At 1.5 degrees, there’s a 1% chance every year that the entire arctic ocean will melt out. At 2 degrees, there’s a 10% chance each summer.
So, if we keep this up, we’ll start starving a lot of the bears. Some will die out. Some will move onto land during the ice free summers, some will starve while they wait for the winter to return.
That should happen in the next 50–100 years.
Also, those coral reefs, that build up tropical islands, are going to die off, as the oceans warm:
Fish relying on the coral will die, fishing communities will suffer. That should also happen in the next 50–100 years.
It’s important to make an effort to fix global warming, to help people in Bangladesh, to help some polar bears, and to keep the oceans healthy. Also, to help coastal cities everywhere.
The problem is huge, but also very slow moving. There’s little urgency. But other people will be dealing with our choices for hundreds of years.
Electric cars should get better and cheaper in the next 10 years or so, and we’ll start to gradually solve the problem.
Until then, I hope that knowing the truth helps you worry a little less that the world’s about to end. If not, you can still talk to your therapist.
