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Comment by brohoolio | Hacker News Reader

“CO2 levels are now comparable to the Pliocene Climatic Optimum, between 4.1 and 4.5 million years ago, when they were close to, or above 400 ppm. During that time, sea levels were between 5 and 25 meters higher than today offsite, high enough to drown many of the world’s largest modern cities. “

Oh boy, that’s not good. I mean it’s going to take a while for it to raise up, but so long Florida.

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Also it is probable that we will see a big extinction event even before +1m sea level mark. Imagine climate changes enough and staple crops start to fail en masse - wheat, rice etc. The hunger and rising costs will likely case a world war, or several.

Where has this water gone exactly? Freezing wouldn’t explain it. Is it lagged thermal expansion of the ocean water as it catches up to the air in heat?

Frozen in glaciers and poles, I would assume...

It's a combination of melting of glaciers and ice-caps and thermal expansion of water as it warms:

https://oceanservice.noaa.gov/facts/sealevel.html

> Freezing wouldn’t explain it.

Pretty sure freezing is exactly it — Greenland and Antarctica contain lots of water stored well above sea level. These fluctuated a lot worth sea levels.

Several people are saying this so I’ll reply to just this one. It wouldn’t make sense for water to freeze up over Antarctica over time as a way to reduce sea level. Water freezing on top of water doesn’t affect water height. And water freezing on top of submerged land would just raise the water levels because it’s more voluminous. There needs to be a mechanism for the ice to accumulate on top of itself far above sea level for sea level to go down beyond just water freezing. As another poster explained, it’s snowfall accumulating semi-permanently in the same spots that moves water to these areas. Excuse my pedantry.

> There needs to be a mechanism for the ice to accumulate on top of itself far above sea level for sea level to go down beyond just water freezing.

It's pretty common in the arctic regions that water forms ice crystals and falls out of the sky, landing on the ground.

Snow, I believe they call it.

I literally acknowledged it was snow it the following sentence

Not pedantic, I think people just didn't realize you were narrowly thinking of seawater freezing directly when you said "freezing wouldn't explain it".

A lot of it is in the glaciers of Antartica, which cover the whole continent with around 1900m of ice. They weren't created from seawater freezing, but from snow getting packed down. But the water will go into the ocean if they melt.

Ah ok that makes sense

There isn't that much water on earth, this image does an amazing job of explaing the scale in the quickest way.

It also sort of explains how easy it would be for earth's water to have come from asteroid impacts.

https://d9-wret.s3.us-west-2.amazonaws.com/assets/palladium/...

I’m not saying it’s wrong, but this graphic is very difficult for me to understand. Is there any geographical significance to the location of the water blobs? No, right? But they also don’t include saltwater which is most of it. Right?

The big bubble is all water, frozen, sea, etc. The small bubble is all fresh (rain clouds etc), rivers, lakes. The smaller is lakes and rivers only.

Here's an article showing comparisons to various moon ice.

https://www.businessinsider.com/earth-water-ice-volume-versu...

It says liquid fresh water.

I don’t think the big bubble includes the ocean.

There are three bubbles.

Ah I see now

By far most of the water is in the oceans, which have an average depth of 3688m. 2% of the water is currently frozen, so one can easily estimate how much would the sea level raise, had it all melt.

EDIT: Corrected km to meters, as comment below points out.

For those reading this with incredulity I believe the above commenter made a typo - the average depth is 3688m, or 12,100ft.

For reference, there's ~6.4M cubic miles of water in antarctica - and ~321M cubic miles of water in the ocean.

I think that’s difficult to utilize for comprehending sea level. The relationship between total volume and sea level won’t be linear.

Surface area of the ocean would be more useful I think?

It's not the volume of the ocean that matters for sea level rise but the surface area, right? Ocean surface area is 140M square miles, so 6.4M cubic miles of water would naively be like a 241 foot rise -- each cubic mile gets split into 140/6.4 slices.

Fine as a rough approximation of course, but don't forget surface area will grow as sea level rises and threatens land.

Additionally the crust is elastic and will move around a bit (especially lifting) with the ice weight off. This is still occurring in many places that were glaciated previously.

I'm a bit stupid, but I understand if we are dropping 6.4M cubic miles from space, there will be an increase in water level, but if the 6.4M cubic miles of water was already present in the system, just frozen, how does it increase the total sea level? There is no net change in volume right?

Antarctic glacier mostly rests on the continent, rather than float in the ocean.

Land ice vs. sea ice.

Sea ice floats in the sea. When melted ... it's still floating in the sea, as water. There's no net sea-level change, at least not from contributed ice.[1]

Land ice sits on land. When it melts, most of it flows to the sea.[2] That increases the total amount of water in the oceans, and hence, sea level. The total rise if all Antarctic and Greenland ice melted would be about 60-70m (200-230ft).[3]

________________________________

Notes:

1. There are other effects, including thermal expansion (water expands slightly as it warms), and centripetal effects (water can flow more than continental crust does, and would spread out slightly more at the equator than the poles with more liquid water in the oceans). Those effects are comparatively small, though not fully negligible.

2. The exception would be enteric basins which have no outlet to the oceans, in which case melted ice flowing into these would form lakes. Examples of enteric basins include the Great Salt Lake and surrounding former Lake Bonneville, the Dead Sea, and Death Valley. Note that as glaciers melt, there's a rebound effect in continental crust, and regions presently below sea level or which would otherwise form enteric basins might not after that rebound effect is taken into account.

3. See: https://science.howstuffworks.com/environmental/earth/geophy... https://www.usgs.gov/faqs/how-would-sea-level-change-if-all-...

Erm, that should be endorheic basins, not enteric, which has a different set of guts.

https://en.wikipedia.org/wiki/Endorheic_basin

https://en.wikipedia.org/wiki/Enteric

Antartica is nearly 10% of total land (it's larger than europe, and half the surface of africa), and the ice depth averages more than 2km over land.

It stores enough ice to raise sea levels by nearly 60m if / when that melts entirely.

It currently loses more than 250Gt/year of ice, up from 40Gt during the 80s. But it holds an estimated 26500000Gt (26.5Pt) of ice.

Additionally to what others have mentioned, there is also the effect of the gravitational pull of ice shields.

https://www.antarcticglaciers.org/question/antarctic-ice-she...

There was a discussion about this effect on hn years ago, the original link send to be dead now. But this is the thread on hn: https://news.ycombinator.com/item?id=23389690

Rigorous explanation of bodies of water on the planet requires accounting for how water and ice deform the crust with their mass.

Glaciers also compress the earth underneath them, which rises back up when they melt in a process called isostatic rebound.

> Is it lagged thermal expansion of the ocean water as it catches up to the air in heat?

I've read that this will be the primary contributor. Might want to double check the answers you're getting here.

Gonna take a while to rise 5 to 25 meters though. I think it's currently rising at like 0.8m per century.

The sea-levels have always fluctuated, no need to worry for Florida, if it happens, it happens. Also, no need to look 4 million years back for similar events, Doggerland [1] was still a thing with actual humans living on it only ~8,000 years ago.

[1] https://en.wikipedia.org/wiki/Doggerland

>> During that time, sea levels were between 5 and 25 meters higher than today offsite, high enough to drown many of the world’s largest modern cities.

That makes for a great headline, but a large part of the Netherlands' coastal areas are currently below sea level, and yet are not "drowned".

Hopefully, this will fix the US's coastal elites problem.

It is clearly a real threat, that's why we have advocates for strict "climate regulations" buying multiple beach-front homes, their own islands, and private jets...

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