The great silence: Just 4 in 10,000 galaxies may host intelligent aliens

[Unclear Engineer]

I don't think a raise of 7 meters in global sea level is possible with a single step with the meltwater dam hypothesis. But, small steps in sea level do seem possible. At the ends of some of the previous ice age peaks, there seem to have been some extreme flooding events that were attributed to failures of "ice dams" in the western U.S. and in western Europe. The one in Western Europe is believed to have broken through where the English Channel is now located, separating what is now England from what is now France.

Here is a link to the thinking about what happened in the western U.S. https://en.wikipedia.org/wiki/Missoula_floods

I did not spend the time to try to find a total discharge volume so as to be able to calculate the ocean level change when the water is spread around the globe. Maybe you can do that for us.

Here is another link that describes some of the ocean and climate effects hypothesized for the western U.S. flood(s?). https://www.livescience.com/873-bursting-ice-dam-flooded-ancient-ocean.html

The time period for that last link is consistent with the estimated timing of the flooding of "Doggerland" which is now the North Sea off the north coast of England. That flooding does appear to have been rather quick, because there seem to be a lot of archeological artefacts in relatively shallow waters there.

 

[Gibsense]

I did not spend the time to try to find a total discharge volume so as to be able to calculate the ocean level change when the water is spread around the globe. Maybe you can do that for us.

Flatterer! But, for a complete melt, it is fantastical. Not a lot of land left, but the redistribution of weight on the crust would probably provide a bit more land via volcanic and the like. Bearing in mind spin and such, no, no chance. But working on a mile or so of ice covering Antarctica and Greenland, the combined surface area etc I suppose a good enough approximation would be possible. I think I read somewhere it would be about 25 meters of sea rise.

 

[Gibsense]

Lifted from one of UE's links:
"In the movie "An Inconvenient Truth," Al Gore mentions the North American lake bursts event and the stalling of the ocean currents. If large parts of Greenland and Antarctica were to melt as some global warming models predict, a similar stalling of the world's ocean currents could occur, Gore said."

This feedback from warming might slow down climate change - at least for the UK - for a while. It probably has already started.

 

[Unclear Engineer]

The projections I have seen for sea level rise due to the total melting of all ice on Greenland is about 25 feet, which is about 7 meters.

Of course, there are still other pieces of land with ice sheet coverage in the north latitudes, which could melt along with Greenland.

The projections I have seen for complete melting of all ice sheets, including Antarctica, is 325 to 350 feet, which is about 100 meters.

But, those melting processes are expected to take many tens to even thousands of years, depending on a lot of assumptions. I agree that there should be expectation of step changes in sea level, but I doubt any single step would be 7 meters.

There are also other step processes which involve ocean currents. Due to Coriolis "forces" on ocean currents, there are differences of several feet in sea level from one side to the other. If the current stops, the Coriolis force is gone, and the sea level equilibrates across the whole surface. For the east coast of the U.S. there would be something like 4 to 7 feet of sea level rise at the coast if the Gulf Stream stopped. That would be sufficient to flood my home, so I am paying attention to that phenomenon.

 

[Unclear Engineer]

OK, the wiki article says the estimated volume of the ancient Missoula Lake that drained by a catastrophic ice dam break was 2184 cubic kilometers (see https://en.wikipedia.org/wiki/Missoula_floods#Further_reading .

That is 533 cubic miles of water.

The surface area of the oceans of the world is 142,200,000 square miles (see https://www.noaa.gov/jetstream/ocean )

So, spreading 533 cubic miles over 142,200,000 square miles makes a layer 533 / 142,200,000 = 0.00000375 miles deep. That is not quite 1/4". That probably would not even be noticed as a "step" in sea level by the time it spread to the other side of the world.

Another source estimated the ancient lake to be about the current volume of Lake Huron, which is about 850 cubic miles, so 3/8" ocean rise?

There are some hypotheses that a much larger lake occupied the Rocky Mountain Trench (see https://en.wikipedia.org/wiki/Rocky_Mountain_Trench ), but that is subdivided into about 4 sub basins, so it is not clear how much of it might have been filled with water that could suddenly drain out one pathway. It is currently drained by 4 river systems, but those might have been blocked at the end of the last ice age.

Anyway, it seems to take an extremely huge melt to make a noticeable step in sea level.

 

[Gibsense]

Anyway, it seems to take an extremely huge melt to make a noticeable step in sea level.

The report I read was not about lakes. It was a major drainage system under a large proportion of Greenland where they discovered a silt/rubble dam build-up. The assumption was that a similar situation may exist in Antarctica. Bearing in mind that the ice thickness on both is often 'miles' that was their concern/point; it would be noticeable.

 

[Unclear Engineer]

In effect, you re still talking about a "lake" of fresh water, sitting on land above sea level, that has to drain rapidly into the ocean in order to make a step change in global sea level. That is really not much different from the scenario where the miles-thick ice sheets in North America and Eurasia were melting and draining lakes that may have also been ice covered at some points, and not in others. So, I would expect similar types of floods as have been detected by geological evidence in the northern hemisphere

We do know that there are currently some fresh water lakes under Antarctic ice sheets, but I don't think any of them are currently large enough to make big step changes in sea level. I don't have time to search for Antarctic sub-glacial lake info right now, but the same calculation could be used for those if we get numbers for their volumes.

Of course, there could be expansion of those lakes in the future, so I don't have a way to calculate the worst possible step change in sea level that might eventually happen as the whole ice sheets melt to nothing. But, 100 meters total rise would be damaging enough if it happened steadily rather than in steps.

 

[Unclear Engineer]

Here are some links to Antarctic sub-glacial lakes:

Lake Vostok - Wikipedia

en.wikipedia.org

Antarctic lakes - Wikipedia

en.wikipedia.org

Note that the largest one is Lake Vostok, with a volume of 1,300 cubic miles. So that much water entering the ocean would raise sea level by a little more than 1/2". Although it is actually already below sea level, presumably the ice on top would be able to make it squirt out if there was an opening to the ocean, and the surface of the ice that is well above sea level would just go down a bit.

It is actually a pretty big lake, the 6th largest in the world by volume.

And, there are other lakes under the Antarctic ice sheets, too.

But, even all of them together don't seem to have a potential for making a very large sudden step in sea level for the entire world.