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I watched a documentary about research on the coasts of Greenland where the sea level has fallen. According to the research, this is because a lot of the glaciers have melted, resulting in less weight on the land and a upward push by the magma under Greenland.

I'm wondering whether the US East Coast could also rise if all the people living there were to move away far enough for the weight to decrease. Since there are some 100 million people living there, I would imagine that's a lot of weight. However, I am thinking that since the inland US is part of a large continent, the magma would not be able to push it up like a "small" island like Greenland.

Do we currently have sufficient metrics to calculate such movements with fairly good accuracy?

Here is an article for reference: https://www.sciencenews.org/article/jacky-austermann-climate-sn-10-scientists-to-watch

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    $\begingroup$ Some thoughts: 1/ this phenomenon is called "isostatic rebound" and has nothing to do with magma (the mantle is solid); 2/ you don't need an island to see the effect, it also occurs in Scandinavia for instance; 3/ we are talking kilometer thick of ice (order of magnitude), so about one million kilograms per square meter. Assuming the average American weighs 100 kg (for the sake of simplicity), you'd need 10,000 people per square meter to get the same effect. New York has 10,000 people per square kilometer. $\endgroup$ Commented Dec 6, 2022 at 21:29
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    $\begingroup$ "I would imagine that's a lot of weight": you could also estimate it, and compare with the weight of a layer of ice on the area that you're considering $\endgroup$
    – njzk2
    Commented Dec 8, 2022 at 21:14
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    $\begingroup$ @JanDoggen "the big mass of ice was pulling water towards Greenland" Are you asserting that the mass of Greenland's ice was exerting gravitational pull on the surrounding water making local sea level higher than the mean, rather than the mass of ice pushing down on the land mass, being the primary driver of relative coast height changes? $\endgroup$
    – Glen Yates
    Commented Dec 8, 2022 at 22:06
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    $\begingroup$ @njzk2: …or, for a quick and visually striking ballpark estimate, just note that the average density of ice is roughly the same as that of humans. (Both float mostly submerged in water.) The US East Coast may be densely populated, but it's not covered in a layer of human bodies several kilometers deep. $\endgroup$ Commented Dec 8, 2022 at 22:09

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The mass of human bodies across the US East coast (about 120 million for the East coast States) is very small compared to Greenland ice mass loss, which (according to Grace Satellite data) has averaged 273 billion metric tons per year since 2002 - about 5 trillion tons over the last 20 years, ie about 1/40,000th based on a generous body mass of 100kg (it is closer to 80kg) if my arithmetic is correct. (I had to edit because my arithmetic wasn't first time around).

I don't know if that would make a measurable difference to the gravity effect of land masses on nearby sea levels - that depends on how accurate the measurements - but it won't be significant.

The mass of buildings, cars trucks, machinery would be larger than that but a lot of that mass would be made out of materials like gravels and sands that were mined locally. Again, it won't be significant.

Greenland Ice Mass Loss

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    $\begingroup$ To add to this, the average weight of Americans in 2020 was 181 lb (82 kg). With 120 M Americans on the east coast, their total weight would be 9.84 Mt. The amount of ice lost in Greenland each year (273 Gt) is 27 744 times the weight of people of the US east coast. The weight of people on the US east coast is insignificant by comparison. $\endgroup$
    – Fred
    Commented Dec 7, 2022 at 0:50
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    $\begingroup$ @Fred - sounds like my 1/40,000th of what Greenland lost over 2 decades is a significant overestimation. $\endgroup$
    – Ken Fabian
    Commented Dec 7, 2022 at 1:27
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    $\begingroup$ If Wikipedia is accurate, 273Gt is about half the world's multicellular biomass, and humans account for about 65Mt. US east coast humans are about 1.5% of that by number, surely more by weight (relatively few children, and fatter adults) but I think less than Fred's estimate which IIUC multiplies adult weight by population. Also, if there were no humans in the region but with an otherwise unchanged environment, there would probably be a lot more plants, ants, etc. so the local biomass would probably be heavier (but still negligibly so)! $\endgroup$ Commented Dec 7, 2022 at 18:26
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While the weight of the population removed might not have much effect, stopping the removal of groundwater could potentially have a fairly large effect on the height relative to sea level. Lots of places are subject to "land subsidence", which is the vertical sinking of land due to groundwater removal and subsequent settling. The most notable example I have heard about recently is Jakarta, but other arid places also suffer from it. The east coast doesn't count as arid, but the USGS claims that 45 US states suffer from it, so it stands to reason that some of those are on the East coast

https://www.usgs.gov/special-topics/water-science-school/science/land-subsidence#overview

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  • $\begingroup$ Yes. I've heard of that living in California with droughts, we have the issue here. $\endgroup$ Commented Dec 8, 2022 at 0:27
  • $\begingroup$ My understanding is that subsidence due to groundwater removal (up to 20 feet in some parts of California) is largely (or even entirely) irreversible. Stopping the further removal of groundwater therefore simply retains the status quo, meaning the effect on elevation relative to sea level would be approximately zero. $\endgroup$
    – njuffa
    Commented Dec 8, 2022 at 21:18
  • $\begingroup$ I would expect some rebound with water extraction ceasing but highly dependent on the local subsoil characteristics; lots of it - most? - will be too compacted. Groundwater replenishment as a cause of land rise seems likely to have only small and localized effects. Loss of mass from groundwater loss ought to have a similar gravity effect (direction) on ocean water and coastal sea levels as ice loss - apparent (but not actual) land rise. Isostatic (actual) rise and from lost and regained mass will have a slow, delayed effect. $\endgroup$
    – Ken Fabian
    Commented Dec 9, 2022 at 21:36

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