Comment by mjfl
2 years ago
2 trillion tons = 1/3 of 1 billionth of the mass of the earth. the distance from the ground to the surface is around 5 millionths of the radius of earth (like 100ft)
So you're talking about a change in angular momentum that is on the order of 10^(-20) times the angular momentum of earth (mr^2 \omega).
A change in the pole of earths rotation by 31 inches (10^(-7) times the radius of earch) corresponds to a change in angular momentum on the order of 10^(-14). So we seem to be off by 7 orders of magnitude.
I call bullshit.
Your computation is very wrong, as explained in the actual research paper and in another comment here. The water is not moved only from under the ground to the surface and the effect on the axis is not due to raising, but mainly to horizontal movement.
post calculation. You're effectively saying, that the movement is due to a change in the center of mass of the earth?
The calculations are in section 2, beneath Figure 2 in the HTML version of the paper.
Changes in terrestrial water (urn:x-wiley:00948276:media:grl65897:grl65897-math-0001) (e.g., Figure 2a) and oceanic (urn:x-wiley:00948276:media:grl65897:grl65897-math-0002) (Figure 2b) mass loads were converted to spherical harmonic (SH) coefficients of the geoid: urn:x-wiley:00948276:media:grl65897:grl65897-math-0003 (1) where urn:x-wiley:00948276:media:grl65897:grl65897-math-0004 is the Earth radius times water density, urn:x-wiley:00948276:media:grl65897:grl65897-math-0005 are latitude and longitude, urn:x-wiley:00948276:media:grl65897:grl65897-math-0006 are normalized associated Legendre polynomials and urn:x-wiley:00948276:media:grl65897:grl65897-math-0007 are load Love numbers. Using the combined mass fields, urn:x-wiley:00948276:media:grl65897:grl65897-math-0008 PM excitations (urn:x-wiley:00948276:media:grl65897:grl65897-math-0009, urn:x-wiley:00948276:media:grl65897:grl65897-math-0010) were computed from the degree 2 (urn:x-wiley:00948276:media:grl65897:grl65897-math-0011) order 1 (urn:x-wiley:00948276:media:grl65897:grl65897-math-0012) SH coefficients via (Chao, 1985; Seo et al., 2021): urn:x-wiley:00948276:media:grl65897:grl65897-math-0013 (2) urn:x-wiley:00948276:media:grl65897:grl65897-math-0014 in which M is the Earth mass, and C and A are Earth principal moments of inertia. urn:x-wiley:00948276:media:grl65897:grl65897-math-0015 and urn:x-wiley:00948276:media:grl65897:grl65897-math-0016 from Equation 2 include effects of rotational feedback. We then estimated monthly (urn:x-wiley:00948276:media:grl65897:grl65897-math-0017, urn:x-wiley:00948276:media:grl65897:grl65897-math-0018) changes from groundwater (storage on land and associated sea level change) plus the other sources (AIS, GrIS, glaciers, dams and soil moisture). urn:x-wiley:00948276:media:grl65897:grl65897-math-0019 and urn:x-wiley:00948276:media:grl65897:grl65897-math-0020 are mostly determined by terrestrial storage changes with minor contributions from corresponding sea level rise due to relatively uniform distribution of ocean mass as shown in Figure 2b.
> the distance from the ground to the surface is around 5 millionths
The water from aquifers is not just lifted a short distance up to the surface. It is used, ends up in rivers, and spreads around the world's oceans. And the places where we are pumping groundwater are not uniformly distributed around the Earth. We are essentially moving mass to the Pacific.
Yeah I don’t know that this even makes sense at a glance. How much mass do we expect just moves around by natural processes? Should we even expect to be able to detect an effect of this magnitude (as in, distinguish it from noise)? I do not think so:
- Based on a cursory search, a large iceberg may weigh in at ~1 trillion tons. The calving process moves mass hundreds of kilometers away from Antarctica. This should be an effect of similar magnitude!
- The water cycle moves ~500 trillion tons of water each year. This water is moved hundreds of kilometers and isn’t evenly distributed over the course of a year. We should expect the impact of seasonal fluctuations in water distribution to thus have a comparatively much larger impact on the earth’s rotation.
I don’t think these numbers add up. I think we’re reading this story because it feeds into a certain kind of “we are hurting Mother Earth” mindset.
The largest iceberg ever measured, B-15, had a mass of about three billion tons. Still enormous, but not trillions, which sounded unbelievable enough to prompt me to look.
https://en.m.wikipedia.org/wiki/Iceberg_A-68
A-68 was part of Larsen C, a section of the Larsen Ice Shelf. Scientists found the crack beginning to form in November 2016. Scientists assess that A-68 "didn't just break through in one clean shot, [but] it formed a lace-network of cracks first."[5] The resulting iceberg was around 175 km long and 50 km wide, 5,800 km² in area, 200 m thick and weighed an estimated one trillion tonnes.[8][5]
I’m not sure if Wikipedia is right here however because the linked sources have changed to remove mentions of mass.
That said we should be able to do a pretty dumb calculation. 1 km3 of water weighs ~1 gt.
5800 km2 x 0.2 km = 1160 km3
1160 km3 of ice x 0.9 (density of ice compared to water) x 1 gt / 1 km3 leaves us with a mass of about 1044 gt or around 1 trillion tons.
Edit: also, I am not able to find an original source for the mass of B-15 being in the few billions range
The calving process results in ice already floating in water moving. Archimedes' law means the effect is very limited ~ the height the ice sticks above the surface and the little change in density.
More importantly, the article mentions removing annual effects on Polar Motion. They are averaging - somehow - to look at permanent changes. The annual effects may be an order of magnitude greater, it is not relevant.
The sea level rise number sounds about right tho.
~8 trillion tons to raise the level an inch based on the simple math of surface area of a 24,000 mile diameter sphere, times 2/3rds water, as seen here:
https://www.answers.com/earth-science/What_Amount_of_water_r...
Won't it take no tons to raise an inch if the water gets warmer?
yes
So we want to calculate the change in momentum caused by moving water from just under the surface to the surface.
Assume earth is a perfect sphere, let m be the mass of earth and r be the radius of earth. Let rdm be the relative mass of the water moved = 3e-8, let rdr be the relative distance of the water moved = 5e-6.
The change in momentum is rdm.m.(1+rdr)².r² - rdm.m.1².r² which is 2.rdm.rdr.m.r²= 3e-14mr². Looks about right for the change of axis you wanted.
There's other counter arguments but at least the oom analysis fits.
Technically speaking I change the tilt of the earth every time I take a piss
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Very nice einstein..now write a paper and get it reviewed by scientists
This sounds truthy to me. Can anyone confirm, and also weigh in on whether uneven distribution of the moved mass might account for any of the missing orders of magnitude?
Uneven distribution is the entire effect.
Groundwater is mostly under ground... And the center of mass of all the surface crust is hundreds of kilometres from the center of mass of all the world's oceans.
That's exactly what I was thinking.
Scientists and researchers collect two decades worth of data and publish peer reviewed papers.
Genius HNer looks as the title, spouts some factually incorrect data to "debunk" it and gets upvoted to the top.
Sadly I'm not just describing this instance but internet discource in general. Critical thinking and analysis is dead in the age of "gotcha" hot takes.
Please make your substantive points without snark or swipes. Otherwise it just makes the thread even worse.
https://news.ycombinator.com/newsguidelines.html
ironically this comment is annoying and pointless cynicism and meta commentary while the original comment is interesting, although apparently wrong - your response should be deadlast but unfortunately I see it before the other comments explaining where in the paper to find why this reasoning is wrong...
This seems to happen a lot here lately, particularly on earth and space science related topics. After you read the comments on a paper in your field, it becomes depressingly obvious that a lot of the most upvoted commentary is armchair skeptics doing back of the napkin math that sounds good but falls apart upon closer inspection. Gell-mann amnesia effect, etc.
Something else is happening in sone cases. You have research that tells you something, the press picks it up/exaggerates it to the point that it's no longer connected to the real thing + now we are all talking whatifs and have opinions without reading the initial claims.
In cases like this it's more than fair to call bullshit.
Appeal to authority.
An appeal to authority would be “Experts disagreeing with you proves you wrong.” That’s not what they’re saying.
They’re taking as read that the top level comment is wrong (as explained by a number of other comments), and making an observation about the circumstances surrounding the incorrect comment, as well as the theme it represents in the broader discourse.
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for the record, nothing in the paper makes me feel better about the issue I raised.
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Even from a “sniff test” perspective, this seems highly suspicious.
- water is heavy
- pumped water gets used and flows to the oceans, far away
- we have pumped a lot of water (look up how much some places have subsided due to groundwater depletion).
- the earth's rotation is something we've worked out how to measure very, very precisely
It sounds completely plausible to me.
Totally. Prima facie bullshit. Problem is that the "I F@#$king Love Science" crowd doesn't actually know how anything about science or math, so they'll buy pretty much anything backed up by even low-quality technobabble.
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The immediate thing that popped into my head is the Earth is a closed system. BS I concur.