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A few years from now, say 2025 or thereabouts, a technologically advanced alien civilization discovers our solar system.
They study us for over a year, using small drones that launch from a vessel in orbit around the moon.
We never get to see the aliens. We just see the drones. They don't interfere with us at all. They don't try to communicate with us.

Then the drones withdraw to the mother-ship and it breaks orbit leaving the solar system. On its way out the aliens send 1 radio-message over a wide range of frequencies. It is the same message, repeated in about 20 of the most important languages of Earth:

"Thank you for allowing us to study your civilization. Your species has potential to become a member of the interstellar community, but isn't quite there yet. So we leave you to evolve further. When you have developed interstellar travel please come visit us. Our nearest base is in the system you know as Wolf 358. We leave you a gift. Make good use of it. Farewell."

Of course everybody on Earth is puzzled about the "gift". It becomes clear 3 days later.
Gradually over a 5 day period the gravity worldwide lowers to 80% of our original gravity and it stays at that lower level afterwards.

However... When you go up into the upper layer of the Stratosphere between 50 and 60 km above sea-level gravity gradually goes back to normal. Above 60 km normal gravity is in place.
Satellites in orbit don't notice any chance. The Moon's orbit is not affected. For all intents and purposes the Earth mass hasn't changed at all.

Obviously the change in gravity will cause all sorts of havoc to equipment that, one way or another, relies on the old value of gravity for it's operation. (I may post a separate question about that, now I think of it.)

What I really want to know is how this gravity change will interact with the weather/climate on global scale.
Are we in for extreme global warming ? Just the opposite: Snowball Earth ?
Or something entirely else ?

I can image that such a drastic change in gravity might cause stability issues with the Earths crust resulting in heavy tectonic activity, which in turn may have a big effect on climate too. Feel free to include that part in you answer.

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    $\begingroup$ Why does traversing the stratosphere restore gravity at sea level? $\endgroup$ – Frostfyre Nov 30 '15 at 13:44
  • $\begingroup$ @Frostfyre I may have phrased that not overly clear. I meant: above 60 km gravity is back to the normal value it is supposed to have at that altitude without the alien interference. $\endgroup$ – Tonny Nov 30 '15 at 13:50
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    $\begingroup$ Somebody call Randall. This is one for the xkcd what-ifs. $\endgroup$ – Sean Boddy Nov 30 '15 at 23:08
  • $\begingroup$ See this one for some of that. $\endgroup$ – JDługosz Dec 8 '15 at 19:19
  • $\begingroup$ This seems like you are describing a gravity suppression field of some kind, i.e., the gravity from the earth is partially suppressed from the ground up to 50 km or so. How far does the suppressed gravity descend into the Earth $\endgroup$ – Gary Walker Dec 15 '15 at 9:07
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Your aliens changed the gradient of gravity. This is/was impossible per Newton's "shell theorem" that says that a spherical body affects external objects as if all of its mass is concentrated at its center. This is pure geometry, nothing to do with the gravitational constant or mass of Earth. If the aliens are so advanced that they can change the fundamentals, why would they need drones and spaceships? They could be at any point in this or any other world at any moment.

What a reasonably advanced alien civilization could try and do is redistribute mass inside the Earth, so that the gravity gets lower on the continents and higher in the oceans. In this case, the gravity converges back to to normal at higher altitude, so that there is no effect on satellites and celestial bodies. Tiny local gravity anomalies of this kind exist on Earth. Aliens obviously can make them stronger and systematic. And I think I understand the aliens' logic: they want to provide palliative care to earthlings suffering from the pandemic of obesity. :)

I can only think of beneficial effects on climate:

  1. Heavier oceans will get calmer and keep in check the sea level rise due to global warming setting.
  2. Air over the continents would get rarer => cooler. Also convection would improve. CO2 - a heavier gas - would be sucked out into the oceans and absorbed there faster.
  3. Lighter machines would use less fuel and thus emissions would go down. Actually, most of them won't be moving at all because of all the smart chips fooled by the gravity change.
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  • $\begingroup$ I don't quite understand your logic here. The aliens changed the magnitude of the gradient of the potential, which is perfectly fine. $\endgroup$ – HDE 226868 Dec 8 '15 at 21:33
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Completely ignoring questions of feasibility, the answer to what would happen at the surface we already have a formula for, the https://en.wikipedia.org/wiki/Barometric_formula :

simplified barometric formula gif

g is changed here, to 0.8 earth's normal g, so as such, the surface pressure changes. Pressure would go down, and as such, all the effects of that would happen. The gravity suddenly changing at the stratosphere would cause a bit of a problem, though. You'd have a point where above it, gravity is higher, and below it, gravity is lighter. Air would naturally move towards this point. You'd likely have a cloud layer form here, since water would be more likely trapped here. This would also play hell with weather balloons, trying to measure soundings, and spacecraft launches. But regardless.

From a pure pressure standpoint, surface pressure would go down, as would all pressure above that up until the break point. This would make the air harder to breathe for a lot of people, and would generally cool things down a bit. The "air trap" in the stratosphere would possibly increase temperatures globally, however, temperature goes up and down as you go up in the atmosphere:

Image Sounding

tl;dr: this would cause a pressure nightmare for atmospheric scientists in the stratosphere.

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However... When you go up into the upper layer of the Stratosphere between 50 and 60 km above sea-level gravity gradually goes back to normal. Above 60 km normal gravity is in place.

THAT is impossible. Quite apart from the fact that lowering the constant of gravity being impossible as far as we know, having it have a discontinuity would be even more impossible.

But forget about that. If gravity goes down, the atmosphere becomes less dense. Which would have a big effect on winds, oxygen levels (thus plant growth, animal life, etc. etc.). Temperatures would go down, further hurting plant and animal life, which likely has even greater negative effects as it would mean more desert style areas.

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    $\begingroup$ Assume the aliens have tech (gravity reduction field) that can do that. Who says they lowered the gravitational constant anyway? For all I know they installed a field generator producing a anti-gravity field at the core of the Earth. However it has limited range, hence the drop-off above 50 km. $\endgroup$ – Tonny Nov 30 '15 at 13:54
  • $\begingroup$ Wouldn't lower density increase evaporation of seawater? More moisture in the atmosphere could maybe balance the loss of heat-capacity due to lower density ? I wonder if someone could make an educated guess about the numbers. It's way beyond me. $\endgroup$ – Tonny Nov 30 '15 at 13:56
  • $\begingroup$ @Tonny the lower gravity would cause the vapour pressure to be lower as well. Would be an interesting thought experiment indeed, but I think you'd end up with something like Mars. $\endgroup$ – jwenting Nov 30 '15 at 14:04
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    $\begingroup$ What kind of negative effects? Why do temperatures go down? Expounding on this answer would be great - the OP already handwaved the transition to normal gravity, so take that as given, and describe the effects. $\endgroup$ – Sean Boddy Nov 30 '15 at 23:39
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    $\begingroup$ @Tonny Just to keep things in perspective, in your first comment above, you are positing a piece of technology that has 100% efficiency at distance 6371 km (gravity is dropped to 80% of normal at the mean radius of the Earth by an antigravity generator at the Earth's core) but this drops to 0% at distance 6431 km (normal gravity at 60 km above the mean radius of the Earth). Here we do often work on the basis of "what the OP says is the constraints we have to work within" but such a sharp fall-off (of anything) after presumably a fairly stable start, would be quite the feat to pull off! $\endgroup$ – a CVn Dec 8 '15 at 21:40

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