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.