I consider this "Flash Fiction" as the story itself is in just one paragraph, and the last two sentences. The extra words are just a list of consequences. So not quite a short story, but a bit longer than a Flash Fiction piece.
Nobody saw the Alien Ship dip down near the ocean just off of New York City. Oh,sure, a few sophisticated (and secret) advanced radar screens caught a brief glimpse of an object piercing the atmosphere only to disappear when it hit the ocean. But they wrote it off as a glitch in the system. After all ablation would remove all the outer layers that glowed on the way down- so it should have been invisible when it hit the water. It wasn’t. It just stopped at the surface - and then disappeared.
Dully noted in the logs, dismissed as an anomaly or glitch- it didn’t fit the profile of any known physical object. Because it was not.
The Tiny Aliens, no bigger than an amoeba,rested in their ship. The descent went as planned. Stephen Hawking couldn’t have kept up with even an Alien Kindergarten child. So much for the “big brain, bigger intelligence” theory common on Earth. You could put all trillion of the Aliens in a coffee cup, never even aware that they were a) There. b)Smarter than you. c)Each an Individual.
“Are you ready, Smrst?”
“Release the DROP.”
Smrst turned the handle a quarter turn. A tiny drop, barely bigger than a Water Molecule- but much, much, much denser, more complicated than the human brain, and more powerful than any Supercomputer on Earth- fell into the Ocean. The reaction was immediate:
1) Sea Temperatures plummeted and stabilized.
2) Acid rain was no longer possible.
3) The Poles were shifted two degrees backwards. Granting the Earth a stable tilt for at least 100 million years. Ice ages would come and go, but none would be “life threatening”.
4) All heavy metals were leached from all the animals in the sea…and eventually that process went onto land.
5) Humans lived out their life free of disease- but birth rates plummeted. Within fifty years there were only 600 million Humans left on Earth. That number would change only slightly (up or down) for the 100 million years granted by the DROP.
“We can go now, Smrst. Set a course for Home.”
“Yes, Velligu. I will be glad to leave this M-Brane. I can’t believe intelligent life messed up their own planet this badly that we had to intervene.”
Velligu smiled at Smrst, letting her Flagella brush up against Smrst’s outer shell:
“Now, now, Smrst, don’t be so harsh. After all they were once pond scum …just like us.”