The Thing From the Future:
As global warming set in and became a more and more real concept scientists started to think about how they could balance out all the CO2 being created. They finally hit on the idea of engineering a plant that could grow quickly and absorb lots of C02 form the air.
They worked and worked and worked at the task. Eventually they managed to create a plant that grew so well and quickly that after a year or so it become an invasive species. It did exactly what they wanted, it ate up so much C02 that it made a notable impact.
They celebrated, they were successful. Then the swarms started. In their hurry to get the plant out there they had not tested what happened if bugs ate the leaves off the plant. At first they didn’t notice anything but about 18 months after they put the plant out the swarms of bugs started drifting towards cities and towns.
There were hundreds of bugs in the swarms and at first people started in wonder. Until the bugs came in and started feeding. On the humans. It turned into a bloodbath very quickly because of the number of bugs.
The scientists had not accounted for how the fast growing nature of the plant might affect the creatures that ate it. What it had done was drastically speed up the reproduction of flying and crawling bugs all over the place. These bugs, with their increased aggression and rapidly growing bodies hungered for something substantial enough to fill them.
That turned out to be human flesh.
Fast forward a few years later and humans had in fact successfully equaled out the impact they had on climate change, dropping the world temperature back down.
And not a single person lived to see it.