Geoengineering: A Horrible Idea We Might Have to Do

By the end of the 21st century, humanity is becoming desperate. Decades of heatwaves and droughts have led to unusually poor harvests, while the warming oceans yield fewer fish each year. In the tropical zones, millions suffer from famine and resource wars have made millions more flee to the north. As things quickly get worse, in an act of desperation, the world's governments decide to enact an emergency plan...
It is far from certain that a grim scenario like this will play out. But the failure of world leaders to effectively address climate change, makes it far from impossible.
So in the near future it might become necessary to try something radical to slow down rapid climate change: Geoengineering. Interventions so massive in scale that they might undo centuries of human behavior. Or make everything much worse.
What is geoengineering, is it really an option and what if it goes wrong?