Expensive geoengineering projects are proposed to keep polar ice sheets cool and save coastal lands from inundation.

If we're going to geoengineering specifically to counter heating, we might as well use the atmospheric methods.

But even if we cancel out global heating, that won't protect the oceans from mass extinction. CO2 doesn't just heat up the ecosphere; it also dissolves in the water making it more acidic. In a few decades, all coral will simply dissolve in the acid, and the thousands of species that depend on coral reefs will have nowhere to go.

Some fish have been observed to engage in dangerous behaviors, likely to get them eaten, in elevated levels of CO2. And it appears that rising CO2 levels are causing grain yields to fall, globally.

So even with a way to get rid of the heat, we still urgently need to stop burning so much fossil fuel. And as a way to reduce future heating, it's probably also cheaper.