Iron, phosphates, Air pumps and light transmitted into the depths where the growthcube rises.
Have a rainforest fall into the depths forever every hour.
Seems scary because we don’t actually know how to do it, and we only have one planet. We could create horrible side effects like killing ocean life we depend on for food, or admire (e.g. whales). We also have evidence for a “snowball Earth” state at times in the past. What if we overcorrect? Lots of good sci-fi stories about that to chill our bones.
The argument against it happening is that it would be expensive for that one nation, but benefits would not be proprietary. Whereas building out an economy of low-carbon power generation, manufacturing, and transportation creates tons of domestic economic benefits like jobs, trade, profits.
Also you probably would want to preserve the Mariana Trench ecosystem, some unique species live there.