Geoengineering could lead to war between countries that have different needs for how to engineer the Earth's climate.

Any geoengineering method that aims to affect temperature directly will not do anything to stop ocean acidification due to dissolved CO2. This threatens to destroy all coral reefs, along with all the other species that depend on coral reefs. The result could easily be the loss of most of the sea food that a billion humans depend on.

In experiments, CO2 affected the behavior of some species of fish too -- they ceased to hide from predators. That could wipe them out.