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.