Mine was to work tax policy to incentivize companies to make all their packaging shiny and white, incentivize people to litter, and disincentivize everybody from recycling.
My friend's was to use a giant rocket to push the earth farther away from the sun
I think it's fair to say that the impact on the ocean of such huge numbers of mollusks would be disastrous.
As for simulating a volcanic eruption, it would have an unpredictable effect on plant and animal life, possibly making global warming much worse once the clouds dissipated.
The solar sail idea is interesting but there is a large potential for abuse.