Experiments on geo-scale? No! No! NO!

For solution to world's problems, I generally take two fair fairly conservative views:
no long term unknown impact and no large scale changes.

Throwing iron into the ocean in the hope of causing phytoplankton growth in order to sequester carbon is one of them. Treehugger reported on a failed experiment. The phytoplankton biomass doubled after the iron source being added. However, the kind of phytoplankton was not what the scientists have predicted.

A failed experiment on a small scale is OK. That's part of the spirit of science. Wholesale adoption of an untested idea is a No No.

The other two ideas I have strong reservations are carbon sequestration in 1. deep sea and 2. deep old mine. Both are untested and long term effects unknown.