I can't help relate this question to a case study I once did on geo-engineering. In the case I'm thinking about, very bright people determined that we could attempt to cool the earth by sprinkling the bottom of the oceans with metallic filings, which would cause light reflection and less heat being stored by the oceans.
This is often the problem we humans face; theory often makes a good deal of sense, but its practical application might not be viable at all. In the case I described, the metal ended up destroying many deep-sea ecosystems, including species that we might not have discovered yet. In effect, our attempt to help the environment ended up doing far more damage than good.
As billups.13 says, the shades could do irreversible damage to native ecosystems, affecting all kinds of flora and fauna. This kind of risk is enormous, especially considering the fact that there is little (if any) scientific evidence to conclusively prove that global warming exists. If we found that it DID exist, there would still be the matter of proving that injury due to inaction, would outweigh the injury caused by action.