In theory, might it be possible to engineer an airborne microbe or virus which could be released into the atmosphere where greenhouse gases are highly concentrated? These microbes would be designed to feed off the gas, but expel harmless gas as a by-product.
Dave: I don't know about an airborne version, but we do already have microbes which do this - algae and plants. In the presence of sunlight, they feed on CO2 and give out oxygen. At the moment, they're not consuming the CO2 as quickly as we add it to the environment.