Geoengineering is any proces by which humans physically intervene in the normal Earth biochemical processes. Its been smeared as dangerous, but if we hear Rex Tillerson of Exxon, some companies have been involved in it knowingly since the 80’s. Still usually it is associated with spraying sulfur particles in the upper atmosphere to deflect sunlight to cool the Earth. This idea although it would work, is highly impractical and would mean sulfur rain all over the planet, so it would utlimately be damaging.
There are other options, and we should not reject any option just -because- it could be called geoengineering. Driving a fossil fuel powered car is geoengineering. it should be judged on its effectiveness and sustainability, as if (big shock) we would like the Earth to be habitable for future generations!
Now that the reuse of rockets has been demonstrated and the cost of launching anything into space has dropped considerably the cost of manned missions to the moon, as well as the establishment of a permanent moon base has become much less. This means that in time there will be a military base there, holding the ultimate high ground over Earth. As we would then have a stabile base observing Earth with massive amounts of dust available, it becomes imaginable that we cause solar dimming by sending moondust to Earth. The energy required for that is modest. If you shoot a rifle bullet up in the sky from the moon it comes down! Basic canons could suffice to shoot material to Earth, or a railgun system driven by solar power.
Sunlight is already being dimmed, even if we don’t notice, by soot from fossil fuels. Strangely if we drop fossil fuel emissons the Earth would warm and the sun would become brighter, causing more warming! Some argue that we should therefore reduce emissions slowly because having a cooler planet is essential for food production, so a sudden drop would spell disaster.
It seems therefore that we would be wise to look for alternatives to soot as a solar dimmer. Many plants can do with 12% of full sunlight and still grow normally, so cooling the Earth by blocking sunlight would mean its plants can suck up a considerable amount of CO2. Sending the moon dust directly to Earth would mean that any effect is realtively short lived. The effect of CO2 emissions takes 20 years to fully express itself. This could be modelled and some experiments could show its effect.
Its not clear how fast the moon will be colonized, but we think it will happen in the next 10 years. It would make sense to not only speak about the military threat of a moon base but also about the opportunity to fight global warming from Earths celestial compagnion.