I believe Professor Crutzen is on the right track. But, rather than placing chemically active, indiscriminately sized particles in the upper atmosphere, one could place specifically sized reflective particles of chemically inert material in the atmosphere.
Specifically, a particle of say 2 or 3 microns could reflect incoming solar energy which is dominately in the sub 3 micron wavelength range. The earth is cooled by thermal radiation to outer space, dominately in the 3 to 300 micron range. Carbon dioxide interferes with that radiation, trapping the thermal energy near the earth, and hence “global warming”. Particles selected with an appropriate diameter could reflect sunlight, but not interfere with earth thermal radiative cooling. If we reflect 2 or 3% of the incoming radiation, there would be a significant coolling effect. Material placed in the upper atmosphere would eventually settle out, so we could tailor the particles (density and thickness) to last say 5 years or so. Side benefits would be beautiful sunsets every night, slightly extended daylight hours, reduced expansion of deserts, and reduced sea level rise.
My estimate of the cost is a few billions of dollars for each one percent reduction of solar flux, a trivial cost compared to the relatively ineffectual approaches of reduced industrial carbon use, wind, solar, nuclear, etc. A few hundred pounds of material taken aloft and released from each commercial airplane flight could deliver the needed material to the upper atomosphere in a year or so. Alternately the material could be taken aloft with inexpensive hydrogen balloons.
With massive effort and expense, we could reduce atmospheric CO2, but we would be missing out on some important benefits. Probably the real benefit is avoidance of a new ice age. Mankind has placed significant carbon dioxide into the atmosphere, and this will probably inhibit the next iceage, due to arrive whenever the sun feels like reducing output. If an iceage were to approach after we reduce CO2 in the atmosphere, there is very little we could do about it (other than increasing atmospheric CO2, something that has taken us a hundred years to accomplish so far). If we take the alternate approach of placing intermediately lived particles in the atmosphere, we could in a few years stop placing the particle to increase the effective solar energy at the earth surface. All around, the particles seem to be a superior approach