Cost analysis of stratospheric albedo modification delivery systems (fulltext PDF) by Justin McClellan, David W Keith, and Jay AptThey quantify the aircraft- or rocket-related expenses needed to undo the global warming.
To make the story short, it was estimated in 2008 that to change the energy fluxes by a watt per square meter which is comparable to what you need to "undo 100 years of global warming", you need to increase the albedo by bringing approximately 1 million tons of \(SO_2\) to the stratosphere. It becomes \(H_2SO_4\) over there.
The authors consider various rocket- and aircraft-based paradigms to bring several millions of tons of aerosols to the stratosphere, about 10-18 kilometers above the surface. The key final result is that such maneuvers would never cost more than $8 billion per year, the ultimate upper bound.
The civilization is already wasting hundreds of billions of dollars every year for policies meant to reduce the production of \(CO_2\) – so far, thankfully and unsurprisingly, with no results whatsoever. To actually reduce \(CO_2\) emissions substantially, we would have to pay trillions of dollars per year.
So the albedo-geoengineering solution is approximately 1,000 times cheaper than attempts to lower the \(CO_2\) emissions. In other words, the proponents of carbon regulation are 1,000 times greater idiots than some people who think about the human effects somewhat more rationally.
This paper is just a proof of the notion that if the mankind needed to influence the temperatures in a measurable way, there would exist plausible ways to do it. However, if you ask me, I still think that it would be absolutely insane to start these geoengineering projects anytime soon simply because there isn't an infinitesimal glimpse of evidence that we are facing any problem arising from a hypothetical warming.
And \(H_2SO_4\) in the stratosphere could have some undesirable side-effects, e.g. when it comes to its interactions with the ozone. Of course, I do believe that even if it were the case, there would exist another acceptable compound that would do the same job safely. Chemists and others should do mostly theoretical or lab-confined research of such things. But I am personally against any attempts to change the global mean temperature and to solve the "climate problem", both totally insanely expensive attempts such as the attempts to regulate carbon as well as cheap attempts such as this one, simply because there isn't any problem.
And that's the memo.
Off-topic: One World Trade Center, formerly known as the Freedom Tower that will replace the Twin Towers, has a complete skeleton and it will be opened in 2014. There are glitches complicating the Three World Trade Center construction.