We've released a great deal of CO2 into the atmosphere in the last 100-odd years, and although it may not account for all of the bizarre weather we've seen in recent years—livestock methane and small changes in the Sun's luminosity almost certainly have some effect—CO2 is the biggie. We've actually begun to talk about deliberately sequestering carbon somewhere other than the atmosphere.
Some of these plans strike me as ridiculous, like a recent Japanese proposal to pump CO2 and store it under pressure underground. Besides being a health risk far worse than a nuclear meltdown (a sudden eruption of massive amounts of CO2 turned loose by an earthquake could quickly asphyxiate tens or hundreds of thousands of people) pumping the stuff underground takes energy, and with only a few exceptions, industrial quantities of energy come from oxidizing carbon. Duhh!
Others strike me as extremely promising, like salting the southern Pacific Ocean with trace amounts (50 parts per trillion) of iron. That part of the ocean is deficient in iron compared to much of the rest of our ocean area, and certain types of diatoms with calciferous bodies breed only slowly there. With only a little more iron in the water, the diatoms reproduce furiously, die, and sink to the ocean bottom, carrying a certain amount of carbon carbonate with them. The chemical leverage is apparently enormous: A few gallons of iron salts can trigger the sequestration—using purely solar power and no altered living organisms—of hundreds of tons of carbon. This has been tested, and I think the opposition from environmentalists stems from the fact that it seems to have no downside and would allow our carbon orgy to continue. (If the ecophonies would quit opposing nuclear and hydro, we might actually make some progress without any help from the poor diatoms.)
Without thinking of it as such, we've actually been sequestering major amounts of carbon for a long time, in a number of odd places. The vast majority of materials buried in landfills contain carbon, and even backing out the plastics (which do not pull CO2 from the air) the amount of paper, grass clippings, and food scraps is immense, and apparently does not decompose very quickly when buried properly. Another place is in housing. Virtually all new detached housing these days uses wood frame construction. (Chicago-style fired clay bricks are energy-intensive and expensive, and are actually an artifact of cheap natural gas energy in the middle decades of the 20th century.) Houses stand for a long time, and when they are razed, the debris is generally buried.
We now have almost as many forested acres in the United States as we did when the Europeans arrived, especially in the eastern states, where marginal farmland has returned to hardwood forest with a vengeance. Oak trees pack a lot of carbon, and even though it's a long-horizon project, we should be planting more of them everywhere we can. (Whatever happened to Arbor Day?) Simply using less gas, coal, and oil is a good thing, but it's not enough to save our bacon at this point. We need to be pulling carbon out of the air anyway we can, in tremendous quantities.
By the way, I've been keeping my eyes open for good figures on how much carbon (in terms of gallons of gasoline, tons of coal, or cubic feet of natural gas) is present in a single average frame house or cubic yard of grass clippings or old newspapers and magazines, but haven't found anything yet. Do send me pointers to data like that if you find it.