"Scheduled shifts in Earth's orbit should plunge the planet into an enduring Ice Age thousands of years from now but the event will probably be averted because of man-made greenhouse gases, scientists said Wednesday.
...
These shifts, called "bifurcations," appear to happen in abrupt series, which is counter-intuitive to the idea that the planet cools or warms gradually.

"You had a big change about a million years ago, then a second change around 650,000 years ago, when you had bigger glaciations, then 450,000 years ago, when you started to get more repeated glaciations," Thomas told AFP.

"What's also interesting is that the inter-glaciations also became warmer."
According to the model, published in the British journal Nature by Crowley and physicist William Hyde of Toronto University, Canada, the next "bifurcation" would normally be due between 10,000 and 100,000 years from now.
...
However, there is now so much CO2 in the air, as a result of fossil-fuel burning and deforestation, that this adds a heat-trapping greenhouse effect that will offset the cooling impacts of orbital shift, said Crowley. "
http://www.physorg.com/news145725882.html
Linked from http://pajamasmedia.com/instapundit/

It will like pissing on a forest fire.

Higher CO2 level will do little to nothing to stop this big chill- which apparently is going to happen 10,000 years from now. But I hope they are right that it is going to be 10,000 years in the future instead of sooner.

Hopeful in 10,000 years we will be in space. And the cooling wouldn't matter much- other than more opportunity to ski in more places.

"Crop spraying" large areas with soot in the temperate regions covered by snow, would probably have a much larger affect than any CO2 levels- unless you want CO2 levels higher than say 2000 ppm- and at these levels or higher [globally] it becomes a serious health concern for humans and even animals. You could be in areas such home or office which the local CO2 was at around 2000 ppm or higher- but if the "freshest air" is 2000 ppm, one will have more of these areas with significantly higher levels:

"Occupants may experience health effects in buildings where CO2 is elevated, but the symptoms are usually due to the other contaminants in the air that also build up as a result of insufficient ventilation. At high levels, the carbon dioxide itself can cause headache, dizziness, nausea and other symptoms. This could occur when exposed to levels above 5,000 ppm for many hours."
http://www.health.state.mn.us/divs/eh/indoorair/co2/index.html

So, if you have global levels at 2000 ppm or higher, cities could be 2 or 3 times higher than that, and offices, homes, or simply crowds of people in the outdoors even higher. Though suppose with a modest amounts of engineering one could have cities so they are instead bubbles of lower CO2.