From my sources, the greenhouse gas concentration is about 387 ppm. There is debate over where the "do not cross" threshold is, 400, and 450 seem to be popular. I have tried to quickly find the ppm we started with in the 1800's but can't, but when we started all this 200 yrs ago, there were 1 billion of us. By 1945 that was 2.3 b, and we have nearly tripled that up to now. All us lovely souls use fuel, fossil and caloric and tend to displace forests and other carbon sinks. The snowball effect is like this: ice caps and glaciers start to melt which causes more heating as the dark land and water beneath. That speeds warming and allows permafrost in the north to thaw, releasing methane trapped in it (someone already noted how bad methane is), making things worse. The higher the GHG concentration, the more acidic the oceans become, reducing it's ability to sequester carbon. The ball is rolling at this point far beyond what we can do. The thing I see is that by the time we see effects, the snowball is rolling down the slop with such momentum we could barely stop it.

Socially, more mouths mean a greater demand for food, and we can do without a lot of things, but not food. Those glaciers that have melted kept many of the major rivers flowing, and when they are gone, the rivers stop flowing, or are greatly reduced. That water is not only needed to drink, but for irrigation. Several major rivers start in India and run through Pakistan and are vital for food in those well populated countries who are sworn enemies and nuclear powers. If India holds back water to feed it's people, what will Pakistan do when it's crops whither and it's people starve?

It seems to me, everyone needs to first walk down to the Shoppers drug mart and buy a box of condoms, and one for people in the third world that can't afford them. Then they need to tell gov't that we need solar/wind/nuke power in much greater amounts, and buy some systems for those developing countries while we are making out our order, because their CO2 is as bad as ours and they can't afford the new tech now. A few other things like limiting urban sprawl and (gawd, I hate to say it..) heavily tax fuel and some other problem items like fertilizer while supporting green technology and grain prices. As a farmer I key on these because there are CO2 efficiencies ther that are uneconomical with either low input prices or low grain prices.

So there is the quick "how to save the world according to me" spiel. Honestly, I think world pop is the #1 enemy, but how many of us are willing to move or take the voluntary dirt nap to sequester our carbon? I also think that if this shift takes place, certain winning technologies and companies will make it's investors VERY rich.

Cheers?