James Hansen and his associates are currently working with a manuscript called "Target Atmospheric CO2: Where Should Humanity Aim?", available in the link above.
In it, they conclude that, to be out of the immediate danger of tipping our life support system into a state where it no longer can be considered a life support system to us (the humanity), the carbon dioxide level in the atmosphere should be below 350 ppm.
The carbon dioxide level in the atmosphere is 385 ppm today.
This means that the atmosphere contains about 74 Gt too much carbon. Every year, about 7 Gt more is released. Say, as a thought experiment, that all emissions, 100%, are stopped within 20 years (don't ask me how), leading to a total breakdown of our current society. Even that would mean additional emissions of about 70 Gt more, leading to a CO2 concentration of 418 ppm (if the oceans don't suck up anything more, perhaps 400 ppm if they are friendly enough to do that).
CO2 levels like that would probably lead us far beyond several tipping points.
Even if not, the resulting climate would not feed the population, not to talk about the potential for floodings.
So, if disaster is threating whatever we do, should we give up?
No, since there is one more possibility: If the efforts to decrease emissions are combined with a massive sequestration of carbon, then it is possible to back away from the ominous carbon dioxide levels.
As the incorporation of plant charcoal in the soil obviously has a lot of benign effects (Google: Terra Preta ), increasing crops, reducing nutrient loses and so on, why shouldn't we start immediately?