Just when you thought that the mitigation challenge was dismal, Matthews and Caldeira publish a paper in GRL suggesting that things are in fact worse than that:

In the absence of human intervention to actively remove CO2 from the atmosphere [e.g., Keith et al., 2006], each unit of CO2 emissions must be viewed as leading to quantifiable and essentially permanent climate change on centennial timescales. We emphasize that a stable global climate is not synonymous with stable radiative forcing, but rather requires decreasing greenhouse gas levels in the atmosphere. We have shown here that stable global temperatures within the next several centuries can be achieved if CO2 emissions are reduced to nearly zero. This means that avoiding future human-induced climate warming may require policies that seek not only to decrease CO2 emissions, but to eliminate them entirely.

Have we mentioned that air capture is coming? And that is whether we like it or not.