Soil. It's soil.

Soil, in prodigious quantities. We need to be making it.

Using human waste - food and faecal from our cities; Using whatever plant fixes the most CO2 in a useful form that we can turn into charcoal (quick trees? bamboo grass? something genetically modified?), releasing heat energy, and then fixing carbon in a useful matrix, for soil regeneration a la Terra Preta.

I sort of made this post elsewhere here, I just wanted to repeat and focus on the soil angle.

We have huge swathes of the globe that due to poor historical environmental conditions have poor soil, so not much can grow. Some have become desertified, perhaps some of which is our fault (eg. Mediterranean deforestation). If we add soil to these places, at least hardier plants can grow and we can expand the reach of our efforts.

Strikes me that the most important thing is that plants can grow, and for that we need soil.

Soil.