If we could convert Sahara back into a savannah or farms, the amount of carbon required for the new soil and plants would exceed the amount of extra carbon we have added to the atmosphere so far. And there are many viable routes to do that. E.g. building large number of ocean thermal energy conversion plants [1] to reduce temperature of the north hemisphere, recreating conditions that have caused green Sahara in the past. There are numerous possibilities to use engineering to revert adverse effects of climate change, and to actually improve the climate. The only scenario when we are doomed, is if population declines significantly so that we get runaway effect from carbon we have already produced melting permafrost, and not have large enough economy to use geoengineering on a large enough scale.

[1] https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversio...