Right, we just need to find the right balance between nuclear winter and global warming!

Bioengineering can have its own negative externalities, and in practice incentives may misalign in such a way that we solve an immediate problem but also create a new one that will manifest later and hence someone else will have to deal with it. I d much rather we address the current problem directly by bringing the climate back to where it would be without human pollution.

Idk. The body seems to adapt to radiation and some excess guardian genes may prevent malignant growth. Of course you cannot live through the gamma blast area, but surely the fallout zones may be manageable.

On the other hand chemistry is highly temperature sensitive and at some point proteins will just denature. Then again you cannot have rapid growth without water, as plants need evaporation to transport minerals and such from the soil into the plant. And what about phosphorus anyway? Soon most soils may be nothing but dirt. You cannot work around the phosphorus erosion. Once it's lost, it's lost. We better start recycling our feces and the dead now. You kinda need to harden all life, not just humans and crops.

just a question of time

Warming is by definition a change