Sounds like a dystopian nightmare, a final complete divorce from lived reality and active participation in real ecosystems. All food growing done by techno-specialists working in lab facilities in the deserts...
The potential unintended consequences off a drastic move like this are off the scale! Besides which it won't solve population growth, it will exacerbate it. Basic ecology: the population of any species grows in direct relation to the amount of food available to it. Because of the capitalist growth imperative frankenfoods like this, if they ever get off the ground (gods forbid), will just raise the carrying capacity to a new high and the population will spike upwards to meet it in short order, just like it did after the 20thC green revolution. Then we'll be in exactly the same situation, except with even more terrestrial space completely dominated by human biomass and sterile living[sic]-spaces. At least pasture land provides space for some wildlife to live alongside the domesticates - a blessing of that 'inefficiency' he complains about. Surely he can't be dumb enough to believe this take-it-all culture will 'set aside' land for rewilding for any meaningful length of time?? If not agriculture they will expand urban areas or find some minerals to mine or cover it in solar farms or some shit. A child could figure this out, yet Monbiot apparently hasn't during his decades of environmental campaigning.
Jevons Paradox: any increase in efficiency in the consumption of a resource will result in a still greater consumption of the same. - https://en.wikipedia.org/wiki/Jevons_paradox
Several commenters call this out as an exaggeration, saying there were a few scattered boos, but mostly polite listening and some applause. Video footage coming out soon apparently...