Moving a few people off the planet and providing "new pastures" will not significantly reduce the current population, nor will it enhance our current ability to constantly improve the food source.
I think there must be an upper limit because ultimate the amount of plant material that the planet can support is limited by the available sunlight. Regardless of what that limit is, if we ever fail at improving the food sources or fail at food distribution, then we unleash devastation and the amount of that devastation gets stronger each year. Our society is actually rather brittle
None of which has to do with global warming...
The bolded parts: distribution is the real problem. My contract job involves looking at the freight logistics of some of our major food distributors. If Americans knew how few mega producers and distributors there are in charge of the majority of what they eat every day, and how much they rely on only two major infrastructure systems (rail and highway) which are too neglected and insufficiently secured, they'd worry about their next meal every day. A third distribution system is of course the seas for getting food from the big producers to countries all over the globe. ALL of these systems rely on fossil fuel with the irrelevant exception of one or two nuclear powered cargo ships.
Without fossil fuels there will be mass starvation in very short order. The continued production of fossil fuels is crucial to support this planet's human population. It would take time for local communities to ramp up heir own production again. The movement away from locally produced food and toward mass distribution has resulted in economies of scale but at the cost of becoming dependent on those very distribution systems.
We are perhaps decades or even centuries away from developing an alternative energy source for freight transportation. Electrified rail, hydrogen fuel cells, etc. all rely on fossil fuels or alternatives to produce and we are a long way away from having the non-fossil alternatives in place to support such schemes. Money and time it will take and in the meantime we are vulnerable to the delicate situation we're in now. Massive amounts of food rotting in one location while people starve elsewhere is a very possible outcome if we don't stay on top of continued fossil fuel production to bridge us to that future.