The year is 2100. While climate change has wrought serious damage to the biosphere, humanity has at last managed to become carbon neutral, and has even developed technology that can be used to reduce the level of carbon in the atmosphere. This has the effect of allowing humanity to set the thermostat, as it were, for the average temperature of the Earth.
Supposing that most of the world will be fed with farming practices similar in nature to what exist today, and that cities and habitation patterns will be built in a similar manner (i.e. not megastructures), what's the optimum temperature for Earth? If humans can control our average temperature, are looking to make the globe as habitable as possible, and aren't overly worried about further damage to the biosphere (because it's already been thoroughly wrecked), how warm would we want the Earth to be?
Climate control in 2100 can only affect the average temperature of the Earth. The gradient of temperatures from the equator to the poles will otherwise settle naturally, and any extreme weather that we'd expect to "naturally" form at a given temperature will be unimpeded. Furthermore, the governments of the future are open to helping populations move about the globe: the goal is to make the Earth, as a whole, as habitable as possible, without regards to maintaining or increasing food production in current population centers.