That doesn't match data I've seen. For example, even the Wikipedia article on carrying capacity (https://en.wikipedia.org/wiki/Carrying_capacity), second paragraph, states there are a number of lines of scientific evidence that we are already over capacity of Earth today. One project (https://en.wikipedia.org/wiki/Earth_Overshoot_Day) does pretty intense data analysis to quantify how far we are over that capacity. The summary is, yes, we are definitely in overshoot, and all economic development to raise standard of living is making it worse.
But it's even worse than that. We aren't just exceeding the carrying capacity, it is diminishing as we reduce biodiversity and biomass through land use (read: obliterating wildlife and forests), overfishing, and over-industrialization of agriculture, which kills healthy soils.
> Between existing but underutilized techniques like fission, renewables, smarter land use and management for agriculture
It's not just about producing more energy. We are consuming everything too much, from biomass to raw materials, even fresh water.
> Fusion power would immediately solve a lot of problems.
It might reduce CO2 consumption but ironically it will increase per capita energy consumption, which encourages even more environmental destruction by making everything cheaper to build and buy. So more land use and more consumption.
That seems unlikely, given that total fertility in wealthy countries is well below "replacement" (2.1 children per woman). Reproduction rate, number of daughters per woman, could only be approaching replacement if the sex ratio is massively skewed in favour of girls.
1. UN chart of TFR for "more developed regions": https://population.un.org/wpp/Graphs/Probabilistic/FERT/TOT/...
Observe the assumed but never yet seen floor to fertility rate.
I agree that's an answer, but is it the answer?
It seems like reproduction rates in wealthy countries trend closer towards replacement levels, so perhaps you "just" need to get everybody's standard of living up to western levels to take population growth out of the equation.
Then you're left with supporting roughly our current population level which seems... entirely feasible?
Between existing but underutilized techniques like fission, renewables, smarter land use and management for agriculture (read: focus on producing more efficient crops and less on the intensive stuff like cows), retrofitting buildings with better insulation, and building out more efficient transit, it seems like we could get pretty close with current levels of technology.
Nevermind the developments we continue to make. Fusion power would immediately solve a lot of problems.