Economists are not normally trained in thermodynamics — the science of putting energy into some economic activity and taking usuable energy out. So when the future production of consumer goods for 7.4 billion people — up from the present 1.4 billion in the advanced countries — are debated then economists do not realise that the energy that would have to be put in every year would have to be raised five-fold from present inputs.
If the further 4 billion or so of young middle aged poor people are going to survive to old age then the world population would rise to 11.4 billion by about 2050. In that case then the energy required would need to be raised seven-fold from present levels
This is the bird’s eye view of the world economy.