How much space do you need?

For some years ago, I was asked the above question by one of my students.
Like most scientists, I started to answer: -- "That depends ... "
Then, I realized that it actually was a rather appropriate answer.
If you are talking about sitting space, it is not the same as what you need for producing your food demand for a year, or for making a good climate for your and your fellow creatures.
It is actually a series of spaces, each depending on the function of the next.
The attached picture attempts to give a notion of that.

The figure is clickable, if you have problems with reading the texts.

The reasoning is quite obvious. You need more space to produce food than you need to sit at the dinner table. And you need more processes and space to maintain a healthy atmosphere than you need to breath (although that area is a quite large, considering the 90 sq. m. inner area of your lungs).

The picture is just a rough outline, just attempting to draw attention to the different spaces needed for our life support, and our dependence of them. But it can also be seen as an attempt to bring figures to the global carrying capacity, with or without fossil fuels. Since the surface area of the planet Earth is estimated to around 150,000,000 km², the figure on the sketch would indicate that the carrying capacity for 'climate production' is exceeded at 7.5 billion people, given our current lifestyle.
It is certainly not sure that the use of fossil fuels will eternally increase the carrying capacity of the globe.
Perhaps the opposite.