It can be combined with overpopulation concerns and stated as "the number of Earths it would take to support every human living exactly the way you do." Ecological footprints have been used to argue that current lifestyles are not sustainable. The concept of ecological footpriniting has been challenged on several grounds. Firstly, many factors of the calculations are based on crude estimates and it is questioned whether the numbers are applicable to other places. Secondly, the model generally does not count multiple uses of land: a forest is there as a carbon sink and the same area is not counted for food production. Finally, some of the processes involved are currently poorly understood.
To counter these uncertainties, the models of ecological footprinting are constantly being refined. Moreover, the use of ecological footprint analysis is considered to be a guide, rather than an exact measure, of sustainability.
Ethically, the number of Earths "consumed" alludes to the categorical imperative, which requires everyone to behave in such a way that they can consistently advise all others to behave.