Consider adding references about the time scale for sustainability

[MatthewHeun]

About Chapter 1, Zeke says:

Some useful references for the time-scale over which sustainability must be assessed: · Soil carbon debt of 12,000 years of human land use https://doi.org/10.1073/pnas.1706103114 · Mammal diversity will take millions of years to recover from the current biodiversity crisis https://doi.org/10.1073/pnas.1804906115

[MatthewHeun]

Covered in #230

[jgvanantwerp]

done.

[jgvanantwerp]

Although humans seem to have inherent cognitive difficulties in planning for time scales significantly longer than the human lifespan, \emph{strong} sustainability is achieved only if the answer is indefinitely'' orforever.'' Our current sustainability crisis has been thousands of years in the making~\cite{Sanderman2018} and will likely require millions of years to recover~\cite{Davis11262}. A \emph{weak} sustainability criterion is sustainability over a long (by human standards) time frame, perhaps 50 years, which is on the order of one human adulthood.