Bloomberg has a fascinating slide show on nine so-called "planetary boundaries." These are biophysical thresholds, such as the amounts of greenhouse or ozone-depleting gases in the atmosphere or the acidity of the oceans, beyond which scientists believe we risk catastrophic environmental change.
A 2009 article by Rockström and colleagues from Nature explains the concept more and argues that the planet has already passed the safe thresholds for greenhouse gases, species extinction rates, and nitrogen loading.
The thresholds are speculative. Scientists can't say for sure where (or in some cases even whether) there are points at which environmental systems tip from one set of dynamic relationships to another. The authors of the Nature article admit to being overly cautious in setting boundaries a safe distance within the catastrophic thresholds (sort of the way that the empty warning light on your dashboard indicates that you at risk of running out of gas but still have some time to get some).
The concept of boundaries fits well with economists' standard methodological approaches of constrained optimization and suggests a formal, practical approach for accommodating sustainability into growth modeling.