What is Natural Capital? "European movements for degrowth, decroissance, are rooted in the idea
  that the perpetual growth assumption built into modern economics fails
  to take into account the depletion of natural capital. Hence the
  importance of sustainability."
  - The end of progress


What is natural capital?
Natural Capital is an analytic accounting approach: it is a system of putting a monetary value on environmental assets such as ancient woodland. It is also the name of those assets, within that approach. So it's one way of internalising the externalities of environmental change, within macroeconomic indicators.

For more, see in particular the work of Paul Ekins, including "A Framework for the practical application of the concepts of critical natural capital and strong sustainability" (pdf)

The philosophy of decroissance - of deliberately engineering a decline in traditional economic growth indicators - is one attempt to find an optimal and sustainable economic path, taking into consideration both the traditional economic indicators such as year-on-year changes in GDP, and changes to the population's wellbeing, together with changes to the national asset base, which includes changes in Natural Capital.