Over the last three decades the biggest concerns have been in relation to the effects of climate change, a factor for which there are no ‘natural’ market prices. In 1992, following the Earth Summit in Rio de Janeiro, the United Nations Framework Convention on Climate Change was born, giving rise to the Kyoto Protocol (in 1997) and the Paris Agreement (in 2015).
In the autumn of that year, Robert Solow, winner of the Nobel Prize for Economics in 1997 for his contributions to modern theories of economic growth, spoke before an audience of environmentalists, stating that:  I do not have to remind you that ‘sustainability’ has become a hot topic in the last few years […] As far as I can tell, however, discussion of sustainability has been mainly an occasion for the expression of emotions and attitudes.