The theory of and the necessary statistics for an eSNI have been worked on since the mid sixties.
(2006) rated the costs of mitigation of climate change to be only one per cent of gross national income as an average over 50 years, much lower than the eSNI figures indicate.
A recent overview of the development of eSNI is given by Colignatus (2008).
From this it follows that eSNI has to grow more than twice as fast as NI in order to reduce the distance between NI and eSNI.
eSNI's are ex post 'greened economy' estimates that show, in combination with NI's, whether or not the gap with environmental sustainability becomes smaller.
Calculating eSNI involves the calculation of the costs of these measures.
The eSNI and the GS approaches can supplement one another, but only under additional conditions.