Back in the late 1990s, indices to chart this kind of investment started to appear and have now proliferated.

In terms of the 'stock universe' to be tracked, some try to measure 'pure-play', investment wholly in environmental and/or renewable energy shares, while others follow companies with a stated portion of their business in the sector.

Everything depends on the criteria, inclduing market capitalisation, on which an index is based. We present two views in the downloadable article on the right of this page.

The first in an interview with Dr Robert Wilder, creator of the Wilderhill Index; and the second from David Harris of the FTSE Group in London.