The promise came in his acceptance speech on 13 September. Mr Duncan Smith said that the Conservatives "will campaign on the issues that matter to people, the things that affect them most in their daily lives, that obsess them - these must be the things that obsess us. The state of their public services, health, welfare, education and the environment."
The pledge came five months after the Tories' election manifesto offered mostly negatives on the environment - abolishing the climate change levy and aggregates tax, cutting fuel duty, scrapping local authorities' powers to introduce congestion charges and parking levies, reducing regulatory burdens on business and the like (ENDS Report 316, pp 33-35 ).
Mr Duncan Smith signalled his intention of a different focus in a speech during the leadership contest on 25 July which was put together after a ring-round to environmental groups.
Part of the speech was backward-looking, claiming credit for the Conservatives for introducing the Clean Air Acts, and genuflecting to Margaret Thatcher, "who brought the environment into the mainstream of British politics."
Mr Duncan Smith also criticised Labour for responding to environmental challenges with "more state direction and control, more taxation and regulation." The climate change levy, he argued, "will drive investment overseas, but do little for the environment." This and other "state-based solutions are bound to fail," he contended.
Mr Duncan Smith sees a narrower role for the state - "to make it easier for people to follow their natural inclination to care for the environment. It is about giving purpose and direction to what people are prepared to do for free."
There was, though, only one concrete idea. The speech was made at the offices of Solar Century, the leading UK supplier of photovoltaic energy systems. The technology, Mr Duncan Smith suggested, could turn every major building into a mini-power station, and every householder aspiring to protect the environment could contribute by producing renewable energy - "this is the glory of Conservatism."
He went on to suggest that people could be helped to invest in photovoltaic systems if energy businesses were given incentives to bear some or all of the capital cost. "By doing this, we would begin to create a marketplace which in turn would mean greater economies of scale from volume production, helping to lower the price." "Net metering", under which householders could sell surplus solar energy into the grid at the same price as they paid for electricity purchases, was another proposal.
The Tory leader's promise to campaign on the environment will, though, need a change of heart within much of his party. During the last Parliament, Conservative MPs did little but attack the Government's environmental measures, and were relentless in their criticism of EU environmental proposals - usually regardless of their merits. The political balance of Mr Duncan Smith's front-bench team will make those habits difficult to break.
The new shadow Environment Secretary is Peter Ainsworth, 45, a former stockbroker who was shadow spokesman for culture, media and sport for most of the last Parliament. He was a member of the House of Commons Environment Committee in 1993-94, but otherwise has no track record on the environment.