From 1988 to 2000, Marshall worked on campaigns for tropical forest conservation and defence of indigenous land rights with the Australian-based Rainforest Information Centre and The Ecologist magazine,[2] specialising in the exposure of corruption and illegal logging in Papua New Guinea.
[6] In 2004, he co-founded (with Richard Sexton) Climate Outreach, a UK charity that specialises in increasing public engagement in climate-change related issues.
[9] His website on the project won a Millennium Award[10] and led The Ecologist to list Marshall as one of their Ten Green Visionaries in 2009.
[11] Marshall has spoken and written extensively on the need to engage new audiences on climate change, especially conservatives[12][13] and people of religious faith.
[14] Marshall is the author of Carbon Detox (Hamlyn Gaia, 2007) on personal action to reduce emissions.