He was an early ecologicalactivist, warning against human-induced climate change before anyone else,and publishing in 1864 Man and Nature: or, Physical Geography as Modifiedby Human Action.

In this context, Marsh invited members of the American public to helpcreate a radical new dictionary of all English which applied the scientificmethod, was collaborative in its making, and was based on written evidence.They were asked to collect current words and especially to read books fromthe eighteenth century – because literature from earlier centuries was harderto get in America at the time. Marsh ended his appeal with the warning thatAmericans would be paid nothing for their help.