I've been kicking this one around for a while now:
Suppose you traveled back in time to Europe and the United States circa 1850. You explain to the political and economic elites of the time that their decision to industrialize through the use of coal and petroleum will set in motion a chain of events that will lead to widespread devastation of the Earth's climate some 200 to 250 years later. Therefore, they must seek to minimize their use of those fuels wherever possible and to develop cleaner alternatives — even if it means higher energy prices and slower economic growth. Assuming that they accepted your version of future events as true, would anybody think that you had made a credible moral claim?
Suppose you traveled to 1900? 1950?