. ♪ ♪ >> narrator: michael maccracken was one of the government's senior scientists investigating climate was in climate modeling. from 1997 to 2002, i was in charge of helping make the first climate assessment on the u.s.-- what would be the impacts. and what we found is that there was no question that it was rising concentrations of co2 doing that. if we really want to do something significant to slow this so that our grandchildren don't face a changing world, we're going to have to do a substantial movement away from the key fossil fuels of coal and oil, particularly. >> narrator: in january 2001, maccracken participated in a headline-grabbing report for the united nations intergovernmental panel on climate change. the i.p.c.c. said there was now "new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities." >> the statement that came out of the i.p.c.c. said, "look, humans are the main cause." and that turned out to be very controversial. >> narrator: exxonmobil's response was severe: a company lobbyist faxed the bush administ