For me, the most compelling short (~18 minute) video about the reality of climate change and the urgent need to address it. From 1981 to 2013, Hansen was the head of the NASA Goddard Institute for Space Studies. He is best known for his testimony on climate change to congressional committees in 1988 that helped raise broad awareness of global warming and his advocacy of action to avoid dangerous climate change — leading on a few recent occasions to his arrest.