Paul Ehrlich is a biologist who wrote The Population Bomb in 1968, predicting that hundreds of millions of people would die of starvation during the 1970s because the earth's inhabitants would multiply at a faster rate than world's ability to supply food.
(Ehrlich's predicted famines never materialized. Indeed, the death toll from famines steadily declined over the twenty-five year period. Though world population has grown by more 50% since 1968, food production has grown at an even faster rate due to technological advances. Ehrlich completely failed to anticipate that the Green Revolution which was then ongoing would increase food production faster than population growth. Norman Borlaug, the "father of the Green Revolution", has since become critical of Ehrlich, noting that Ehrlich was one of the "worst critics we had" and had claimed that Borlaug's work at increasing food output wouldn't make a dent in world hunger, when in fact Borlaug's work succeeded.) 
Ehrlich lost a famous bet with Julian Simon having to do with the scarcity of mineral resources. Simon's thesis, rooted in classical economics, was that the market will always seek substitutes for scarce resources, and therefore catastrophic predictions of high prices of resources due to depletion will not come to pass. Ehrlich had bet a basket of commodities would increase in price, Simon had bet they would decrease.
Despite being famously wrong about his two most important predictions, he is still cited as an authority on the environment (by liberals, anyway).
- Despite his lengthy green-left credentials and his terrible track record as a prognosticator, NBC contracted with Ehrlich in 1990 to produce a 12-part series on global ecological issues for NBC News. NBC cast the radical professor as a disinterested scientist dispassionately explaining global warming and environmental issues. Ehrlich was later nominated for an Emmy Award for his TV propaganda and received a five-year MacArthur Fellowship. PBS subsequently made him the subject of a puff-piece documentary.