Lack of Group Memory
John Hawkins has collected some of the best ecowhines over the years in Environmentalist Wacko Quotes. There are so many juicy ones, it's hard to choose, but I have a lot of affection for:

Every time you turn on an electric light, you are making another brainless baby -- Helen Caldicott, Union of Concerned Scientists

Named Helen?

And I also rather like the global cooling fad of the 70's:

The continued rapid cooling of the earth since WWII is in accord with the increase in global air pollution associated with industrialization, mechanization, urbanization and exploding population. -- Reid Bryson, "Global Ecology; Readings towards a rational strategy for Man", (1971)

This [cooling] trend will reduce agricultural productivity for the rest of the century -- Peter Gwynne, Newsweek 1976

This cooling has already killed hundreds of thousands of people. If it continues and no strong action is taken, it will cause world famine, world chaos and world war, and this could all come about before the year 2000. -- Lowell Ponte "The Cooling", 1976

Brrrr! I'm feeling right chilly!