The current hype among environmentalists is global warming but over the years they have hyped one non-sequitor after another- Alar, DDT, power lines, overpopulation, ice ages, famine, GMO's, ozone, ad nauseum. It's always an apocalypse for these people (which is why I think it's closer to religion than science).
Any solution to an environmental problem involves trade-offs. These could be economic trade-offs or environmental trade-off. The replacement of the horse with the horseless carriage eliminated the problem of manure in the streets and disease carrying flies but created a new set of problems. I wrote about the large environmental impact the Prius has. Here's another post about the impact of the new fluorescent lights. Some clamor for use of wind power while others fear impact of windmills on birds. The banning of DDT helped save some birds at the cost of millions of dead children in Africa.
There is no free lunch. The only good alternative to fossil fuels creates (a small amount of) hazardous radioactive waste. It seems the only solution the environmentalist really believe in is population reduction.