And, frankly, environmentalists have shot themselves in the foot too many times. Too much negativity, too little in the way of creative solutions and too many lost jobs blamed on environmental regulations have taken their toll.
But, contrary to a divisive debate that has embroiled the environmental community in recent months, the movement isn't dead.
Greenberg said that's clear in the same surveys that show support shrinking.
"The public still feels that the environment is a major issue for the future," Greenberg said, even though some who used to think it should be the 'top' priority now think it is only 'important.'"
"The numbers [in those two categories combined] haven't changed much in the last 20 years," he said. "That remains very strong, and that's why I think this is a temporary fluctuation."
But when and if it does reemerge near the top of the worry list, the movement will be different. It will be less dominated by white, upper-middle class suburbanites. And in a sense, it will return to its roots.
The modern environmental movement wasn't born on the green lawns and in the upscale homes of suburbia. And it didn't come from the wilderness or the countryside or any of the idyllic places that the movement labors so hard to protect.
It came from the urban jungles, where people knew what they had lost. That's why the polls show that the strongest supporters of the environmental movement today are African-Americans, Greenberg said.
"It's a big issue for them," he said. "They are there with the traffic density and the air pollution, and it's an in-your-face issue for them." Their No. 1 environmental concern, he noted, is water quality.
They know what environmental degradation really means. Maybe from their ranks, a new John Muir for the 21st century will emerge.