"The environmental movement has moved from a back-to-land, campus-based effort to something in corporate boardrooms, in national organizations. It's something that's really taken on a full-scale implementation in our society," he said.
From solar panels and green buildings to green school curriculums and corporate green efforts, he said "environmentalism is very much in vogue."
At every level of society, Miller said, Americans are taking steps to lighten their carbon footprints, re-use and recycle materials, use less energy and more. The tailpipe emissions from cars, says the EPA, are 98 percent cleaner than they were in 1970.
But though environmentalists have helped solve national problems, Miller said they haven't been as successful on a global scale. World leaders could not agree on a global climate treaty at the Copehagen summit last year.
To address climate change, the key environmental issue of the day, he said, we need to transcend national boundaries.
"What matters now is taking that original message of the first Earth Day and bringing it into a 21st century context," he said.
Some who recall that first Earth Day say today's movement needs a motivating vision comparable to the one it had 40 years ago.
"I think the real difference is that today, the environmental movement is really being led by environmental activists, scientists and some enlightened members of the political class," said Robert Stone, director of the documentary "Earth Days," which premiered on PBS this week. "It's less of a grassroots movement, and I think that's largely because the issue of climate change is more abstract than air pollution and water pollution."
In the 1970s, people could see the smog blocking their view of the skies. They could see the oil and sewage spoiling the water. But, Stone said, the greenhouse gases that threaten the planet, its inhabitants and their future generations today are invisible.
"There is this hidden thing of CO2 which is slowly killing us, but it's not something we can see or smell," he said.
While there was significant action in the decade after the first Earth Day, Stone said that over the past thirty years, the political climate has shifted.
The country started investing more faith in the power of market forces to enact change and people starting turning inward instead of opting for collective action, he said.
While individual efforts are not futile, he said, they don't really match the scope of the problem.
"I'd like to see the environmental movement focus more on the big picture, an inspiring energy proposal that the nation can rally around," Stone said. "We need to be inspired to do great things again."
Environmental organizers today acknowledge that the problems today are increasingly complex and global in scale. But they say they draw hope from the generation that founded the movement and the generation that will carry it into the future.
"We truly have come a long a way," said the NRDC's Goldstein. "Young people in America really seem to get this. They understand the environmental challenges, they have a grounding in these issues and, in many instances, they have a sense of entitlement to the basic environmental right of clean air, clean water and green spaces in which to recreate."