On April 22, 1970, the very first Earth Day, more than 20 million people joined in demonstrations across the United States.
Colleges organized teach-ins to draw attention to the pressing environmental issues of air and water pollution. A part of New York City's Fifth Avenue was shut down to accommodate the marching crowds. Congress recessed so that members could join the days' activities.
Organizers said it was the largest national protest in U.S. history.
Today, Earth Day participants are more likely to carry reusable shopping bags than picket signs. They're more likely to pledge their commitment online than in a line outside city hall.
Forty years later, the tone may have changed, but Earth Day organizers say about 1 billion people worldwide are taking part in the international day of environmental awareness. Many will continue to green their shopping habits, homes, cars and more.
But some environmentalists who remember the first Earth Day -- and the political will that was so palpable then – say they wonder if those individual changes will be enough, considering the massive challenges facing the planet on this Earth Day.
In 1969, the Cuyahoga River in Ohio was so polluted it caught fire. An oil spill off Santa Barbara, California, killed thousands of birds and other marine wildlife. City skies across the country were darkened by smog.
The environmental movement, energized in 1970, helped not only to clean things up but to change the public consciousness.
"There have been advances on almost every front in terms of reduced pollution in our air and water, millions of acres of land protected and parks created, a wide range of new technologies introduced and environmental sensitivities adopted in virtually every walk of life," said Eric Goldstein, director of the Natural Resources Defense Council's New York Urban Program.
In the decade after the first Earth Day, the Environmental Protection Agency was created by executive order and Congress passed the Clean Air Act, the Clean Water Act, the Endangered Species act and other key pieces of environmental legislation.
According to 1997 EPA figures, the number of rivers, lakes and estuaries safe for fishing and swimming doubled in the 25 years after the first Clean Water Act. Smog levels have decreased about 20 percent and lead in the air is down about 90 percent. More than 600 animal species and nearly 800 plant species are listed as threatened or endangered in the U.S.
But the movement itself has changed too.
"The environmental movement in the early days very much rode on the back of the early protest against the Vietnam war," said Sean Miller, education director for the Earth Day Network.
When Sen. Gaylord Nelson, D-Wisconsin, suggested in September 1969 that the country have an Earth Day demonstration to force the environment to the top of the national agenda, Miller said college activists quickly adapted the model of protest they were already using.
But the movement has changed. Instead of local pollution, Miller said, environmentalists today talk of a sustainable society.
"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."
As they reach the age of the 1970 Earth Day protesters, they'll start to run businesses, seek positions in government and make decisions that will shape the country's environmental and economic future, he said.
"There is reason to hope that having grown up with some of the benefits of these first decades of environmental progress, that they will take things to the next level," he said.
Stewart Brand, the founder of "The Whole Earth Catalogue," who urged NASA to publish the first satellite images of Earth, said that though environmentalists and the general public may not always agree on how to take on the next 40 years of environmental problems, they'll have to remember that at the climate level, everything is convergent.
"We're all in this together," he said. "Climate happens to everybody and the changes that we don't like are being caused by everyone."