THE GENIUS OF EARTH DAY: How a 1970 Teach-in Unexpectedly Made the First Green Generation. By Adam Rome. Hill and Wang. 346 pages. $30.

Monday marks the 43rd Earth Day. Most Americans living today were not yet born when Earth Day’s planners heralded a cultural awakening, the coalescing of an environmental groundswell that had been mounting for decades.

Adam Rome records the history of Earth Day’s momentous beginnings in this thorough evocation.

“Earth Day,” asserts Rome, “was an educational experience as well as a political demonstration.”

Conceived by Gaylord Nelson, a Democratic senator from Wisconsin, as a national teach-in, the first Earth Day took place over weeks and months, a happening of more than 12,000 speeches, rallies and protests, leading up to April 22. On that day in Manhattan, a quarter-million people gathered on Fifth Avenue and 14th Street. Fifty blocks were closed to traffic.

Prominent speakers, including New York Mayor John Lindsay, proclaimed the new awareness, and singers, Pete Seeger among them, urged the crowd to action.

Nationally, Earth Day events varied in scope and venue, but pollution and overpopulation were at the top of most agendas.

Some 20 million Americans participated in the day’s events, and Rome cites a conservative estimate that 35,000 individuals addressed the crowds nationwide.

The environmental movement before Earth Day was not actually a movement but a collection of fragmented efforts at grass-roots organizing.

Rome describes in detail the different groups that came together as a social and political force and conveyed an ecological ethos to the nation.

Scientists, notably Rachel Carson, Barry Commoner and Paul Ehrlich, played a crucial role in illuminating the environmental predicament to a universal audience.

Progressive political thinkers, including Arthur Schlesinger Jr. and John Kenneth Galbraith, advocated a “qualitative liberalism” focused on enhancing public goods such as fresh air, clean water and unspoiled countryside.

The National Audubon Society and the Sierra Club intensified the drumbeat for conservation as their memberships surged. America’s youths, the baby-boomers who fought for civil rights and to end the Vietnam War, found in environmentalism a third platform for their activism.

Mayors, governors, senators, congressmen and even presidents listened to the grass-roots coalitions and proposed legislation in response.

“The Genius of Earth Day” explains what made Gaylord Nelson’s teach-in a momentous success. Earth Day symbolized a new conception of humanity’s connections to the natural world and of human beings’ relationships with one another.

The events of April 22, 1970, changed the thinking of many Americans, some of whom Rome profiles in the book’s epilogue. In Earth Day’s wake came shifts in national policies, most notably the Clean Air Act and the Clean Water Act of 1972, as environmental groups gained a foothold in Washington’s inner circles.

The enthusiasms of Earth Day 1970, wrote Gladwin Hill of The New York Times at the end of the decade, “have been ‘institutionalized’ in legislation, regulation, litigation, political dynamics and new personal values, and woven into the fabric of national life.”

These accomplishments are impressive and should be remembered.

But what about today? What is Earth Day’s relevance in 2013? “The Genius of Earth Day” does not address these questions, and Rome at no point acknowledges the mammoth environmental problems and political obstacles that we face.

Elected officials seem less responsive to the public will now than they were in the giddy days of the 1960s and ’70s. It is striking to read the progressive language of that era, bolder and without equivocation, in contrast to the timid and cowed liberalism of today.

Environmentalism was not a partisan issue in the 1960s; both Republicans and Democrats supported policies designed to protect the environment. Compounding political stagnation are the pressures from the staggering increase in the world’s population: There are 50 percent more of us today than there were in 1970. Threats to the planet’s livability and to the Earth’s remarkable biodiversity are more complex and harder to solve than simply cleaning up polluted streams and freshening the air we breathe.

Yet Rome’s retelling of the hopeful origins of Earth Day and its early successes contains an important lesson for today. The social movements and anti-war crusades that swept through the country in the 1960s and ’70s and the movement to promote respect for the natural world demonstrate the tremendous power of activism and grass-roots organizing.

Forming coalitions with like-minded groups and individuals and acting collectively for the greater good is a wonderful thing. Striving to make the world better suited to sustain life of all kinds is acting on our greatest virtues. Every year, Earth Day should remind us of this truth.

Reviewer Carlin Rosengarten is a writer and environmental consultant based in Charleston.