Finis Dunaway, an associate professor of history at Trent University in Peterborough, Ont., is writing a book on the history of environmentalism. He spoke with Muriel Draaisma of CBC News.

What led to the first Earth Day in the U.S. on April 22, 1970?

About 20 million Americans participated in the first Earth Day in 1970. (iStock)Dunaway: Rachel Carson's book, Silent Spring, published in 1962, warned about pesticides and their effects on wildlife and on human health. That helped to popularize ecological ideas to a larger audience. The book created a lot of controversy and there was a backlash from the chemical industry.

In the late '60s, there was increasing coverage of environmental problems and air pollution became more talked about. In 1969, there was a dramatic oil spill in Santa Barbara, Calif., that received extensive media coverage. The stories showed sea lions, western grebes covered in oil, struggling to survive, suffocating and dying along this beautiful beach in Southern California.

This oil spill made an impression on Senator Gaylord Nelson from Wisconsin. It was Nelson who came up with the idea for Earth Day. It was partly from going to Santa Barbara. He had been in environmental issues before but this really seemed to have affected him.

Nelson had read about students who had been organizing teach-ins against the Vietnam War. These are events that took place on university and college campuses where students and others would gather together and talk about the war and organize and take action against it. He thought a similar model could be used for the environmental movement. It was a pretty modest idea to begin with. So the idea came from Nelson. He started to promote the notion of an environment teach-in, but because of growing ecological concern, because of protest movements taking place during the 1960s, the idea really captured the imagination of many, many people, far more than Nelson could have ever have predicted.

In a sense, he turned over the organization of the events to a staff primarily of graduate students and people in their 20s, and they helped to co-ordinate activities nation-wide. This was done up from the grassroots.

When the event took place on April 22, 1970, it was estimated that about 20 million American participated in some form in Earth Day events in their communities, which made it the largest protest in U.S. history, larger than any protest against the Vietnam War. A lot of people see it as a pivotal moment into making environmentalism into a mass movement. It put it on the radar screen of the media.

What kind of events took place on the first Earth Day?

Dunaway: The events were very diverse. There were a number of community cleanup campaigns. Schools were very involved and there were a lot of educational fairs on the environment. Even at the beginning, Earth Day had this dual quality, both a celebration and a protest. It was an opportunity for people to gather together to celebrate the Earth but also to engage in some sort of protest activity. They varied. Some were more radical than others. There were lots of speeches in different communities.

In New York City, where some of the biggest events took place, they actually shut down some of the major streets for a few hours to hold ecology fairs.

How has the day evolved over the past 40 years?

Dunaway: My impression is, for the most part, there were local efforts here and there on an annual basis over the years, but there wasn't necessarily the intent to make this into some sort of holiday. There were scattered efforts during the 1970s. In 1980, there was a greater effort on that day to make it into a bigger event to mark the 10th anniversary. It was really in 1990 that Earth Day became institutionalized. There were huge efforts to make it into a huge extravaganza and a global event.

It was estimated 200 million in over 140 countries took part. It became a worldwide event.

It was also criticized at the time by a number of environmentalists for becoming overly corporate in terms of lot of sponsorship. It seemed to be a much more top down affair than it was in 1970. There was lot more celebrity involvement and the media really embraced the cause in 1990.

Would you say there was, in 1990, broad support for Earth Day, but that it was more corporate and didn't necessarily mean it was going to lead to changes in government policies that would protect the environment?

Dunaway: Exactly. During this time, the first George Bush would call himself an environmental president.

How useful is Earth Day?

Dunaway: As the day has developed over the years, it's hard to appreciate how significant it was in 1970. It is something that we look at now and think, oh, it's Earth Day, it's somewhat trivial, maybe we should go take a walk and make some overtures about why we care about the state of the planet.

It doesn't strike me that as an annual event that it has that much effect on politics or that leads to meaningful change.

It's a double edge sword: it makes it possible for environmental ideas to reach a mass audience in a significant way, but also there are some important issues that are left out or obscured.

But there are examples of people who became more involved in environmental causes because of Earth Day in 1970. I wouldn't want to close off the possibility that Earth Day could make a difference in a person's life and make him or her think about environmental issues in a new way.