Thirty nine years ago, responding to widespread environmental degradation, Gaylord Nelson, a United States Senator from Wisconsin, called for an environmental teach-in, or Earth Day, to be held on April 22, 1970. Over 20 million people participated that year, and Earth Day is now observed each year on April 22 by more than 500 million people and national governments in 175 countries.  Senator Nelson, an environmental activist, took a leading role in organizing the celebration, hoping to demonstrate popular political support for an environmental agenda. Senator Nelson selected Denis Hayes, a Harvard University graduate student, as the national coordinator of activities.

 
One of the organizers of the event said: "We're going to be focusing an enormous amount of public interest on a whole, wide range of environmental events, hopefully in such a manner that it's going to be drawing the interrelationships between them and, and getting people to look at the whole thing as one consistent kind of picture, a picture of a society that's rapidly going in the wrong direction that has to be stopped and turned around.

 
Senator Nelson stated that Earth Day "worked" because of the spontaneous response at the grassroots level. Twenty-million demonstrators and thousands of schools and local communities participated. He directly credited the first Earth Day with persuading U.S. politicians that environmental legislation had a substantial, lasting constituency. Many important laws were passed by the Congress in the wake of the 1970 Earth Day, including the Clean Air Act, wild lands and the ocean, and the creation of the United States Environmental Protection Agency.

 
It is now observed in 175 countries, and coordinated by the nonprofit Earth Day Network, according to whom Earth Day is now "the largest secular holiday in the world, celebrated by more than a half billion people every year." Environmental groups have sought to make Earth Day into a day of action which changes human behavior and provokes policy changes. 

 
As 1990 approached, a group of environmental leaders asked Denis Hayes to organize another big campaign. This time, Earth Day went global, mobilizing 200 million people in 141 countries and lifting the status of environmental issues on to the world stage. Earth Day 1990 gave a huge boost to recycling efforts worldwide and helped pave the way for the 1992 United Nations Earth Summit in Rio de Janeiro.

 
As the millennium approached, Hayes agreed to spearhead another campaign, this time focused on global warming and a push for clean energy. Earth Day 2000 combined the big-picture feistiness of the first Earth Day with the international grassroots activism of Earth Day 1990. For 2000, Earth Day had the Internet to help link activists around the world. By the time April 22 rolled around, 5,000 environmental groups around the world were on board, reaching out to hundreds of millions of people in a record 184 countries. Events varied: A talking drum chain traveled from village to village in Gabon, Africa, for example, while hundreds of thousands of people gathered on the National Mall in Washington, D.C., USA.