Earth Day was first observed in Spring of 1970. An estimated 20 million people nationwide attended festivities out of which came the largest grassroots environmental movement in U.S. history, and the impetus for national legislation like the Clean Air and Clean Water Acts. By the twentieth anniversary of that event, April 22, 1990, more than 200 million people in 141 countries participated in Earth Day celebrations.
Former U.S. Senator Gaylord Nelson, Earth Day's co-founder, modeled Earth Day on anti-Vietnam War demonstrations, called "teach-ins," that were common on college campuses. "At a conference in Seattle in September 1969, I announced that in the spring of 1970 there would be a nationwide grassroots demonstration on behalf of the environment," says Nelson. "The response was electric. Telegrams, letters, and telephone inquiries poured in from all across the country." As many as 20 million Americans participated in environmental rallies, demonstrations and other activities in the 1970 Earth Day.
Since the first Earth Day, however, the environmental movement has increasingly transformed itself from a largely grassroots, citizen crusade to a professionally-organized, established special interest.
世界地球日(World Earth Day/International Earth Day)：
1970年4月22日，美国首次举行“地球日”活动，宣传环境保护、要求政府保护和治理环境污染。此活动普及到其他国家，逐渐演变成全球性的环境保护宣传活动，1990年4月22日，全球140多个国家开展了地球日活动，每年的4月22日正式被联合国确定为世界地球日。世界地球日推动了许多国家的环保立法。
世界地球日历届主题(1974-1984)：
1974年世界地球日主题：只有一个地球 Only one Earth
1975年世界地球日主题：人类居住 Human Settlements
1976年世界地球日主题：水，生命的源泉 Water: Vital Resource for Life
1977年世界地球日主题：关注臭氧层破坏，水土流失 Ozone Layer Environmental Concern; Lands Loss and Soil Degradation; Firewood
