Sunday is Earth Day, observed every year since 1970.

Earth Day organizers have called on people worldwide to pledge “one billion acts of green.” Those pledges range from residents turning off lights in their homes, to using mass transit for travel to work, to eating locally grown food.

Earth Day was first celebrated in the U.S. on April 22, 1970. It was the brainchild of Senator Gaylord Nelson, who called for a national environmental “teach-in” day. Organizers now say Earth Day is celebrated in one form or another in nearly every country.

Much has changed in the U.S. since the first Earth Day 42 years ago – a time when many factories still were allowed to belch considerable smoke and pollutants into the air, and cars burned fuel containing lead, a dangerous automobile-exhaust pollutant. Over the following decades the nation's Environmental Protection Agency was established, and federal and local jurisdictions enacted many laws protecting air and water supplies, and endangered species of animals and plants.

Environmental regulations nevertheless still cause controversy in much of the U.S. The two main political parties, Democrats and Republicans, frequently contest each other's claims about the effect of greenhouse-gas emissions on the warming of the planet. As the nation has struggled to recover from a disastrous economic downturn, some have questioned whether environmental laws contributed to the loss of jobs and whether the resulting rise in energy costs is worthwhile.