Parents, students and millions of others in communities nationwide focused this past weekend on the greening of our planet at events marking Earth Day -- observed every April 22 to encourage environmental awareness. It’s a chance to think about ways we can make the environment cleaner and safer.

Groups in cities, towns and campuses use the occasion to speak out for nature-related causes and to promote energy-efficient products, such as hybrid-fuel vehicles that use electricity at highway speeds. Free concerts in New York, Chicago and San Francisco featured more than 200 performances by artists including Dave Matthews, Willie Nelson and Bob Weir. Those “Green Apple” shows featured cups, straws, napkins and paper towels made of recycled materials, compostable garbage bags and biodegradable cleaners.

When Earth Day began 37 years ago, the environment was becoming so polluted that thick smog blanketed most major U.S. cities. On April 22, 1970, more than 20 million Americans participated in rallies, marches and other gatherings designed to educate people about the damaging effects of environmental carelessness. The public made it clear that they wanted the government to help make America a cleaner country. In response, President Richard Nixon created the Environmental Protection Agency. What started as an attempt to clean the air and water has become an important movement.

Success stories: Thanks to all the attention and action, the United States is actually in better shape than it was when Earth Day began. Congress has passed laws encouraging a cleaner environment, such as the Pollution Prevention Act, the Clean Air Act, the Clean Water Act and the Endangered Species Act. State and local laws protect wetlands, establish nature preserves and create incentives for developers to leave green spaces untouched as natural zones.

Current priority: Many activities this year focus on global warming, a pressing environmental issue spotlighted by An Inconvenient Truth, an Oscar-winning 2006 documentary produced by former Vice President Al Gore. Efforts to reduce emissions from gasoline-powered vehicles and power plants that use coal or oil coal are aimed at reducing erosion of the upper atmosphere that lets in more solar rays, changing climate patterns and melting polar ice.

Four words to remember: "Think globally, act locally." That slogan, attributed to the founder of a group called Friends of the Earth, has been used since the 1970s as a reminder than small environmental actions can make the planet better or worse.