This year's theme is The Face of Climate Change, chosen to highlight the "mounting impact" of global warming on people worldwide, according to the Earth Day Network, the group founded by the first Earth Day organizers.
The Earth Day Network is asking people to become their own "climate reporters" by sending their pictures and stories of people, animals, and places threatened or affected by global warming, according to the group's website. (See a map of global warming effects.)
On or near Earth Day, an interactive display of the images will be shown at events around the world, including next to federal buildings in countries that produce the most carbon pollution.
"The Face of Climate Change will not only personalize and make real the massive challenge that climate change presents, it will unite the myriad Earth Day events around the world into one call to action at a critical time," Franklin Russell, director of Earth Day at Earth Day Network, said in a statement.
Earth Day has come a long way since its founding in 1970, when 20 million participated across the U.S. (See pictures: "The First Earth Day-Bell-Bottoms and Gas Masks.")
Find out when it is, how it started, how it's evolved, and what you can do.
When Is Earth Day?
Every day, the saying goes, is Earth Day. But it's popularly celebrated on April 22. Why?
One persistent rumor holds that April 22 was chosen because it's the birthday of Vladimir Lenin, the founder of the Soviet Union.
"Lenin's goal was to destroy private property and this goal is obviously shared by environmentalists," the Capitalism Magazine website noted in a 2004 article perpetuating the theory.
Kathleen Rogers, president of Washington, D.C.-based Earth Day Network, said in 2010 that the rumored communist connection is untrue.
Instead, April 22, 1970, was chosen for the first Earth Day in part because it fell on a Wednesday, the best part of the week to encourage a large turnout for the environmental rallies held across the country, Rogers said.
"It worked out perfectly, because everybody was at work and they all left," she said.
Earth Day is now celebrated every year by more than a billion people in 192 nations around the world. (See pictures of quirky Earth Day stunts.)
Mad People and a Frustrated Politician
Earth Day's history is rooted in 1960s activism. The environment was in visible ruins and people were mad, according to Rogers.
"It wasn't uncommon in some cities during rush hour to be standing on a street corner and not be able to see across the street" because of pollution, she said.
Despite the anger, green issues were absent from the U.S. political agenda, which frustrated U.S. Senator Gaylord Nelson of Wisconsin, whose campaigns for the environment through much of the 1960s had fallen flat.
First Earth Day "Took off Like Gangbusters"
In 1969 Nelson hit on the idea of an environmental protest modeled after anti-Vietnam War demonstrations called teach-ins.
"It took off like gangbusters. Telegrams, letters, and telephone inquiries poured in from all across the country," Nelson recounted in an essay shortly before he died in July 2005 at 89.
"The American people finally had a forum to express its concern about what was happening to the land, rivers, lakes, and air—and they did so with spectacular exuberance."
Nelson recruited activist Denis Hayes to organize the April 22, 1970, teach-in, which today is sometimes credited for launching the modern environmental movement.
By the end of 1970, the U.S. Environmental Protection Agency had been born, and efforts to improve air and water quality were gaining political traction.
"It was truly amazing what happened," Rogers said. "Blocks just tumbled."
Earth Day Evolves
Earth Day Network is pushing the Earth Day movement from single-day actions, such as park cleanups and tree-planting parties, to long-term commitments, such as this year's theme of climate change.
For those whose inner environmentalist speaks loudest on April 22, Earth Day Network's Rogers encourages them to make a public commitment to take an environmental action.
Ideas promoted by the Earth Day Network include pledging to educate friends and family on global warming or buying green products such as energy-saving compact fluorescent light bulbs (CFLs).
According to Rogers, everyone is part of this generation, which marks the transition from the industrial revolution to the green revolution.
"It is also about the green generation of energy and the generation of green jobs. ... The name [Green Generation], whenever I say it to people, they have their own idea of what it means, which is exactly what we want."
(Check out National Geographic and Google's live Hangout on Earth Day at 12 p.m. on April 22, 2013.)