Today marks the 39th annual Earth Day, an idea hatched by Wisconsin Senator Gaylord Nelson in 1969 to "shake up the political establishment and force this issue into the national agenda," according to the Earth Day Network, a nonprofit that helps organize the day.

But way back before global warming was a household term and canvas totes were a fashionable alternative to shopping bags, environmental supporters started with the basics: recycling, energy use, pesticides and population growth, to name a few.

So how much have actions and attitudes about saving the earth changed since then? Mark Fischetti, managing editor of Scientific American Earth 3.0 magazine, reflects that, "Back in the '70s, Earth Day was kind of this quirky, one-day grassroots event. It raised a little awareness, but the next day it was gone… Now it's on the radar every single day, it's in the headlines every single day."   

The first Earth Day saw 20 million people turn out for rallies and demonstrations in defense of the earth, and on the 20th anniversary of the first Earth Day, in 1990, 200 million people in more than 100 countries celebrated the event. These days, Earth Day Network focuses on coordinating volunteering and events year-round, boasting an association of more than 15,000 organizations in 174 countries.

But Earth Day isn't just about just about getting your hands dirty picking up litter or foregoing that plastic grocery bag. Organizers have rounded up more than 2,000 events around the country. In an early celebration on the National Mall in Washington, D.C., this past Sunday, 100,000 people turned up to see bands, speakers and celebrities. In honor of the day today, Earth Day Network's streaming video site will present the international premiere of Walden: The Ballad of Thoreau, a play dramatizing a conversation between famous 19th-century naturalists Henry David Thoreau and Ralph Waldo Emerson.

Today the nonprofit is also launching the Green Generation Campaign, an initiative to up awareness for next year's 40th anniversary of the first Earth Day. But unlike the so-called Greatest Generation that came of age in the mid-20th century, "The Green Generation is open to everyone: people of all ages and all nationalities," the organization notes on its Web site.

The awareness generated around Earth Day must carry over into every other day, notes Fischetti. And in addition to keeping up with the three R's (reduce, reuse, recycle), he says, people should continue putting pressure on corporations and politicians to take real action on climate change.

"Right now, most of the rest of the countries are literally waiting for the U.S. Congress to act to pass climate legislation," says Fischetti. With the Copenhagen talks looming just eight months away, he notes, international leaders have said that if the U.S. shows up to the talks without having instituted climate legislation in its own backyard, the talks will likely fall apart, leaving the globe without a replacement for the 1997 Kyoto Protocol.

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Image of the Earth Day flag courtesy of BL1961 via Flickr