2010 marked the 40th celebration of Earth Day, a holiday that helped spark America's modern environmental movement when it was founded on April 22, 1970, by then-Sen. Gaylord Nelson of Wisconsin. In honor of that historic day, MNN produced this quick look back at the last four decades of planetary appreciation.

earth day
1970: Twenty million people celebrate the first Earth Day on April 22. A few months later, the U.S. Environmental Protection Agency opens its doors for the first time.

1971: Amtrak is founded, even though gas costs just 33 cents a gallon.

1972: The EPA bans DDT, which was thinning bald eagles' eggshells.

1973: A Mideast oil embargo sparks a U.S. gas crisis.

1974: Congress passes the Safe Drinking Water Act, shamelessly pandering to the water-drinkers lobby.

1975: Congress sets emissions and efficiency rules for cars, leading to the introduction of catalytic converters.

1976: The EPA starts phasing out PCBs, which can cause cancer and other health problems.

1977: The U.S. adds the first plants to its endangered species list — despite their disturbing lack of cuteness.

1978: Congress bans CFCs in aerosol sprays after scientists realize they can deplete the Earth's ozone layer.

1979: A partial meltdown at Pennsylvania's Three Mile Island nuclear plant ruins an otherwise good day.

1980: Congress creates the Superfund program to clean up toxic waste sites. Those expecting "super fun" sites are quickly disappointed.

1981: Acid rain intensifies over the Northeastern United States and Canada.

1982: Dioxin contamination forces the U.S. government to buy homes in Times Beach, Missouri — not the last time it would have to buy up toxic assets.

1983: A long failure to clean up the Chesapeake Bay begins.

1984: 8.6 million acres of protected wilderness are established in 21 states. Somewhere in the distance, a coyote howls.

1985: Scientists discover a giant hole in Earth's ozone layer. During the next year's NBA All-Star Game, Spud Webb dunks through it.

1986: Congress declares the public has a right to know when toxic chemicals are released into the air, land or water. The public breathes a sigh of relief — and a little sulfur dioxide.

1987: Medical waste washes ashore in New York and New Jersey, forcing beaches to close. Efforts to rebrand the area don't work out.

1988: Congress bans ocean dumping of sewage sludge and industrial waste, ending a cherished American tradition.

1989: The Exxon Valdez spills 11 million gallons of crude oil into Alaska's Prince William Sound, one of the worst environmental disasters in U.S. history.

1990: The EPA's Toxic Release Inventory tells the public which pollutants are being released into their communities.

1991: The U.S. government begins using products made from recycled content.

1992: The U.S. Energy Department and the EPA launch the Energy Star program to label energy-efficient products.

1993: A cryptosporidium outbreak in Milwaukee sickens 400,000 people and kills more than 100, raising awareness of microbes in water supplies.

1994: The first genetically modified tomatoes hit the U.S. market.

1995: Wolves are reintroduced into Yellowstone and central Idaho. The initial awkwardness quickly fades.

1996: Public drinking-water suppliers are required to inform customers about chemicals and microbes in their water.

1997: The U.S. joins other countries in Kyoto, Japan, to negotiate a global climate-change treaty it winds up rejecting.

1998: Earth has its warmest year since record keeping began in 1880.

1999: The EPA announces new rules to improve air quality in national parks and wilderness areas. Somewhere in the distance, a coyote coughs.

2000: High temperatures and low rainfall spark the worst U.S. wildfire season in 50 years.

2001: The U.S. formally rejects the Kyoto treaty. The treaty suffers brief self-esteem issues before hooking up with Europe on the rebound.

2002: The U.S. suffers its second-worst wildfire season in 50 years.

2003: The EPA retrofits 40,000 school buses nationwide to cut back their tailpipe emissions.

2004: The EPA requires cleaner fuels and engines for farm and construction equipment.

2005: The 2005 Atlantic hurricane season produces a record number of tropical cyclones, including Hurricane Katrina, which devastates the Gulf Coast.

2006: An Inconvenient Truth is released, winning Al Gore an Oscar, a Nobel Prize and a lifetime of being criticized every time it snows.

2007: The bald eagle is removed from the endangered species list.

2008: The EPA releases a list of "eco-fugitives." Captain Planet comes out of retirement.

2009: Something happens in Copenhagen, but no one is sure what, if anything, it is.

2010: People around the world celebrate the 40th Earth Day, once again dedicating a full day to the planet's health. The Earth is touched, even though it creates days in the first place by rotating, which means "Earth Day" is a regift. But it's the thought that counts.

Sources: EPA, U.S. Energy Department, U.S. Fish and Wildlife Service, National Oceanic and Atmospheric Administration, NASA

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Earth Day: An animated tribute
In honor of Earth Day, MNN takes an animated look back at several decades of U.S. environmentalism in this video.