Nearly half a century ago, when smog choked numerous U.S. cities and toxic waste fouled many waterways — most infamously, Cleveland’s chemically contaminated Cuyahoga River, which spontaneously caught fire — a small group of activists founded Earth Day principally to help curb air and water pollution.

Sen. Gaylord Nelson of Wisconsin, a leading organizer, modeled the event after teach-ins that protested U.S. involvement in the Vietnam War, and persuaded some 20 million Americans to attend thousands of peaceful demonstrations at colleges, universities, schools and parks across the country.

That first Earth Day, 45 years ago today, is considered the birth of the environmental movement, and within a decade it inspired landmark legislation:

[naviga:ul]

[naviga:li]Formation of the U.S. Environmental Protection Agency in 1970.[/naviga:li]

[naviga:li]Approval of an amendment to the Clean Air Act in 1970 to set national air quality, auto emission and anti-pollution standards.[/naviga:li]

[naviga:li]Passage of the 1972 Clean Water Act, limiting sewage and pollutants from entering rivers, lakes and streams.[/naviga:li]

[naviga:li]Passage of the 1973 Endangered Species Act to protect plants and animals threatened with extinction.[/naviga:li]

[naviga:li]Passage of the 1974 Safe Drinking Water Act.[/naviga:li]

[naviga:li]Passage of the 1976 Resource Conservation and Recovery Act, regulating hazardous waste from production to disposal.[/naviga:li]

[naviga:li]Passage of the 1976 Toxic Substances Control Act, restricting the use of chemicals in food, drugs, cosmetics and pesticides.[/naviga:li]

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Earth Day has grown to an international event observed in 192 countries, continuing to draw attention to some of the same issues but now focused on topics that were far off the radar in 1970 — most notably climate change and the concept of sustainability.

The Earth Day Network has organized thousands of activities today that range from planting trees to rallies, under the theme, “It’s our turn to lead.”

This overarching message is particularly appropriate at a time when too many people seem content to follow practices and policies that could doom our future. Simply put, we no longer can continue denying that a buildup of greenhouse gases has raised the Earth’s temperature and intensified storms, droughts, wildfires and other extreme meteorological phenomena.

“There’s no greater threat to our planet than climate change,” President Barack Obama said in his weekly presidential address on Saturday, adding that “2014 was the planet’s warmest year on record. Fourteen of the 15 hottest years on record have all fallen in the first 15 years of this century… Climate change can no longer be denied – or ignored.”

This newspaper agrees, and urges the United States to put its money where its mouth is by finally signing the Kyoto Protocol, an international treaty that commits nations to reduce greenhouse gas emissions. Since its 1997 authorization by the United Nations, 192 countries have ratified the treaty, but to our great shame, the United States has not.

This country has always taken pride in its role as a global leader when it comes to democracy, the economy and military strength, but unfortunately we also lead in less-desirable categories, such as energy use.

Americans constitute 5 percent of the world’s population but consume 24 percent of global energy. We must reverse this trend.

Last year, when gas prices spiked above $4 a gallon, more Americans bought smaller, fuel-efficient cars, but now that fuel prices have dropped to about $2.50, sales of SUVs and trucks have begun to soar.

It’s some consolation, though, that cars today are far more fuel-efficient than they were at the time of the first Earth Day in 1970, when the average auto got about 15 miles per gallon. Of course back then gas cost about 37 cents a gallon.

Also in 1970, the global population stood at about 3.5 billion. In 45 years it has more than doubled, and experts predict the number of people on the planet will reach 8 billion by 2026.

Even Pope Francis has underscored the moral responsibility of not reproducing “like rabbits.”

All in all, the United States and the rest of the world would be much better off if we adhered to the principles embraced by that first Earth Day on April 22, 1970, and articulated by the late Sen. Nelson: “The wealth of the nation is its air, water, soil, forests, minerals, rivers, lakes, oceans, scenic beauty, wildlife habitats and biodiversity… that’s all there is. That’s the whole economy. That’s where all the economic activity and jobs come from. These biological systems are the sustaining wealth of the world.”