Back when Sen. Gaylord Nelson of Wisconsin first proposed that April 22 be designated annually as “Earth Day,” the Cayahoga River that runs through Cleveland, Ohio had recently caught fire. The burning river quickly became the symbol credited with sparking the birth of the environmental movement in America.

But why that river at that time caught the attention of the entire nation when so many other American rivers were also badly polluted tells us something about how a nation changes its mind. Like so many other watershed (pardon me) events in American political life, the story played big because serendipity met the zeitgeist.

The Cuyahoga River actually had burned many times prior to the 1969 fire, which is why no one in Cleveland paid particular attention to the event; the following day the fire on the river was barely reported in the city’s primary newspaper, Cleveland Plain Dealer. But Carl Stokes, the first black mayor of a major American city, understood the symbolism of the event and held a news conference on the banks of the river to call for more aggressive pollution control. No one paid much attention to the news conference either.

But then serendipity raised its unexpected head. Writers from Time magazine happened to be nosing around Cleveland gathering background information for a major profile they were planning to run on Stokes’ historic election. And some smart, unnamed editor back in New York recognized that when a new black mayor of one of America’s largest cities elevates the issue of water quality over the threat of inner city job loss, perhaps the spirit of the times, that indomitable zeitgeist, had shape-shifted.

Ironically, the spectacularly eerie image of the night fire on the Cuyahoga that Time ran the following month was not a picture of the 1969 fire, but of the earlier 1952 fire on the river that had been much larger and more damaging. For those who love to hate the media, this part of the story might be viewed as a classic example of the media biasing its news coverage.

In fact, Time’s prose was highly provocative, describing a river that “oozes rather than flows,” and if someone falls in, they will “not drown, but decay,” and pointing out that not a single fish lived in that part of the river.

But a year later, the picture of the burning river helped turn Earth Day into a mass movement across the country.

In the 1950s, my parents proudly bought an 80-year-old falling down stucco house on the banks of the Hudson River, 20 miles north of New York City, from the estate of a pair of ancient spinster sisters who had lived there for half a century.

For a kid, the most glorious part of the new-old house was a winding path down to a little sandy beach at the river shaded by the branches of a huge tree. You could not quite jump from the outstretched tree into the river, but you could imagine it while splashing in the warm shallow water under its branches. When a few years later my father told us we could no longer swim in the river because it was unhealthy, like almost everyone else, I just shrugged. It was not until Earth Day many years later that I began to realize what had been lost.

Both the Hudson and Cuyahoga rivers have been cleaned up enough that you can swim in them and eat the fish you catch. After the expenditure of $3.5 billion, there are now 44 species of fish in the Cuyahoga, but since the two most common are hogsuckers and shiners, maybe you won’t be eating a lot of what comes out the river. But the clean up of the Cuyahoga is one of the signature legacies of the 1972 U.S. Clean Water Act, perhaps the most successful piece of environmental legislation in the history of the country.

I wish I could say the same for the Clean Air Act, which was also passed in 1972. Although the air we breathe has notably improved in terms of the tons of soot, sulfur dioxide and mercury that have been removed, the most dangerous pollutant in the history of human civilization continues to increase unabated. The problem is that most of us don’t think of carbon dioxide as a pollutant. And why should we? We exhale it everyday from our own lungs and plants inhale it to grow.

Since 2009, the Environmental Protection Protection Agency has officially classified CO2 as a pollutant, but the wise people of Washington figured that the EPA, which is the government agency many Americans love to hate for its supposed history of choking off economic growth, could not effectively lead on this issue. Instead, Congress would enact a grand bargain to limit CO2 emissions based on market-oriented legislation that proponents called cap and trade and opponents labeled tax and trade. Of course, we have since learned what has happened to grand bargains in Washington these days.

So now we are back to the prospect of regulating CO2 through the EPA. Because power plants emit about 40 percent of the carbon dioxide pollution in the United States, it may come as a surprise that there are no federal limits on utility emissions of this potent greenhouse gas.

When the EPA began drafting regulations to limit CO2 emissions from new power plants, the result was a national advertising campaign, headlined: “Carbon dioxide: they call it pollution, we call it life!” Should we be surprised to learn that fossil fuel companies funded this campaign?

It is impossible to know when the zeitgeist might offer us a new symbol equivalent to the burning river published 44 years ago or what serendipity will enable the symbol to penetrate the clutter of our lives. But a good first step is for all of us to call a spade a spade, by referring whenever we can to carbon dioxide as a potent pollutant.

Words matter.

Philip Conkling is founder and president of the Island Institute.