(I spoke at a climate change conference in Seattle the other day. Several participants asked for elaboration on a point I made from the podium. Here it is.)

If you tend to get discouraged by the news these days, you might enjoy Adam Hochschild’s article “Against All Odds” in the January/February “Mother Jones.” I sure did. It describes the campaign to end slavery in the British Empire, a movement that began as a meeting of twelve people in a London bookstore in 1787.

At the time, the British Empire was as dependent on slavery as northwesterners are today on—say—automobiles and fossil fuels. Three quarters of the world’s people were in legal bondage of one form or another: slavery, serfdom, debt peonage, or indentured servitude. Worse, such bondage was as accepted then as the consumer lifestyle is today. Slavery had been a fixture of human existence throughout history, predating even money. And avenues for protest were scarce: at the time, fewer than one in ten Englishmen—and no Englishwomen—even had the vote.

In the United States, as noted by Nobel Prize-winning economist William Fogel in his book “The Fourth Great Awakening & the Future of Egalitarianism,” the abolition movement also faced stupendous odds. The Constitution not only sanctioned slavery and disenfranchised slaves, it also stipulated that seats in the House of Representatives be allocated among the states based on their populations, with each slave counting as three-fifths of a person. Thus, slave owners’ power in the federal government increased in exact proportion to the number of slaves their states held. That’s a lock on power even Big Oil and the automakers would envy.

Still, within 50 years of the bookshop meeting, slavery was illegal throughout the British Empire, and within a century, the same was true in the Americas. Slavery became untenable, and its end—long thought impossible—became inevitable. What explains this extraordinary progression? The same two things that explain the success of history’s other great social change movements, such as those for democracy, women’s suffrage, workers’ rights, and civil rights. Each of these movements was organized on an unarguable moral principle—freedom, democracy, equality—by a corps of people, initially few, who would not rest until the world conformed to that principle. As their movements grew to the thousands and millions of followers, the impossible became the inevitable.

Our cause is no less compelling: to reconcile ourselves with our planetary home and thereby secure the future. Its moral power is no less rooted in ethical and religious teachings. And our numbers are already swelling into the tens of millions worldwide. The political currents sweeping the continent may sometimes obscure the fact, but we are living through (indeed, are bringing about) the transformation of another impossibility—an environmentally sound economy—into an inevitability.