In 2000, James R. Milkey, an environmental lawyer for the state of Massachusetts, became increasingly concerned about his state’s eroding coastline. It was obvious to him what was happening. Increasing concentrations of heat-trapping gases, like carbon dioxide, were causing sea levels to rise, slowly sinking his state. Milkey moved to Denmark for a year with his wife, and spent his time there pondering what could be done to avert catastrophic global warming. Before he returned home, in 2001, he had an idea: Massachusetts would sue the Environmental Protection Agency.

The case seemed ludicrous at first. Milkey insisted that, under the Clean Air Act, the E.P.A. was required to regulate greenhouse gases in the same way that it regulated air pollutants, and its failure to do so was causing irreparable harm to Massachusetts. Many conservatives laughed at the idea of the federal government treating carbon dioxide the way it treated asbestos, hexachlorobenzene, titanium tetrachloride, or any of dozens of other chemicals with scary-sounding names.

By 2006, Milkey was standing before the Supreme Court arguing his case, and he no longer seemed crazy. But the idea that carbon dioxide, a substance that all humans exhale, should be treated like a poison still met resistance.

“Your assertion,” Justice Antonin Scalia, who was skeptical about the link between greenhouse-gas emissions and Massachusetts’s disappearing coastline, said, “is that, after the pollutant leaves the air and goes up into the stratosphere, it is contributing to global warming.”

“Respectfully, Your Honor, it is not the stratosphere,” Milkey responded. “It’s the troposphere.”

“Troposphere, whatever. I told you before, I’m not a scientist. That’s why I don’t want to have to deal with global warming, to tell you the truth.”

Scalia was not alone in not wanting to deal with global warming. Until Monday, when Barack Obama’s E.P.A. announced new regulations to combat climate change, many environmentalists questioned the President’s commitment to the issue.

In fact, Scalia’s line of questioning has haunted climate policy for years. “But is it”—carbon dioxide—“an air pollutant that endangers health?” Scalia asked Milkey. “I think it has to endanger health by reason of polluting the air, and this does not endanger health by reason of polluting the air at all.”

Scalia was right, of course, in the sense that, if you stand around in a room full of humans exhaling carbon dioxide, you are in no danger, at least not from the air. And yet Milkey politely explained that Scalia’s example was nonsensical. If the Clean Air Act meant anything, it had to take into account the over-all effect of emissions. For example, he said, the E.P.A. regulated sulfur dioxide not because it was harmful to the air around us but because it was harmful after it left the air and washed out as acid rain. Carbon dioxide was harmless—that is, until it became so concentrated in the troposphere that it threatened to cook the planet.

In a five-to-four decision, the Court adopted Milkey’s logic and ordered the E.P.A. to treat carbon dioxide as an air pollutant. In 2009, Obama’s E.P.A. did exactly that, but it wasn’t until Monday, more than seven years after the case was decided, that the Obama Administration placed serious restrictions on carbon emissions, limiting carbon pollution from coal-fired power plants, one of the largest sources of greenhouse-gas emissions.

The new regulations are relatively modest, and they alone cannot stop runaway global warming. But the regulations mark a historic moment in Obama’s Presidency. He has broken a cultural taboo by firmly declaring that carbon dioxide could indeed destroy us, and a statement like that from the President might just make it more likely that the public, which intuitively and understandably has a more Scalia-like view of the effects of carbon dioxide, will begin to think of carbon pollution in the same way that it thinks of other kinds of toxic substances spewed into the air.

“Right now, there are no national limits to the amount of carbon pollution that existing plants can pump into the air we breathe,” Obama said on Saturday. “None. We limit the amount of toxic chemicals, like mercury, sulfur, and arsenic, that power plants put in our air and water. But they can dump unlimited amounts of carbon pollution into the air. It’s not smart, it’s not safe, and it doesn’t make sense.”

Obama’s evolution on carbon pollution—from timid to bold—is similar to his evolution on other cultural issues that attract a disproportionate response from his political opponents: gay marriage, marijuana policy, and gun control. Public opinion on all these issues has been gradually moving in a liberal direction, though Obama has approached each one warily, waiting until circumstances ripened before taking strong action. It was only when public opinion in support of gay marriage reached the threshold of fifty per cent—and was publicly endorsed by Joe Biden—that Obama publicly reversed his own position and came out in favor of allowing same-sex couples to marry. It took successful grassroots legalization campaigns in several states for Obama to mellow his drug-war view of marijuana laws. And it took an unfathomable slaughter of innocent children in Connecticut before the President proposed serious gun-control legislation.

On global-warming policy, Obama has been a more consistent advocate of action, but after his comprehensive climate-change legislation died in Congress, in 2010, his E.P.A. was slow to fill the policy vacuum with its new regulatory regime. Yesterday, he silenced most of the doubters, perhaps strengthening his bargaining position in the next round of global climate-treaty negotiations and surely making it difficult for a future President to reverse the federal government’s treatment of carbon pollution.

It’s hardly unheard of for a President to be cautious about pushing social change, and it would be more surprising if a President didn’t move in the direction of shifting public opinion. Obama and his aides like to see him as someone who plays a long game. They sometimes suggest that his movement on these issues is all part of a grand plan. More likely, Obama is what might be called a “left conservative,” a phrase that Norman Mailer briefly popularized when he ran for mayor of New York, in 1969. Obama obviously shares the outlook of the left on these cultural issues, but he’s temperamentally cautious and rarely believes that it’s worth his effort to act until his own liberal base has moved the country along with it. And, even then, he sees his job as moderating the passions of the activists.

But this interaction between Obama and an activist left that is slowly pushing the country in its direction—especially among younger Americans—is becoming the main subplot of the Obama years. While people like James Milkey push for change at the bottom, they are increasingly finding an ally at the top.

Photograph: Charles Krupa/AP