On March 17th, the International Energy Agency announced that 2016 marked the third year in a row that global carbon emissions had stayed at the same level while the world’s economy grew. This three-time repeat has put to rest any lingering suspicions of gremlins in the data. Something new is happening. The global economy has now grown nearly ten per cent without any increase in the annual CO 2 emissions that are the principal human contribution to climate change. In the parlance of sustainability, growth and emissions appear to have “decoupled.”

Nearly as remarkable was the star of the latest announcement: the United States. Long decried as a climate-action laggard, America led the world in reducing carbon pollution in 2016, with a decline of three per cent. More important, this improvement was not tied primarily to the evolution of its economic system. Wealthy nations have long been moving toward economies anchored in finance and services, which produce less pollution, but, by continuing to consume, they effectively outsource carbon pollution from manufacturing to other countries. While the Global Carbon Project estimates that U.S. emissions should still be adjusted upward by about ten per cent to account for products consumed in America but made elsewhere, recent gains within the United States have been achieved by burning less coal and more natural gas, which is a cleaner fuel, and, to a lesser extent, through an increase in renewables. If that’s not news enough, G.C.P. scientists reported, in a paper published in January, that current trends in energy are “broadly consistent” with Paris Agreement targets for 2030, which aim to limit global warming to less than two degrees Celsius.

This is shout-it-from-the-rooftops stuff. Since 1972, and the publication of “The Limits to Growth,” there has been debate over whether economic growth and a sustainable environment are incompatible. Can humans live with the comforts to which we have become accustomed (or aspire to)—air-conditioning, cars, constantly updated wardrobes—without doing environmental harm? “The Limits to Growth” both warned against infinite growth on a finite planet and acknowledged that “it is success in overcoming limits that forms the cultural tradition of many dominant people in today’s world.” That dominant world view has not shifted. Decoupling economic activity from its ecological consequences is central to the goals of international sustainability and development. It’s the foundation of American faith that technology can resolve climate change without the need to substantially change our life styles. It’s the holy grail of “green growth.”

If the I.E.A.’s announcement didn’t quite become a eureka moment, it’s in part because the Trump Administration's plans suggest that current trends are about to shift. On Friday, the Keystone XL oil pipeline from Canada received a Presidential permit, while a proposed budget released by the White House earlier this month would end spending on federal climate-change research and prevention programs. As Amy Davidson and others have described, the Trump Administration is hostile toward efforts to combat climate change, and it has promised that energy and climate policies will focus on reviving the coal industry, reducing carbon-emission standards for vehicles and power plants, and revisiting requirements that climate change be considered, and its costs accounted for, in decisions by federal agencies. Critics have pointed out that at least some of these efforts are unlikely to succeed (the I.E.A. emphasized that America’s 2016 emissions drop was driven by markets and new technologies as much as by policy), but any one of them would increase carbon emissions. A historic advance against climate change is threatened with reversal.

The about-face is compounded by the scope of what still needs to be accomplished. Laura Cozzi, the head of the I.E.A.’s energy-demand-outlook division, told me that the over-all slowdown in emissions “underlies a three-speed story.”

Moving fastest toward reducing emissions are the world’s richest nations, the U.S.—for now—among them. The second of Cozzi’s speeds is represented by China, the world’s greatest carbon polluter. (America is No. 2, and among the highest polluters per capita.) Switching from coal to gas accounted for most of the decrease in Chinese carbon, too, but so did what Cozzi called a “tremendous push” toward renewables and nuclear power. Grinding in the lowest gear is most of the rest of the world, where emissions are still rising. In short, some of the world is decoupling, but much of it is not.

Despite the uplifting talk of decoupling, it is probably more accurate to say that economic growth and carbon emissions remain rather tightly coupled. Until recently, emissions had stood still or declined only during economic downturns. That’s no longer the case, but the slower pace of growth, especially in China, the U.S., and Europe, has been an important silent partner in the stalled emissions of the past three years, narrowing the ground that clean energy and efficiency need to cover in order to achieve decoupling.

“We’re at a fragile stasis in emissions,” said Rob Jackson, a Stanford University environmental scientist and chair of the Global Carbon Project. “If the global economy were booming, emissions would not be flat.”

Few climate scientists believe that we’ve reached peak emissions. The Chinese government has reserved the right to increase its carbon pollution until 2030 as it plays developmental catch-up with North America and Europe. In the U.S., Trump’s anticipated policy shifts aren’t the only concern; so is the Trump Bump—an outbreak of investor-class optimism that could accelerate growth to a point that puts emissions back on an upward trajectory. Then there is India, which is rich in coal and counts three hundred million people just on its list of citizens waiting for household electricity. “India’s emissions now are about what China’s were in 1990,” Jackson told me. “If India develops in the same way, using the same fuels that China did over the past twenty-five years, we’re in trouble.”

The sheer scale of emissions also remains a problem within the decoupling calculus. When emissions flatlined, they did so at the record high set in 2013. The best estimates suggest that we need to cut emissions in half just to keep CO 2 from continuing to accumulate in the atmosphere. In other words, decoupling remains a staggering, colonies-on-Mars-scale task, and is likely to get harder as straightforward solutions, such as switching from coal to natural gas, reach their limits. Recent research, using Australia as a case study, applied optimistic projections of decoupling against a modest rate of growth and still found that, by the year 2100, demand for energy would increase by more than two hundred and fifty per cent—all of which would have to come from energy sources that produce next to zero carbon pollution.

“We didn’t seem to be able to have high growth without growing emissions, which suggests it’s hard, at the very least, to get very major reductions in emissions without some change in the scale of the economy,” Richard York, an environmental sociologist at the University of Oregon who studies resource consumption and pollution, told me. We seem to have found a painless way to slow the increase in emissions; now we only need to find a similar solution for growth itself.