Let’s imagine for a moment that we’re at 2100, and the atmospheric CO2 level is slowly subsiding back toward 350, and the worst is over. Let’s try to figure out how we got there—reverse-engineer a century of halting but ultimately decisive progress.

The first decision, clearly, was the most important. In 2011, after 22 years of hemming and hawing and circling, the world’s governments—moved by a series of devastating floods on every continent that galvanized the already growing climate movement around the globe—grudgingly took the initial steps toward imposing a cap on carbon emissions. The fight was by no means easy: developing countries insisted, with reason, that the cap couldn’t hit them yet, and China insisted that it was still a developing country. Still, the climactic political battle with big oil and bigger coal ended decisively—it would be many years before they ceased to be powerful parts of the economy, but the fossil fuel era began to end on that day when the parties signed on at the Nairobi conference center.

A few things happened, and more quickly than anyone but the economists had dared hope. For one, anyone looking at a spreadsheet quickly figured out that future investment had better be green—that coal-fired power was going to get steadily more expensive until it made no sense at all. And so the trajectory of the future began to shift: money started to fall in the direction of the new economic gravity. It started to pool around railroads, and insulation manufacturers, and all the other businesses that featured relatively low energy costs as a selling point.

The effect on consumers was not quite as strong, since few households had CFOs charged with plotting the bottom line return of future expenditures. Still, every family was now getting a rebate check each month for its share of the permits for putting CO2 into the atmosphere, which meant a steady flow of capital. Some of it went for flat-screen TVs, but a nontrivial amount ended up buying solar hot-water heaters, plug-in hybrids, and local tomatoes.

Meanwhile, governments started figuring out where the future was headed too, and the political demand for greater investment in basic research began to grow. Helpfully, with each passing legislative session, the pockets of the wind and solar barons got a little deeper, and they began to exert more and more pressure for switching subsidies away from the “technologies of the past.”

None of it, though, happened anywhere near fast enough to slow down the momentum of the heating. Year after year saw catastrophe after catastrophe. Human-caused temperature change, barely one degree when the decade began, edged toward two degrees, and the toll of damage steadily mounted. Some of it was insidious and daily—like the steady drip-drip of lost agricultural yield as temperatures climbed and water evaporated and the continuing spread of disease-bearing mosquitoes, which damaged not only ever-larger populations but also the development budgets of one nation after another.

As time went on, it became increasingly clear that there was no way simply to pull the internal combustion engine out of the world’s economy, toss in a few solar panels, and continue on as before. Not only was the drag on old economies from changes in the weather creating real friction, but the logic of renewable energy began to assert itself. Sun and wind were everywhere, but diffuse. And so a new kind of power grid began to grow—based on many million rooftops, not a few thousand centralized power stations. Other commodities began to go in the same direction. After a century of agricultural consolidation, for instance, local food networks were spreading fast, replacing much of the oil-starved, monocultural, industrial food system that had dominated the planet since World War II.

If it sounds as if this happened smoothly, however—well, it didn’t. The immense gulf between the rich and poor worlds was the most intractable problem, as people across Asia, Africa, and South America felt themselves being denied the fruits of modern development. The outbreaks of chaos were ugly, as migrants from low-lying areas tried to move inland across India and China and refugees from newly formed African desert edged outward onto land already fully occupied. And there were always the floods, now a perennial feature of any wet part of a planet whose atmosphere was much moister—adding constantly to the woes of already stressed populations.

Europe, Japan, and America—and increasingly China and India—did a little to help, but the technology assistance and crisis aid never came close to matching the damage their carbon emissions had caused. Mortality rates climbed all over the planet, and life expectancy dropped. There was some of the Malthusian horror long predicted, and skirmishes and wars were constantly breaking out. But there was also a much more popular and general political uprising of people around the world who insisted that the push toward climatic stability go faster, no matter the cost. The iconic number 350 came to mean one thing above all: shutting down the coal mines and the tar sands, keeping the carbon in the ground. And within a few decades, this had—more or less—happened. The world was running rough, but still running, with the Internet providing the kind of links that jet planes had once allowed.

At last, the level of carbon in the atmosphere began to plateau. Smaller increases—measured at the station on the side of Mauna Loa where this science had begun in the 1950s—gave way to tiny decreases, as forests and oceans slowly began to suck some of the carbon back below the surface. This did not “make the problem go away,” and, in fact, as the century wore on, researchers began to show that even 350 ppm of CO2 was too much, that we needed to retreat closer to the 280 ppm level that prevailed in the days before the Industrial Revolution. There was no way to refreeze the Arctic, and ocean productivity continued to dwindle because of elevated levels of acid. But at a certain point, the volume of crises began to slowly diminish, both because temperatures had nearly stabilized and because society had been rebuilt in ways that made it more resilient, less vulnerable.

The most essential things—a culture, a civilization, some semblance of the natural world—had come through the bottleneck more or less intact. It had been a miserable century, but not, in the end, a completely impossible one.

That this is a good news scenario should give us pause. It would be easier, and perhaps more plausible, to write a much uglier forecast. A few things worth noting here: First, action to change the price of carbon comes very early in this scenario, in 2011. It’s pretty clear we need to tip this system quickly in another direction. Second, the decisive interventions aren’t technological as much as political—in many ways, the outcome will be decided by whether people pull together or are pulled apart as a result of the forces we’re unleashing in the atmosphere. There are many variables we can’t predict, including that one. But at least we can have an influence—by building a political movement right now, across borders, faiths, ideologies, and languages, that allows us to understand our novel global predicament.