Earlier this month, scientists announced that humanity has kicked off the first major “tipping point” — in which an Earth system dramatically transforms, often permanently — as warm-water corals die en masse due to relentlessly rising temperatures. Think of such events like driving off a cliff: There’s no reversing back up to the edge, and the impact will be terrible.

For all the attention these disastrous milestones get from scientists and the media — and rightfully so — less discussed is the fact that they also work the other direction. Positive tipping points can unfold on a wide range of scales, from a community garden helping a neighborhood eat healthier, all the way up to the global energy system switching from fossil fuels to renewables. An individual person can even reach one, like if they decide to do more and more walking instead of driving. 

People can influence communities, communities can influence cities, and cities can influence nations. These critical junctures, then, can spread like a contagion — in a good way. “It’s rather a mirror opposite of the damaging Earth system tipping points that we want to desperately prevent,” said Steve Smith, a researcher at the University of Exeter’s Global Systems Institute, who studies how to encourage the phenomenon. “Because the positive tipping points are changes that we really need to promote.”

What makes environmental tipping points so insidious is that they self-perpetuate as threats amplify other threats — in the case of corals, high temperatures combining with ocean acidification and marine pollution. Luckily, their happier counterparts also accelerate under their own momentum and feedback loops, as benefits beget other benefits. There are many ways that cities, for instance, can encourage the adoption of green technologies like electric vehicles, according to a new report from C40, a global network of mayors tackling the climate crisis. “It is possible to move pretty quickly, at the moment, because of the wide availability of these technologies,” said Cassie Sutherland, managing director of climate solutions and networks at C40. “You can actually get — with a kind of dedicated intervention — quite a significant and ramped-up change.”

By wielding both carrots and sticks, policymakers can juice these social and technological systems to get things rolling. In the parlance, a “pull” policy makes a sustainable technology more affordable, accessible, or attractive using incentives like tax rebates, drawing people toward it. It’s carrots made of money, basically, that can create unstoppable market momentum. The stick, by contrast, is the “push” policy that, say, makes fossil-fuel technologies more expensive, less convenient, or unavailable, like banning new natural gas hookups in buildings. Push and pull policies are not mutually exclusive, and indeed policymakers can use them in concert for maximum effect.

Cities are uniquely positioned to do this kind of work, Sutherland says. Globally, they’re home to more than half the human population — and rapidly growing — yet are responsible for 70 percent of carbon emissions. But they’re also much more nimble than national governments because they set building energy efficiency codes, manage public transportation systems, and pursue their own targets for reducing emissions overall. By contrast, the U.S. government is now actively hostile to climate action, so it’s up to cities to increase their ambition. “They’re the crucibles, the test beds, the ones that have the ability to go further, faster, and particularly go first,” Sutherland said.

Municipalities, for example, have the power to embrace — and ideally push to a tipping point — one of the more powerful climate solutions: the e-bike. Pedaling instead of driving slashes greenhouse gas emissions, alleviates urban congestion, and improves public health, both because of the additional exercise and reduced air pollution. (E-bikes also require less exertion for people who might struggle on a traditional two-wheeler.) In a given city, a vocal group of dedicated bicyclists might start out advocating for basic infrastructure, like dedicated lanes. “Bike commuting has a big positive feedback effect,” said Cameron Roberts, a social scientist at Carleton University in Canada, who studies active mobility. “Once they start winning victories, this leads to better road regulations, better infrastructure, which brings out more cyclists, which then snowballs.” The data backs this up: Bike commuting in Washington, D.C. and New York City doubled in four years, in part because of improved infrastructure.

Which is not to say that cities can’t also encourage the switch from gas to electric vehicles, like Oslo, Norway has done with extraordinary speed: In just the past decade, the market share there for new EV sales grew from 13.6 percent to 95.8 percent, C40’s report notes. (Ironically, Norway is one of the world’s biggest exporters of fossil fuels.) That was thanks to the government providing financial incentives, thus making EVs more affordable, then mandating that all new car sales be zero-emission by 2025. (Cities can do this with public transportation, too: Mexico City, for instance, gave operators of gas-powered buses an eight-year heads up that starting in 2025, it would only procure electric versions.) Oslo then layered on top of that sweetener by expanding charging infrastructure, thus making it more convenient to own an EV. “It’s cities that have a lot of the control over the bits that make it accessible and attractive,” Sutherland said.

Oslo did much the same for heat pumps, which are now in 63 percent of Norwegian households. Instead of burning fossil fuels, these electric appliances extract heat from even frigid air and pump it indoors, then reverse in the summer to cool a space by expelling indoor heat to the outside. To encourage their adoption, Norway slapped a carbon tax on heating fuel, making it increasingly expensive not to go fully electric, and provided financial incentives to make the switch. Oslo, once again, layered on its own subsidies, streamlined the permitting process for installing the devices, and implemented stricter energy efficiency standards for buildings. (In the U.S., market share has been growing too, as heat pumps now outsell gas furnaces. States have also formed a coalition to accelerate adoption.) 

Policymakers can push emerging technologies to tipping points, too. For instance, last year in Framingham, Massachusetts, Eversource Energy commissioned the nation’s first networked geothermal neighborhood operated by a utility. This technology also exploits heat pumps, which use liquid coursing through a network of underground pipes to cool and heat homes. Other states and cities could encourage more projects like this with the stick approach, like by banning natural gas in new buildings and continuing to pressure utilities to switch from fossil fuels to cleaner technologies. With the carrot, they could provide financial incentives for communities to transition to networked geothermal, which remains expensive. 

Thinking bigger, the various parts of the global energy system must tip from fossil fuels to renewables. Whereas oil and gas are a stagnant technology, renewables benefit from “economies of scale” and “learning by doing,” meaning the more you make something, the cheaper and better it gets. Accordingly, the price of solar panels has plunged more than 99 percent since the 1970s. The explosion of wind power, too, enabled the United Kingdom to pass a tipping point as it rapidly retreated from coal, helped along by the country putting a price on carbon. “The price of coal just became quite quickly uncompetitive and uneconomical, to the point where last year the final coal-fired electricity generator was shut down,” Smith said.

The rapid development and adoption of renewables, Smith added, is due in large part to a “positive tipping cascade” made possible by batteries, which get better and cheaper year after year. That’s created a domino effect that is boosting all kinds of sectors: electric vehicles and trains, home energy storage, and the grid, which uses huge packs to store solar and wind power for when the sun isn’t shining and wind isn’t blowing. Utilities are also experimenting with “vehicle-to-grid” technology, in which EVs both draw power and send it back to the system, providing a vast network of backup energy to further accelerate decarbonization. These intertwining market forces behind renewables and batteries are so powerful that the Trump administration can only hope to slow the green energy revolution in the U.S., not stop it. 

But cities and nations can’t just encourage positive tipping points and call it a day — that needs to happen alongside the urgent mitigation of the pollutants that are causing warming in the first place, said Kiff Gallagher, a climate strategist and founding executive director of the Global Heat Reduction Initiative. Carbon dioxide gets all the attention, but humanity can dramatically and quickly reduce warming by tackling “superpollutants,” greenhouse gases that are dozens, hundreds, even thousands of times more powerful than CO2. 

Food waste rotting in landfills, for instance, as well as fossil fuel infrastructure, produce clouds of methane, which is 80 times more potent, yet disappears much faster from the atmosphere. (Agricultural waste, like corn stalks, is often left to rot too, so increasingly farmers are turning it into biochar, which instead captures carbon from the atmosphere and improves yields when applied to fields.) Ironically enough, heat pumps are essential for weaning off of fossil fuels, yet a common refrigerant that lets them work their magic is 2,000 times more powerful than CO2. So the industry is transitioning to more sustainable alternatives, even using CO2 itself as a refrigerant because it doesn’t cause as much harm in the unlikely event of a leak. “These short-lived climate pollutants cause almost 50 percent of the warming, but they’re only receiving 5 percent of climate finance right now going to directly address them,” Gallagher said. “So that could be an amazing high leverage point for the world.”

Tipping points, then, are both an environmental curse that we desperately need to avoid, and an essential phenomenon we need to exploit to keep more of Earth’s systems from tipping. Individual sectors can positively tip, and in turn influence other aspects of the clean-energy economy — momentum building upon momentum. “Once those tipping points have then been reached,” Sutherland said, “and there’s the positive feedback loop, you’ll start to see the other elements of the solutions of the system clicking into place a bit more easily.”