After millennia of human existence, in 2008, we hit a tipping point. For the first time, half of the human population lived in a city. That trend continues growing, and by 2014 that was up to 54 percent .

But that growth is not without a cost.

Urban areas eat up two thirds of the world’s energy, and account for 70 percent of our collective carbon emissions. If we want any chance at fighting climate change, cities need to be front and center. Thankfully, they are. Not only are cities around the world rising up to meet the climate change challenge — but they’re finding smarter, cheaper and faster ways forward than thought possible.

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From Shenzen, China’s 16,000 buses going electric by 2017 to Hamburg, Germany’s partnership with UPS to deploy bicycle deliveries, to Vancouver, Sydney and Salt Lake City going 100 percent renewable, cities around the world are experimenting with novel approaches to zero out emissions.

With the fewest hurdles to trying policy and the most ground to cover in terms of reducing emissions, cities have become the place where transformative climate policies are happening fastest. In just the last three years, hundreds of cities have begun implementing transformative solutions that aim to zero out carbon from their buildings, transportation and electricity systems.

Looking across the wide sweep of municipal actions on climate change, it is now possible to piece together a “playbook” of game-changing policies. While few adopt all the solutions, it is now possible to see the required playbook being implemented piece by piece.

After 25 years of incremental experimentation since the 1992 Rio Earth Summit, there has been a sudden increase in the ambition of climate commitments. Advocates have termed these targeted policy proposals “science-based commitments and policies,” an unglamorous name for an otherwise exciting development.

In California, cities are following Santa Monica’s lead in requiring all new homes be zero emissions. Even better, all new commercial buildings in the state will be zero emissions by 2030 — a big deal since buildings are generally the largest single carbon source in cities.

Another major source is transportation. Here we see another set of solutions. This includes bike deliveries in Germany that eliminate idling delivery trucks fouling the air and taking up space. It’s mass transit investments and electric busses in L.A., a carbon neutral transportation system in Vancouver by 2020 and the phase out of gas engines in Beijing.

Going a level deeper into infrastructure, some cities are cutting off fossil fuels at the source, like Portland and Seattle’s decision to prevent any new or expanded fossil fuel infrastructure. No new pipelines, port facilities or rail lines to ship out fossil fuels, and no new power plants to burn them.

Those are straightforward changes. But they’re far from the only ones. Five cities and three provinces in China have initiated a carbon cap and trade system to limit and gradually reduce pollution. This has been an explicit test for the Chinese government as it weighs its options for scaling up the system nation-wide.

Making polluters pay is one way to raise the funds to finance emissions reduction projects. But it’s not the only one. The City of Washington, D.C. established a Green Bank in 2017, aimed at funding climate-friendly projects. Congestion pricing, where drivers pay extra when streets are most crowded, has started in Milan, Stockholm and London, sending revenue to mass transit improvements.

As these cities test the low-carbon waters to find what works best, groups like C40 and the Carbon Neutral Cities Alliance work to spread the ideas, helping cities learn from one another. It’s no small feat to take the gas out of your transportation system, so connecting the engineers and city planners with their peers means each city doesn’t have to recreate the wheel.

Because when it comes to carbon emissions, we don’t exactly have time to waste or money to spare. We need to find the best answers, fast. But with hundreds of cities across the globe all working toward a common goal, we can filter out what’s least efficient, and focus on what works. As economies continue to turn from manufacturing to service, and service to information, cities are only becoming more central to the human experience.

We’ve never been a species to shrink from a challenge or shy away from the pursuit of knowledge. By focusing our talents on the climate challenge, we’re putting that to good use, each city its own source of light, shining on a hill.

Michael Northrop is the program director for the Sustainable Development grantmaking program at the Rockefeller Brothers Fund, where he focuses on climate change and energy. The views expressed here are solely those of the author and not of his employer.