What makes a city?

The answer to that question has been in a state of flux for centuries, prompting endless academic, philosophical, and aesthetic debate. Among other challenges, the distinct identities of urban centers around the world make generalization almost impossible.

It’s becoming increasingly clear, however, that sustainability must be a part of that definition. The global population is becoming ever more urban: By 2050, two-thirds of the world’s population will live in cities, according to the United Nations. Faced with the prospect of all those people crowding into the same geographic space, consuming energy, and producing waste, major cities are already demanding innovation in sustainable and green practices.

As Gregory Hayes, president and CEO of United Technologies Corp., put it at an Atlantic Live conference on “Growing Cities,” sustainability “is not an option for our business going forward. It’s an imperative.”

Every sector imaginable is responding to that imperative—from urban planners, city officials, and business owners to artists and chefs. All those actors contribute to a city’s DNA, bringing ideas and technologies old and new to the task of inventing the future of cities.

Take, for example, acclaimed chef Dan Barber, who has won multiple James Beard awards and is co-owner and executive chef at the high-end New York restaurant Blue Hill. He also founded WastED, a community of participants in fine dining and food production who have committed themselves to a vast reduction in the city’s food waste. Since 40 percent of food served in the U.S. goes to waste, our future cities are going to need people like Barber, who creates appealing dishes composed of ingredients that would otherwise be discarded.

Photo courtesy Elena Olivo Photography.

Barber argued at the Growing Cities event that what he does isn’t a new practice. Chefs have always made it their job to use their material efficiently, he said. They just haven’t “worn it on their sleeves.”

“These are things that people all over the world are already doing to some extent,” he said, referencing dishes like pot-au-feu and bouillabaisse as examples of traditional cuisine that have always been based on leftovers. Barber just wants to transform that traditional practice into an ethic, a movement, and the basis for a new and wonderful cuisine. “How do we transition?” he said later. “It seems like we have a really great opportunity right now to create a culture that makes that transition a lot more palatable, a lot more friendly, a lot more delicious.”

Barber’s approach is being applied in a very different sector: green, sustainable architecture and building construction. According to Russell Unger, executive director of Urban Green Council, a New York-based non-profit that focuses on making buildings more environmentally friendly, the best path to sustainability will be through upgrading current infrastructure rather than counting on future projects to bring the green revolution to our built environment.

“If you think of the building stock as an ever-evolving organism, we’re going to get more efficient,” says Unger, who believes that the need for sustainable cities is sufficiently urgent to drive best practices in future renovations and other construction projects.

Since established cities are unlikely to be rebuilt from the ground up, the Atlantic Live panelists agreed that moves toward sustainability in whatever sector need to be continuous and incremental. Installing triple-paned windows, high-efficiency cooling systems and LED lighting may not be flashy, but they are steps in the right direction. Unger’s approach, much like Barber’s, is founded on making sustainability a core value of our culture and economy.

“My hope is that in 50 years, [sustainability] becomes a part of our economy, a standard in our economy, and a standard expectation in our culture,” says United Technologies Corp. chief sustainability officer John Mandyck. In his view, the movement toward a sustainable future doesn’t require the invention of new technologies: “We have nearly all the technologies we need today to achieve sustainable urbanization.”

What it does require, as Unger puts it, is a sense of urgency: “In our world, the question is not whether we’re making things green. It’s ‘Are we making things green fast enough?’”

Nilda Mesa, director of the New York City Mayor’s Office of Sustainability, agreed that upgrading and retrofitting building stock, as well as partnership and collaboration between the private and public sectors, are critical for the future of cities. The information base for an urban green revolution already exists—in data collected by buildings, users, and local governments. But cities can also mobilize action through goal-setting, as New York City has done by committing to cut 80 percent of greenhouse-gas emissions by 2050. Mandyck believes the private sector can and should respond to those trends. “The private sector can do what it does best,” he says. “It can innovate. It can find ways through creativity and new technology to meet those goals.”

The most compelling force for progress—cited across the food, architecture, financial, and municipal sectors—is public demand. The green movement has been driven largely from the bottom up, as citizens and residents have incorporated the vocabulary of sustainability into their everyday lives and what they ask from their employers, governments, and products. “The only way we can make change at this scale,” says Unger, “is to use the market forces that exist now, hijack them for our own social purposes, and make our cities and our buildings better over time.”

At Growing Cities, United Technologies’ president and CEO Gregory Hayes made the point that private corporations can help lead the way. “We need to translate that understanding into investments and into future innovation,” he said. “It will come from a groundswell of support from the world’s population and the need for sustainability for our employees and our customers.”

The message emerging from the Growing Cities event was that the ideas, tools, and demand necessary to create sustainable, energy-efficient cities already exist. What’s needed is a sense of urgency about how little time there is to do it. As Unger put it, “We can do this. There’s really no question. The challenge is: Can we do it in the timescale that we have available to us? We have no choice right now but to do our best.”