Biking Through Denver, a Sustainable City

Mayors of more than 1000 American cities pledge to reduce greenhouse-gas emissions

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This podcast is part of the Sustainable Design radio program, a collaboration between IEEE Spectrum and the National Science Foundation.

Susan Hassler: Denver is one of over 1000 American cities whose mayors have pledged to reduce greenhouse-gas emissions. And it’s at the city scale that any national targets on sustainability will be carried out. But what does sustainability mean for a city, and how do you measure it? Lisa Raffensperger took to the seat of a bicycle to find out.

Andrew Duvall: You just tap your card on top of the dock; a second or two later it starts beeping. Pull the bike out and it’s ready to go. So all the seat posts are adjustable so you can raise the saddle height to where you like it, and the seat post is numbered so you can remember from one time to the next where to set it.

Lisa Raffensperger: Andrew Duvall is a Ph.D. researcher at the University of Colorado Denver. He’s intimately familiar with Denver’s public bike-sharing system, since he helped found its predecessor back in 2008.

Andrew Duvall: It’s got a bell, too, [dings] built into the…left handbrake, and it’s one of the most popular features of the bikes.

Lisa Raffensperger: Duvall’s research is within the Sustainable Urban Infrastructure project at UC Denver. The program is interdisciplinary, bringing together health science, engineering, economics, and other fields, to answer one big question: What makes a sustainable city? To see the answer up close, Duvall’s taking me to the streets of Denver on a rented bike, in search of a definition of sustainability. And we’ll end up at an experiment in sustainability being built right now in the heart of Denver’s downtown.

Lisa Raffensperger: Duvall and I start out on a bike path near a canal, then ride up onto the street, sharing it with cars. Once we get into downtown, though, the cars thin out, replaced by light rail and buses. The hub of all this action is a massive Romanesque building surrounded by sleek rail lines. And it’s our first stop.

Andrew Duvall: So we’re at Denver Union Station…and this is one of the most popular spots because it has connections to bus and light rail lines.

Lisa Raffensperger: The bike system links up to trains and buses to spin a whole web of alternative transit in the city. This saves on gasoline—and it has other benefits, explains Duvall.

Andrew Duvall: It also reduces reliance on a single source of transportation. There is more resilience built into the system.

Lisa Raffensperger: Reducing carbon emissions, alleviating traffic jams—those are definitely parts of the sustainability puzzle. As we continue our bike journey, though, it becomes clear that a lot of things that sustain cities come from outside—from the trains bringing out-of-town commuters to the very materials the city is built on.

Andrew Duvall: Here’s a cement truck carrying a load coming through the intersection. That cement was produced somewhere else…but it’s coming into the city, and it’s now likely being built into part of the city structure.

Lisa Raffensperger: And the concrete in that truck has historically been left out of measures of a city’s greenhouse-gas emissions, since it came from outside the city. Another oversight has been air traffic. Modern cities need air travel to keep them alive, but airplane emissions have never been included in cities’ carbon tallies.

Lisa Raffensperger: For more explanation, we head to the UC Denver campus, not far from downtown. There, Anu Ramaswami, director of the urban sustainability program, shows me a pie chart that represents Denver’s carbon emissions. A quarter of the segments are crossed with hatch marks.

Anu Ramaswami: These hatch things are things that were not being counted traditionally. So you see food has a big impact…, cement production, fuel processing. We were counting tailpipe pollution, but not counting what it takes to make your gasoline.

Lisa Raffensperger: It’s because old metrics were based on a simple geographic boundary—either something was in or it was out. But cities don’t work like that—they’re porous, explains Ramaswami. Which is why the carbon emissions from cities never seemed to add up right. Now Ramaswami’s team has come up with a much better measure, what they’re calling the “hybrid method.”

Anu Ramaswami: The hybrid methodology that we’re using uses life-cycle analysis for all these important material and energy consumption streams that are coming into cities and says, let’s look at them not just in terms of what happens within the boundary, but their full life cycle.

Lisa Raffensperger: For now, it’s being used to calculate carbon dioxide emissions. It could eventually do water and energy requirements or pollutants produced. Ramaswami’s team plans to add in economic measures such as GDP, too. Cities around the world are already interested in using the tool. But sustainability often involves balancing competing needs. How does measurement help with such hard decisions?

Anu Ramaswami: You know, if you’re in a place with very little water and your water footprint is being, you know, projected to be at stress because of climate change, you can start doing scenario thinking. You know, how can I shift from a water intensive mode of producing electricity to a different mode that at the same time reduces my greenhouse-gas footprint and creates employment?

Lisa Raffensperger: Having all the measures in one place allows you to visualize not just trade-offs but synergies. Ultimately, Ramaswami says, it’s up to the people who are residents of the area to define sustainability. It’s a very cutting-edge approach to sustainable engineering, says Ph.D. researcher Krista Nordback.

Krista Nordback: I think we need—as engineers, we need to know more about social science. You don’t need to take any classes in psychology or social science to graduate with your civil engineering degree, and yet you are practicing in an area, especially with transportation, where you’re not dealing with water molecules; you’re not dealing with the behavior of solids; you’re dealing with human behavior.

Lisa Raffensperger: That’s the blending of disciplines at the UC Denver program. Engineers work alongside social scientists, for instance, to study not just how to make a new smart meter but how people will use it. The teamwork reflects the nature of sustainability: It’s about the environment, and about people too—both today’s city dwellers and tomorrow’s.

Lisa Raffensperger: It’s for a glimpse at this “social” sustainability that we set off on our bikes for the final time. When we arrive, we’re in a shaded alley, surrounded by tall brick buildings.

Andrew Duvall: We’re in the alleyway of the development of the Living City Block. Let’s go check it out! All right.

Lisa Raffensperger: The Living City Block will be a block-sized oasis of sustainability in downtown Denver. The nonprofit behind the work will use UC Denver’s measurement tools to track the “footprint” of the block as it undergoes deep retrofits to make it, eventually, energy neutral.

Chad Riley: So this is the Living City Block from the rooftop perspective.

Lisa Raffensperger: Chad Riley is director of finance and strategy for the project. He points out the surrounding rooftops, describing alternative energy, community space and rooftop gardens, a pedestrian mall through the alley, bike lanes and car shares.

Chad Riley: We really believe that we’re trying to do something innovative and new here. It’s a very complex puzzle, and it’s a very complex project, but it’s a great deal of fun at the same time.

Lisa Raffensperger: It’s sustainability—healthy for the environment, and healthy for us. In Denver, I’m Lisa Raffensperger.

This podcast is part of the Sustainable Design radio program, a collaboration between IEEE Spectrum and the National Science Foundation.

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