To maintain a fighting chance of limiting warming to 2 degrees C, global emissions need to peak this decade, and then decline steadily and rapidly to roughly 1 ton per person annually by the middle of the century. Globally, we’re currently at 6 tons per person. The U.S. is at 23. China, the world’s most populous nation, is also at 6, and growing about 10% per year. We need to bend the curve, and fast.
We critically need to restructure our energy system. Out with fossil fuels, in with renewables. Use less energy in buildings and factories. Create low-carbon transportation. Easy, right? Beyond that, well… it gets (even) harder.
An increasingly prevalent notion is that one major climate solution is close at hand but often overlooked: the city. The world is rapidly urbanizing, with nearly 70% of the world’s population expected to live in urban areas by 2050. The way that cities are built and rebuilt can (in part) determine the greenhouse gas footprint of their residents. If we channel the global migration to (and growth within) cities into low-carbon development, will it help?
The shape and layout of cities has particular sway over emissions from resident transportation. Cities that provide residents ready access to goods and services through proximity rather than extensive networks of cars, roads, and parking lots lead to lower carbon emissions. When home, shopping, work, school, and entertainment are all close by, car trips are fewer, and shorter. Looking across cities, as population density goes up, auto travel, and associated greenhouse gas emissions go down. The difference is up to 5 tons per person compared to the most sprawling regions: say, between Denver (with over 6), to New York City (with less than 2), to Barcelona (with about 1). By building up, and filling in, cities create the conditions where walking, biking, and public transit can take the place of car trips. Of course people have to want to live in these cities, and these neighborhoods. Urban “grain” and vibrant streetscapes hold human interest, which tends to mean a mix of uses and building types rather than monolithic skyscrapers.
Building cities that are, on average, denser, also brings other energy and greenhouse gas emissions benefits. Apartment buildings, with their shared walls and economics of scale, need less energy to heat and cool than detached houses. And smaller homes have less room for lots of furniture and stuff, meaning fewer goods are needed, avoiding the emissions associated with making these goods. Of course, accommodating growth within existing cities through in-fill development also helps avoid emissions associated with clearing new land for development.
Taken altogether, how much do denser, pedestrian- and transit-friendly cities reduce greenhouse gas emissions relative to the alternative? It’s an elusive question, and a complicated one. Even if a city enabled emissions associated with resident transportation and housing below 1 ton per person (an ambitious goal in itself, and one that Seattle is exploring, that wouldn’t help much with the emissions associated with other stuff we still buy, including food. And what if giving up the car, and downsizing the home, means more money to spend on (perhaps even more climate-damaging) airplane trips, as has been shown to happen in at least one city?
People are moving to the city, especially in developing countries, by the millions – even billions. How that growth gets accommodated has a significant impact on how greenhouse gas emissions-intensive cities will be (not to mention other, critical local issues, such as health and safety of housing the new residents, many of them desperately poor.) Will global cities sprawl, locking in car use for decades to come, or create vibrant, people-focused places?
By their very nature, cities can provide an important piece of a low-carbon future if they can orient us towards low-carbon lifestyles. It’s clear they can do this for buildings and (especially) transportation, and for this reason they should be a central piece of the discussion about how to rapidly reduce global greenhouse gas emissions. Whether they can be a crucial lever for emissions from other parts of the economy – such as from food, or industry – is an entirely open question.
Peter Erickson is a Staff Scientist in the Climate and Energy program in the Seattle office of the Stockholm Environment Institute. His research focuses on climate change policy, with particular interests in the role of offsets in cap-and-trade programs, contribution of consumption and behavior change to reducing greenhouse gas emissions, industrial policy, and cities.