From New York to Houston to Anchorage, hundreds of mayors reacted to President Donald Trump’s announcement to withdraw the U.S. from the Paris climate agreement by reasserting or even intensifying their commitment to fighting climate change. They represent a diverse group of cities that is potentially large enough to significantly reduce global greenhouse gas emissions.
This is encouraging, but America must recognize that city efforts to curb emissions are not substitutes for national climate policy, and they pose several unique challenges and drawbacks.
First, due to their limited size, city climate initiatives neglect major emissions-producing sectors. Agriculture was responsible for 9% of U.S. emissions in 2015, and it overwhelmingly takes place outside of city limits. Similarly, some of the most emissions-intensive industrial activities, such as cement production and natural resource extraction like fracking, are not concentrated in cities. Even many power plants that generate electricity are many miles from the cities they serve. Inter-city freight transportation and air travel comprise a substantial fraction of national transportation emissions, but are unlikely to fall within the purview of city regulations.
Second, city policies would be rendered ineffective if sources of emissions relocate to evade them. Researchers use the term “carbon leakage” to describe a scenario in which stringent climate regulation in one country causes companies to move their production to countries with weaker regulations. If some U.S. cities adopt ambitious climate policies, a similar dynamic could unfold of companies relocating to other cities or to independent municipalities on cities’ peripheries. The latter could exacerbate urban sprawl and increase vehicle travel, thereby raising emissions.
Climate change is fundamentally different from local environmental problems, such as air pollution, that cities have historically confronted. The impact of carbon dioxide is the same no matter where on Earth it is emitted, so merely moving emissions around accomplishes nothing.
Finally, given their limited scope, cities often pursue untraditional mitigation strategies that are unlikely to be cost-effective. For example, Austin is currently overhauling its land development code in hopes of reducing emissions by creating denser urban neighborhoods that decrease vehicle travel. But research shows that higher population densities are associated with lower emissions only in places with densities far exceeding those present in Austin.