As world leaders negotiate rules for cutting greenhouse gas emissions at the COP24 meeting in Poland, U.S. cities have a vested interest in the outcome. About 85 percent of Americans live in cities, and urban areas produce some 80 percent of our nation’s greenhouse gas emissions. Many cities are highly vulnerable to climate change impacts such as flooding and heat waves.
Cities are central to shaping effective solutions, too. After President Donald Trump announced in 2017 that he planned to withdraw the United States from the Paris climate accord, more than 400 U.S. mayors – representing 69 million people – pledged to fulfill it. Some cities are going further and aiming for net carbon neutrality by 2050. Their efforts are sorely needed: According to a Dec. 5 report from the Global Carbon Project, global carbon dioxide emissions grew 2.7 percent in 2018, the largest rise in seven years.
Mayors believe they can make a climate impact while making their communities greener and cleaner. To succeed, they will have to take bold policy actions and demonstrate that emissions are declining. However, tracking greenhouse gas emissions requires models, forecasting tools and lots of data. Today most of that information is organized at the national or state level, not at city scales.
Now, though, this is changing. Over the last decade, our work on urban greenhouse gas emissions has shown that with the right combination of instruments, data and modeling techniques, it is possible to independently quantify carbon dioxide and methane emissions from urban areas. Just as researchers measure local smog concentrations for public health, they now can measure local greenhouse gases for climate action.
Move fast and measure carefully
Many steps cities need to take to hit their 2050 targets must be undertaken within the next decade. Investments in energy systems, transportation infrastructure and new housing stock can lock in consumption and emissions patterns for decades to come.
As an example, by reducing urban and suburban vehicle travel, making electric vehicles more affordable, and expanding use of public transit, cities can cut carbon emissions and also reduce local air pollution. But such actions require very long lead times. The much-anticipated Westside subway extension project in Los Angeles has a construction timeline of 14 years, but was preceded by decades of planning and negotiation.
To track actual progress over time, urban agencies also need to generate and share local-scale information. Nongovernment organizations like the Carbon Disclosure Project and ICLEI have produced tools that make this kind of data more accessible and transparent, but it is still challenging to accurately quantify city-scale emissions. Resource constraints, data gaps and shifting local priorities make it difficult for many cities to regularly report emissions in a consistent way.
We work with a growing network of U.S. cities where researchers are quantifying greenhouse gas emissions across urban regions. For example, in Boston we are analyzing how trees within the city influence greenhouse gas levels, and how local traffic congestion increases emissions of carbon dioxide as well as air pollutants. And in Salt Lake City, researchers have outfitted train lines with sensors to measure greenhouse gases and air quality in real time.
Remote sensing is another promising data source. Several greenhouse gas observing satellites are already in orbit, with more to follow. The Orbiting Carbon Observatory 3 (OCO-3), soon to be installed on the International Space Station, will focus on observing cities and urban areas across the globe. It will provide scientists and city governments regularly with vital data on local carbon dioxide concentrations.