Seeing the progress made by international treaty negotiations, cities are taking steps to tackle climate change. Los Angeles, California, where nearly 4 million people live, has one of the most ambitious goals: to reduce greenhouse-gas emissions by 35% from 1990 levels by 2030.

The city has calculated its carbon ‘footprint’ and found that road vehicles constitute 47% of total carbon dioxide emissions, and electricity consumption is 32%. So how should Los Angeles be targeting its policies?

Knowing where certain roads, types of vehicles or parts of a city dominate road emissions and why people drive at specific times will tell city planners where and how to reduce emissions efficiently. Improvements in traffic congestion, air quality, pedestrian conditions and noise pollution can all be aligned. But tracking emissions from roadways and from building construction is beyond the capability of most cities.

Fortunately, scientists are gathering data that city managers need—in studies that match sources of CO2 and methane with atmospheric concentrations. Now the research community needs to translate this information into a form that city managers can use.

Emissions data needs to be merged with socio-economic information such as income, property ownership or travel habits, and placed in software tools that can query policy options and weigh costs and benefits. And scientists should help municipalities raise awareness of the power of detailed emissions data in formulating climate and development policies.

carbon hotspot

Cities account for more than 70% of global fossil-fuel CO2 emissions, the main driver of climate change. If the top 50 emitting cities were counted as one country, that ‘nation’ would be third in emissions after China and the United States. Urban areas are set to triple globally by 2030 (ref 3).

Much of this emitting landscape is controlled by mayors, town planners, businesses and community groups who are responsible for the health and welfare of residents. A 2014 survey listed 228 global cities – representing nearly half a billion people – that have pledged to cut the equivalent of 454 megatons of CO2 per year by 2020 (see go.nature.com/inaxr4) .

For example, Shenzhen in China aims to have an additional 35,000 electric vehicles on the road by the end of 2015. The German city of Munich aims to produce enough green electricity to meet all its electricity needs by 2025.

Yet such pledges account for only 3% of global urban emissions and less than 1% of total global emissions projected for 2020 (Reference 4). Wealthy cities dominate these pledges, yet low- and middle-income countries are experiencing the most urban growth.

Emission reduction requires them to be mapped to the finer scales of space and time that reflect the human dimensions on which carbon is emitted: by individual buildings, vehicles, parks, factories and power plants. These should be tracked at least annually.

Such fine-grained estimates are necessary for several reasons: to verify emissions rates; To confirm progress towards reduction and support of carbon trading, permits or taxation5; enable more targeted and financially efficient decisions about mitigation options; and to identify and fix unintentional releases, for example, gas pipe leaks in landfills or malfunctioning methane-capture equipment.

Cities are already approaching air quality improvement, regional development, transportation planning and home or street scale waste disposal. All of them could benefit from adding low-carbon policies to these efforts. For example, reducing traffic congestion will reduce air pollution and traffic accidents and improve traffic. And targeting the immediate needs of residents broadens public acceptance.

problems

Although methods for community-scale emissions have been designed by non-profit organizations such as the World Trade Council for Sustainable Development and the World Resources Institute (see go.nature.com/q7wjeb), most cities have independent, comprehensive and comparable data sources of.

The expertise and staff required to create this information is expensive. Transparency of data and methods is also important to enable verification by third parties and to build trust.

Scientists are beginning to face these challenges. Over the past five years, fuel reporting, traffic data, building information and ‘bottom-up’ estimates of carbon emissions from human activity are being merged with ‘top-down’ atmospheric measurements on cities of CO2, methane and 14CO2 – An isotope of CO2 which characterizes the combustion of fuel7.

Such efforts began in the late 2000s in Paris and the American cities of Indianapolis, Boston, Salt Lake City and Los Angeles; More are planned for So Paulo, Brazil and cities in Australia, China, the United Kingdom and Canada. These studies cost millions of dollars, and involve at least a dozen monitoring sites and remote sensing data and modeling systems.

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