Cities are taking steps to combat climate change, given the scant progress made by international treaty negotiations. Los Angeles, California, home to around 4 million people, has one of the most ambitious targets: to reduce greenhouse-gas emissions by 35% below 1990 levels by 2030. The city has calculated its carbon 'footprint' and found that road vehicles constitute 47% of total carbon dioxide emissions, and that electricity consumption constitutes 32%1. So how should Los Angeles target its policies? Knowing that certain roads, types of vehicle or parts of a city dominate road emissions and why people drive at specific times would tell city planners where and how to lower emissions efficiently. Improvements in traffic congestion, air quality, pedestrian conditions, and noise pollution could be aligned. But tracking emissions road by road and building by building is beyond the capacity of most cities. Luckily, scientists are gathering the 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 need 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 up costs and benefits. And scientists should help municipalities to raise awareness of the power of detailed emissions data in tailoring climate and development policies.