When we think of GHG/carbon emissions, we often think of smokestacks and tailpipe emissions that we can see in our own cities. These are known as Scope 1 emissions. These are relatively easy to measure, and relatively easy to assign responsibility for.
However, many goods and services that we purchase in cities have a more complicated footprint. For example, imagine that we're buying a new motorized scooter. We know that while we're using the scooter it will burn gasoline and create carbon emissions, and those will be added to the emissions from all other vehicles and sources in the city.
But the process of manufacturing the scooter also used energy and produced emissions, and it probably happened in a different city, or even across a border in a different country. Now it's harder to measure and harder to assign responsibility for. We should be trying to account for the entire lifecycle emissions of goods and services at the place where they are used!
This is just one example of how carbon emissions and other kinds of pollution can be exported to producer cities. For a lot more detail, I suggest reading the 2018 report from C40 cities about consumption-based accounting of GHG emissions.
Consumption-based GHG accounting is an alternative to the sector-based approach to measuring city GHG emissions. This focuses on the consumption of goods and services (such as food, clothing, electronic equipment, etc.) by residents of a city, and GHG emissions are reported by consumption category rather than GHG emission source category.