Measuring Consumption-Based Emissions of Cities

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For most cities, consumption emissions exceed production emissions

By David Roberts:

"C40, a coalition of cities around the world committed to sustainability, issued a report on the “consumption-based GHG emissions of C40 cities.” It attempted, for the first time, to estimate the consumption emissions — “scope three” emissions, in the lingo — of 79 participating cities.

In this post, I’m going to quickly review consumption emissions and how they’re measured, discuss ways to reduce them.

What counts as consumption-based emissions and how they are measured ?

A city’s attempt to measure, or at least estimate, its consumption-based emissions is known as a “consumption-based emission inventory” (CBEI).

The first thing to note is that a CBEI is not an alternative to a traditional sector-based production emissions inventory, the kind that virtually every city has done at this point.

Indeed, the emissions a CBEI tallies somewhat overlap with those counted by a sector-based inventory.

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A sector-based inventory counts all GHGs produced in a city, no matter where the resulting goods and services are consumed; a CBEI counts all embodied emissions consumed in a city, no matter where they were produced. The former captures exports; the latter captures imports. The two inventories overlap in that some goods and services are both produced and consumed within the city.

To point out the obvious, there’s no way that every city can be responsible for both its production and consumption emissions at the same time. All emissions would be double-counted!

Rather, a CBEI is a complementary analysis, a different lens through which to view a city’s contribution to climate change.

The second thing to note is that consumption emissions are devilishly difficult to measure. Production emissions are, comparatively speaking, easy. It’s possible to know exactly how much electricity is used and how much gasoline is burned, what industrial processes are running and when. Most big facilities are required to report their emissions annually. A city can track sector-based emissions with some precision; it’s like accounting.

Consumption emissions are different; they are modeled, not directly tracked. To directly measure them, a city would need to know about every single material good imported or purchased in the city and its exact lifecycle emissions. And because those things change constantly, a city would have to recalculate constantly.

Most of that data simply isn’t available, especially on supply chains outside the US, and even if it were, it’s too much to track. So by necessity, consumption emissions must be estimated using broad categories of goods and their average carbon intensities.

That kind of modeling is inevitably going to be imprecise. And it’s going to miss a lot. For example, there are wide variations in emissions within consumer categories. Organic, grass-raised beef involves lower embodied emissions than factory-farmed beef. If a city engineered a large-scale shift from the latter to the former, it wouldn’t show up in a CBEI because there’s generally just one category, “beef,” with a rough average. Variations can’t yet be tracked with any precision.

This means two things for cities. First, a CBEI will be, at best, an informed estimate — directionally suggestive but far from precise. And second, a CBEI will be unable to closely track changes over time. This substantially complicates the task of measuring success. In order to know whether it is reducing consumption emissions, a city will need to find proxy measures to track — the amount of food waste it reduces, vehicle miles traveled, etc. Reducing consumption emissions is as much art as science.

“The tools and the data are going to evolve over time and become more accurate and precise, to meet the need,” says Babe O’Sullivan, who has worked in sustainability in Eugene, Oregon, as well as with the Urban Sustainability Directors Network, a coalition of city-level sustainability professionals across the US and Canada. “But in the meantime, they have a really important and valuable story to tell.”

The C40 report marks the first attempt at a CBEI for a broad range of cities. Here is what it found:

- Of the 79 participating cities, 63, or 80 percent, have larger consumption emissions than production emissions. There are 16 “producer cities” where the inverse is true, mostly in South and West Asia, Southeast Asia, and Africa, but for most cities, especially in the developed world, consumption emissions dominate." (https://www.vox.com/energy-and-environment/2019/7/1/18743992/climate-change-cities-food-cars-emissions)


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