What greenhouse gas emissions are you responsible for? That question is not as easy to answer as it might appear.

Say you drive to the store, buy a soda, and drive home. Your car (unless you have an electric vehicle) spewed a certain amount of carbon dioxide. Those are the only greenhouse gases you produced.

But consider: Manufacturing the bottle, making the soda, shipping the bottle to the store, and disposing of the bottle all produce greenhouse gases. By purchasing the bottle of soda, you didn’t directly produce those emissions, but you induced them. You are the demand for which they were created; they are embodied in the bottle of soda (thus the somewhat unwieldy term “embodied emissions”).

And while we’re at it, the gasoline that was burned in your car was drilled as oil, refined, and shipped to the gas station. That’s a few more GHGs emitted. The car itself, along with each of its thousands of parts, was manufactured and shipped. More GHGs. The clothes you wear, your sunglasses, the phone you used to Google map the store were all manufactured and shipped and they all must be disposed of. They all represent embodied emissions.

If you view your trip to the store through this lens — the GHG emissions you induced, not just those you produced — it looks very different. It could be that the “consumption emissions” in the goods and services you consumed were equal to or even greater than your “production emissions.”

Say you took some responsibility for, and wanted to reduce, both your production and consumption emissions.

For the former, it’s easy. Put a solar panel on your roof, use it to charge an electric vehicle, and boom, a guilt-free trip to the store for soda. You produce no GHGs. You are virtuous.

But for consumption emissions, it’s not quite so easy. The only way to have any effect on the number of sodas manufactured and shipped is to ... not buy the soda. Don’t drive to the store. Don’t own the car. In other words, the only way to reduce your consumption emissions is to change the way you consume — to consume less, reuse more, repair more, and recycle.

Climate optimists argue that it is possible to reduce our production emissions without substantially changing our lifestyles. (Electric cars are just as good!) But what if those lifestyles — in which consumption is perpetually rising — are driving increases in production emissions elsewhere? What if they are making it more difficult for other countries, the ones producing the goods we consume, to reduce their emissions?

Spoiler: They are.

If climate change really is a crisis, then surely we — especially the “we” in the wealthy developed world, doing most of the consuming — must take some responsibility for our consumption emissions. This is especially true if we want to make some room for the millions now living in grinding poverty around the world to reach something close to the lifestyles we now enjoy.

These are not abstract ethical questions. Cities around the world are beginning to seriously grapple with the question of their consumption emissions — how to measure them, how to make policy to reduce them, and, perhaps most importantly, how to communicate about them.

These are politically explosive issues that bump up against personal choices about where and how we live, even who we are. There’s a reason climate policy has focused so exclusively on production emissions; it is safer ground.

But things are beginning to shift. A few weeks ago, 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, and then chat with a fellow in Portland, Oregon, who has been working on this issue for the city, to get a local perspective. It’s gonna be great!

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. Here’s a helpful graphic:

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.”

For most cities, consumption emissions exceed production emissions

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

As I said, these results should be considered extremely approximate. But directionally, the result is clear: The consumption emissions of C40 cities are half again as large as their sector-based emissions. Wealthy cities consume more embedded emissions than they produce directly.

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.

As you can see from the graph above, at least half the consumer cities have consumption emissions twice as large as production emissions, a couple up to eight times as high. (Notably, Scandinavian cities, so celebrated for their work cutting production emissions, have huge consumption emissions, because they import so much of what they consume.)

Who is doing most of the consuming? Here is a chart of consumption emissions per capita, in major global regions:

As you would expect, the wealthy cities of Europe, North America, and Oceania (Australia and New Zealand) have the highest per capita consumption emissions.

What goods and services represent most of the emissions? Here’s a breakdown:

It’s a little difficult to see, but the two big categories on the left are capital (“business investment in physical assets such as infrastructure, construction, and machinery”) and utilities and housing. Note that some of these categories, like utilities and housing, public transport, and private transport, will also be captured by a sector-based inventory.

Those broad results can hide some wide variations, even within categories. For instance, here’s food consumption emissions, broken down by subcategory:

If they want to reduce consumption-based emissions around food, South, West, and Southeast Asia will go after rice. Just about everyone else — hello, Latin America! — will go after meat.

C40 acknowledges that its data is extremely coarse and its conclusions extremely broad. As the data improves, the ability to more precisely target cities will increase as well. “The aim should be to obtain complete and consistent energy use and final demand data for all cities without the use of proxy data,” the report rather boldly suggests. It will be a long time before that data is available.

In the meantime, what should cities do?

A toolkit for cities that want to do a CBEI

The Urban Sustainability Directors Network (USDN) has developed a Sustainable Consumption Toolkit for cities to use to run a CBEI, make policy based on it, and communicate about policy to stakeholders.

Within it, there is a CBEI Guidebook, developed by the Stockholm Environment Institute, that walks cities through some of the nuts and bolts of running a CBEI. (SEI’s model is one of about five bouncing around, each with slightly different methodologies and carbon intensity estimates. It is early days for this kind of research; presumably, data will improve and models will converge over time.)

Alongside the guidebook is a report called “Smart Shift,” done on behalf of the USDN by the research consultancy Climate Access, about how to communicate the results of the CBEI and organize action around it. It contains recommendations for how to communicate to both internal and external stakeholders, in both cases walking through the opportunity, the challenge, and the choices.

In most cities, there are already programs ongoing to reduce food waste, share and repair tools, or reduce individual driving — various ways, broadly speaking, of reducing consumption. What a CBEI can do for a city is help stitch those programs together and give them some focus, so that the biggest consumption emission targets (things like housing size and food) get proportionate attention.

“Emissions from car travel, building heating, and power consumption each average at least 2 tons CO2e per person in the US,” USDN writes. “But this is already well known from cities’ standard, territorial GHG emissions inventories. The unique contribution of a CBEI perspective is instead the expanded emphasis placed on goods, food, and services, each of which also is generally responsible for another 2 tons or more of CO2e per person.”

Taking on consumption emissions in Portland

Back in the early 2010s, Oregon was one of the first states to undertake a statewide CBEI, which came out (alongside a more traditional sector-based inventory) as part of the state’s 2015 emissions report. It found that as Oregon’s population and economy have grown and shifted from a resource base to a more service base, its consumption emissions have risen even as its production emissions have fallen.

Afterward, the Oregon Department of Environmental Quality (DEQ) worked with the Portland Bureau of Planning and Sustainability (BPS) to develop a CBEI more narrowly targeted at Multnomah County, where Portland is located, using local input data. It was released (alongside a more traditional sector-based inventory) as part of Portland’s 2015 Climate Action Plan — and marked the first CBEI done for a US city.

Kyle Diesner, a policy analyst at BPS, was responsible for overseeing the CBEI and is now thinking about Portland’s consumption emissions more generally. That original CBEI was based on 2011 data; Diesner says an updated CBEI using newer data will be released as part of the city’s 2020 climate plan — though, he stresses, the basic information hasn’t changed much.

Remember, the data is fairly coarse and cannot closely track changes over time. It just uses broad consumption categories and multiplies local spending by estimated carbon intensity factor.

What the data can do, Diesner says, is highlight the big sources of consumption emissions, and where within their life cycles most of the emissions occur. That can help reveal “where we should be focusing policy or programmatic intervention.”

For instance, the biggest source of consumption emissions in Portland is automobiles, but the data shows that most of those emissions are in their use, not their manufacturing. That means “we need to work on reducing [vehicles miles traveled], we need to work on fuel economy, and we need to support people in upgrading to electric vehicles.”

The second biggest source is food and beverages, but there, 90 percent or more of the emissions are in production, not transportation, cooking, or waste disposal. So “it’s less about where your food comes from and more about what type of food you’re eating.” As as the chart above shows, the highest emissions foods in the US are meat and dairy.

Almost every city has scattered programs meant to reduce consumption; a CBEI can help “connect the dots,” Diesner says.

He points to Resourceful PDX, a program to encourage sharing and reuse, and stresses that, per the Climate Shift report, none of the messaging is focused on climate change or reducing emissions. Instead, it stresses the benefits of greater community and lower costs.

Diesner acknowledges that “behavior change is exceedingly difficult,” so he prefers to focus on systemic changes, things like establishing a “corner store” model for repair-and-sharing shops, so that it is as quick and easy to fix something as it is to buy something new. And he cites Portland’s efforts to limit home size, accompanied by efforts to cluster housing around transit.

Both examples are efforts to change local circumstances to make it cheaper and easier to reduce consumption.

Diesner also stresses the inequality in consumption. “Today, we have tens of thousands of Portlanders sleeping on the streets, who don’t have basic health care,” he says. “So this strategy really has to tackle both problems.” Portland also has programs to “build wealth in lower-income communities through direct ownership of homes or renewable energy systems,” among other policies (including a fund, paid for by a corporate tax, meant to target greening efforts on vulnerable communities).

He cites economist Kate Raworth’s “doughnut economics” — the notion that human flourishing is best found above a lower limit on individual consumption but below an upper limit on collective consumption. Part of the city’s ongoing work with C40, Diesner says, will be to “try to measure that ceiling and floor for Portland.”

The larger implications of a consumption emissions perspective

Consumption emissions are the final frontier of climate policy — and are likely to be the most fraught and difficult.

Is it worth taking them on at all? According to the Intergovernmental Panel on Climate Change, for any hope of avoiding temperature rise of more than 1.5 degrees Celsius, the world must reach net-zero carbon emissions by 2050. It is nice to imagine that every nation in the world could reduce its production emissions to zero by 2050 without anyone needing to worry about their consumption emissions, but it seems unwise to bank on it. Every bit of avoided consumption lowers the target that emissions cuts must hit.

Alongside efforts to reduce production emissions must come some effort on the part of wealthy consumers to reduce their consumption emissions, i.e., to reduce their consumption. That means more compact homes, closer together, with less driving and more sharing and repairing.

That kind of message is absolute rage-bait for right-wing politicians and media, out in the red, suburban, and rural areas of the country. That is why the consumption focus is taking root first in blue cities, where liberals tend to cluster. Portland and Eugene (which has done similar work) are not Atlanta or Tulsa.

It will be fascinating to watch in coming years as consumption data and inventories improve and that side of climate policy begins to spread to more cities. It’s a challenging message in a US culture and economy that run on consumption spending, to say the least. But who knows, sometimes local politics and local community can bypass the polarization that has eaten everything else.

At the very least, city dwellers should ask their municipal leaders to do a CBEI. “It’s early in the process of understanding this connection between consumption and climate,” says O’Sullivan, and “these inventories are a great way to start that conversation.” At the very least, it is better to know what effect consumption is having than not to know.