Estimating emissions

Creating a CBEI for your city

Cities interested in estimating their consumption emissions have several alternatives. Before deciding on an approach, it is helpful to first consider why the city needs a CBEI, and what data, resources, and technical capacity the city has to create an inventory.

If the city is just aiming to understand and communicate consumption emissions, and has limited capacity to create a CBEI, existing estimates may be sufficient to meet needs. See for instance:

If no estimates are available, or more detail is needed than is available in these analyses, it may be necessary to create (or commission) a new CBEI.

Choosing a CBEI estimation approach

There are several different approaches that can be used to estimate a CBEI.

All approaches follow the same basic methodology: estimate how much of each good or service is consumed, and multiply that by how many GHG emissions are released in the course of producing, transporting, consuming, and (in some cases) disposing of each unit of good or service. Where CBEI approaches differ is in how they define and estimate these two basic quantities.

Estimating consumption

Broadly speaking, consumption can be estimated in one of three distinct ways:

  1. Use spending data (e.g., dollars spent on a good or service)
  2. Use data on actual units of a good or service consumed (e.g., tons of cement, pounds of broccoli, number of attendees at ballet performances, etc.)
  3. Using data on the mass or quantity of a good that is disposed (e.g. as refuse in a landfill)

Each approach has its strengths and weaknesses. Spending data, for example, might be simple to collect, but is difficult to disaggregate into policy relevant categories (e.g. categorizing types of food consumed). Unit-based data may give an accurate picture of the types of goods and services consumed, but is very challenging to collect across all consumption categories. Waste disposal data is useful for tracking consumption changes, and is often already collected by cities for waste management purposes, but it may miss important non-waste consumption categories (such as air travel).

Estimating emissions associated with consumption

Two approaches are commonly used to estimate emissions from consumption data:

Input-output models typically use spending data to map the types and relative amounts of intermediate goods and services (“inputs”) needed to produce other goods and services that are ultimately consumed (“outputs”) in a particular region. By associating GHG emission factors with a range of different inputs, the models can be used to estimate total GHG emissions per dollar spent on final goods and services, and can also help identify the most emissions-intensive phases of a good or service’s life-cycle (e.g. production, transportation, or use). 

A key advantage of the input-output approach is that models can generate consumption-based GHG emissions estimates for a full range of goods and services consumed within a community, making them particularly suitable for constructing a full CBEI. However, a significant disadvantage is that these models often lack resolution at the city level, because they often rely on statistics collected at a national level. These models also tend to be technically complex, so they are often created by researchers or consultants on behalf of cities, rather than by internal city staff.

Life cycle analysis estimates the full emissions associated with producing, using, and in some cases disposing of goods and services.These analyses need to be conducted for each good or service in question, making them cumbersome for constructing a full CBEI. However, the use of actual consumption (or disposal) data combined with LCA can sometimes yield a more accurate picture of consumption-based emissions, as well as insight into alternative production processes, which in turn may be useful for designing policies tailored to particular types of consumption.

In practice, most cities tend to use a hybrid of the approaches above, combining different methods to produce a comprehensive CBEI with more resolution for key consumption categories.

CBEI examples

San Francisco and London estimated consumption-based emissions with an input-output analysis using spending data.

Vancouver and Iowa City used local resource use and waste data to calculate an “ecological footprint”. The emissions component of the footprint was used to generate the city’s CBEI.

Portland and Eugene adapted consumption-based estimates created for the State of Oregon, by adjusting State-level estimates using local data where available.

Minneapolis and Lake Oswego used estimates from the Cool Climate tool to generate a simple estimate of consumption emissions.

Developing a policy relevant CBEI

A key lesson from cities who have developed their own CBEIs is that thinking about policy relevance in the development stage is critical for ensuring the CBEI is useful for policy and planning purposes.

Two key questions to ask to ensure the CBEI is disaggregated in a way that is policy relevant are:

What categorization of emissions will be most useful for identifying and developing policy actions?

Different inventories, developed using different methods, will frequently report emissions using different groupings of consumption. While there is not a single prescribed approach for categorizing CBEIs, the following general guidance may be helpful to consider:

  • Travel or transportation: break into different modes, e.g., air travel, personal cars and trucks, and public transportation.
  • Housing or home: distinguish between construction, home appliances, (non-energy) utilities, and energy consumption for heating and cooling.
  • Goods: break up into defined categories, e.g. clothing, furniture, electronics.
  • Services: try to divide into distinct categories, such as transportation services (e.g., car rentals, air travel, and public transit), home and vehicle maintenance, communications, health care, banking, and entertainment.
  • Food and beverages: separate out food and beverages with high emissions (particularly meat and dairy), and restaurant food consumption from other food purchases.

These are only general guidelines, and data limitations may prevent disaggregation of all of these categories. Overall, the goal should be to categorize the CBEI into groupings that provide insights needed to develop appropriate policy solutions. The CBEI prioritization tool that accompanies this toolkit can help to explore CBEI data at different levels of aggregation and prioritize areas of policy focus.

Who is doing the consuming?

The majority of consumption in most cities will be from households, and households will usually be the primary target of consumption-based emissions policies. However, other key sources of consumption include local government itself, and local businesses and industry. There are good reasons to treat these emissions separately. Government purchasing decisions can have an outsize effect, in terms of supporting markets for sustainable goods and services, so tackling these as a distinct category is important. Likewise, business and industry have unique opportunities to reshape production and consumption. Although the guidance in this toolkit is primarily aimed at household consumption, separating out these other categories of "consumers" in a CBEI can help identify additional targets for policy action.

A deeper dive

For a more detailed description of different types of CBEIs, and tips for creating a policy relevant CBEI, download the SEI guidance on estimating consumption emissions using a CBEI

  1. Wiedmann, T. O., Chen, G. and Barrett, J. (2016). The Concept of City Carbon Maps: A Case Study of Melbourne, Australia. Journal of Industrial Ecology, 20(4). 676–91.