Allison Bishins and Nicolae Duduta of EMBARQ (the producer of this blog) today released a working paper, “Citywide Transportation Greenhouse Gas Emissions Inventories: A Review of Selected Methodologies.” As the report highlights, the transportation sector accounted for 14.3 percent of the worldwide GHG emissions in 2005, making it one of the three highest emitting activity sectors in the global economy. In the United States, the transportation sector’s share of emissions is much higher than in the rest of the world, accounting for 33 percent of nationwide CO2 emissions in 2008.

“The ability to perform accurate transportation emissions inventories at multiple geographic levels and update them regularly is critical for identifying opportunities for emissions mitigations activities, as well as for measuring their progress over time,” Bishins and Duduta explain.

To download the full report, click here. For a deeper explanation of the importance of tracking transport-related emissions in cities from San Francisco to Mumbai, read our Q&A with Duduta below.

What is a citywide transportation emissions inventory and who develops it?

It is an estimate of the total emissions from a city’s transportation system, based on different estimates such as fuel sales, vehicle miles traveled, etc. An emissions inventory is usually developed by cities who want to learn the magnitude of their transportation emissions and the relative contribution of each transport mode, such as cars, buses, metro. A precise inventory allows cities to know what sectors or modes to target for emissions reductions.

The paper talks about a lot of different inventories and methodologies. Why are there so many?

An inventory at the city scale needs to look at transportation emissions associated with a particular city. But cities are part of large urbanized regions where many trips cross city borders so there isn’t a single way to account for all the trips that occur partly within city boundaries and partly outside it. Examples of such trips include long distance commuting, inter-city passenger trips, freight transportation and even shipping waste to distant landfills. Depending on how all these trips are accounted for and what percentage of their emissions is attributed to each city, an inventory can report very different results.

Why is this study important? What’s new about it as opposed to other work on GHG inventories?

This study takes a detailed look at the key methodological issues that need to be addressed when developing a citywide transportation inventory: which emissions to count, how to count them, how to attribute emissions from longer trips to multiple cities. We use examples of existing inventories developed in cities around the world, from San Francisco, to Lisbon, and even Mumbai, and show how the different inventories addressed or dealt with these difficult questions.

Who is the target audience?

Generally, the audience is those interested in how inventories are developed. Researchers currently working on inventory methodologies might be particularly interested in looking at the methodological challenges. The study would also be relevant to city officials who are interested in creating an inventory and would like to know which methodology might be best suited for their particular city.

Have similar studies been done before?

Similar studies have been done before, both in the U.S. and Europe. This study synthesizes some of the findings from previous studies, looks in greater depth at the challenges of developing an inventory and uses more recent inventory examples. Transportation inventories are still an emerging field. A lot of new inventories are being developed all over the world, and there is no consistent methodology used or agreed upon for creating it.

How were the inventories presented in this paper chosen?

The examples used in this paper were chosen to illustrate the diversity and range of inventories and show how the local context (especially the data available) influences what method can be used. Some examples, such as the inventory used in Lisbon, use very sophisticated methods to estimate emissions (aerial imagery, traffic counts, GPS, travel demand models.) While others, such as the one in Mumbai, have very little data to work with and must rely on a lot of assumptions. The International Vehicle Emissions (IVE) model is a tool specifically designed for the developing world and can work in places where data aren’t readily available. The European COPERT model, on the other hand, is a more sophisticated tool, but also more data intensive.