• Ramsey Cook

GHGs & GPC - Quantifying Local Greenhouse Gases

In 1998, the Intergovernmental Panel for Climate Change (IPCC) established its Task Force on National Greenhouse Gas Inventories (TFI). The goals of the task force were simple: define methodology and software acceptable for greenhouse gas (GHG) calculations and make those resources available to countries across the world. As a task force of the IPCC, which was established by the United Nations as a means of analyzing the science behind climate change, the group draws from an international network of scientists. Since its founding, the TFI has published several peer-reviewed papers detailing the internationally accepted methods for national greenhouse gas inventories.

The most recent of these reports was published in 2019 as a refinement of the 2006 edition. The report details methods of estimating national greenhouse gas emissions within the categories of Energy, Industrial Processes and Product Use, Agriculture, Forestry and Other Land Use, and Waste.

Although this report proves useful for national reporting, the protocols detailed in the report do not directly apply to smaller-scale greenhouse gas reporting. With that said, some 75% of the energy-related carbon dioxide emissions come from cities. Therefore, developing a GHG reporting standard at the city-scale represented a valuable impact area. This is what the World Resources Institute, the C40 Cities Climate Leadership Group, and ICLEI - Local Governments for Sustainability had in mind when they decided to compile the Global Protocol for Community-Scale Greenhouse Gas Emissions (GPC).

As a coalition of internationally recognized organizations, the GPC synthesized the methods detailed in the TFI publication to reflect the unique challenges of small-scale reporting. Not only does it focus on reporting standards, the GPC also delves into how GHG emission inventories can be used to pursue climate mitigation in cities.

The report, much like the TFI report, breaks emissions into the categories of Stationary Energy, Transportation, Waste, Industrial processes and product use, and Agriculture, forestry, and other land use. These emissions are further split between scope 1, scope 2, and scope 3 emissions. Scope 1 emissions are emitted within the city boundary. For example, greenhouse gasses emitted from cars within the city are considered scope 1 emissions. By contrast, scope 2 emissions are those emissions generated outside of the city to supply energy to the city, and scope 3 emissions are any other emissions that are generated outside of the city in order to support city functions. While electricity generation and the heat, steam, and cooling associated with those processes are classified as scope 2 emissions, waste that is transported from the city to a nearby landfill is classified as scope 3. Reference Figure 1 for more details on the classification of emissions as scope 1, 2, or 3.

Scope 1 includes agriculture, industry, stationary fuel combustion, in-boundary transportation, and in-boundary wastewater/waste. Scope 2 is grid-supplied energy. Scope 3 is out-of-boundary waste/wastewater and transportation, transmission and distribution, and other indirect emissions.
Figure 1. Classifications for scope 1, 2, and 3 emissions as provided by the GPC

The purpose of scopes is to ensure that cities do not double-count emissions. For example, without the classification of scope 2, a city with a power plant in-boundary might include it in their total emissions. Meanwhile, a city nearby, which sources its electricity from that same power plant may also include the emissions from that plant in their total emissions. Perhaps this same classification is made by several neighboring cities. When a regional analysis of GHG emissions is desired, the total regional emissions will be much higher than reality because cities report a lump sum for their emissions regardless of source. Scope classifications subvert this issue and allow more valuable conclusions to be made with the resultant inventories.

In an effort to make some of these valuable conclusions, the Fall 2021 Sustainable Cities Studio at the Georgia Institute of Technology undertook a 2019 greenhouse gas inventory for the City of Atlanta. Led by Dr. Jairo Garcia, who published Atlanta’s first Climate Action Plan and Greenhouse Gas Inventory as the Director of Climate Policies and Renewables with the City of Atlanta, the team of undergraduate and graduate students spent a semester diving into five different emission categories: Energy and Buildings, Transportation, Water and Waste, Food, and Green Spaces. The following is a summary of their findings. The complete report can be viewed at this link.

Note that the following data is provided in million metric tons (MmT) of carbon dioxide equivalent emissions (CO2e). There are a variety of greenhouse gasses, including, but not limited to methane, nitrous oxide, and carbon dioxide. Each of these greenhouse gasses has a different global warming potential (GWP). Because carbon dioxide is chosen as the reference gas, its GWP factor is 1. However, methane, which is a much more potent greenhouse gas, has a GWP factor of 25. For the sake of reporting, all emissions are converted to a carbon dioxide equivalent mass. The totals presented in the report are for the year 2019.

The Energy and Buildings team quantified the emissions from electricity and natural gas usage, and provided recommendations for data collection necessary to calculate emissions associated with refrigerant leakage. The total emissions from the Energy and Buildings sector was found to be 4.8 MmT CO2e with electricity use making up 82% of that total. The emissions from Energy and Buildings, by far, represent the largest emission source for the City of Atlanta.

The Transportation team calculated a total of 3.6 MmT CO2e. The analysis quantified emissions from on-road vehicles, bus transit, local and national passenger rail, freight rail, and aviation. While aviation emissions were calculated to be 0.2 MmT CO2e, the sum was not included in the total transportation emissions due to limited access to data as the emissions were calculated by scaling up the aviation emissions from 2015 based on the increase in air travelers. The calculations in this report show that aviation represents a significant portion of Atlanta’s transportation emissions, second only to on-road vehicle transportation (3.2 MmT CO2e). Transportation is the second-largest emitter of greenhouse gasses in the City of Atlanta.

The Water and Waste team estimated emissions from electricity use at water and wastewater facilities, direct emissions from wastewater treatment, and direct emissions from anaerobic decomposition of organic waste in landfills. Both wastewater treatment and decomposition of waste in landfills result in the release of methane and nitrous oxide, two of the most potent greenhouse gasses. The team found that the Water and Waste sector generates 0.25 MmT CO2e annually. The overwhelming majority of those emissions came from direct (scope 1) emissions from wastewater treatment and direct (scope 3) emissions from waste.

Emissions from food waste and production were also included in this analysis. The Food team analyzed food emissions using two metrics: emissions associated with meat-heavy diets and emissions from anaerobic decomposition of food waste in landfills. The impact of land use was not included in the analysis because, with the exception of urban farms and community gardens, there is limited agricultural land within the City of Atlanta. In total, the Food Team estimates that Atlanta produces 2.2 MmT CO2e per year. Note that this number is significantly larger than the emissions from waste because the Waste and Water Team calculations included only organic waste that was collected by the city and not by private companies.

Considered the City in the Forest, Atlanta has a significant carbon sequestration potential in its green spaces. Trees, which absorb carbon dioxide during respiration and convert it to mass and energy, are recognized as carbon sinks. As such, the Green Spaces team performed an analysis of the sequestration potential for public and private green spaces in the city. It found that annually, green spaces sequester 0.2 MmT CO2e.

In total, this means that the City of Atlanta produced an estimated 8.4 MmT CO2e in 2019. How does this number compare to other cities?

As a lump sum, Atlanta’s greenhouse gas emissions are relatively low. However, in normalizing for population, it becomes clear that Atlanta does not stack up well. As shown in Table 1, while the total emissions for Atlanta are less than both Austin, Texas and Chicago, Illinois, their per capita emissions are higher. If Atlanta had per capita emissions equivalent to that of Chicago, the City would achieve a 27% reduction in greenhouse gas emissions (nearly 2.3 MmT CO2e).

Table 1. The total and per capita emissions for Atlanta, GA, Austin, TX, and Chicago, IL


​Total Emissions (MmT CO2e)


Per Capita Emissions (mT CO2e/person)

Atlanta (2019)




Austin (2016)




Chicago (2015)




All three cities have goals to reduce greenhouse gas emissions, though some are more ambitious than others. While Austin plans to be net-zero by 2050, Chicago hopes to reduce its GHGs by 90% from its 1990 levels by 2050. In 2015, Atlanta set the goal to be 40% below 2009 levels by 2030. Figures 2 through 4 show the GHG emission trends for each city over the span of their climate efforts.

Bar graph showing approximately 1 MmT CO2e reduction from 2015 to 2019
Figure 2. Greenhouse gas emissions over time for the City of Atlanta

Histogram showing about 2 MmT CO2e reduction over 5 years. Austin is above its net zero target goal
Figure 3. Greenhouse gas emissions over time for the City of Austin

Histogram shows 1.5 MmT CO2e reduction from 2005 to 2010 and a 3 Mmt CO2e reduction from 2010 to 2015
Figure 4. Greenhouse gas emissions over time for the City of Chicago

Both Austin and Chicago are making progress towards their GHG emission targets. Atlanta has also seen GHG reductions (10%) between 2015 and 2019. It is important to note that the 2015 total emissions calculation for the City of Atlanta included emissions from the Hartsfield-Jackson International Airport. While the 2019 report calculated aviation emissions by scaling the value up from 2015 based on an increase in air travelers, the value was not included in the total emissions for the City of Atlanta due to limited data. As such, the emissions estimate for 2019 might be significantly lower than the true value. Additionally, different methods for natural gas emission calculations in the 2015 and 2019 reports may have resulted in inconsistencies between the two GHG values.

If the City of Atlanta wants to achieve greenhouse gas emissions reductions, it would do well to look to Chicago and Austin for ideas. Table 2 shows the emissions by sector for each city. Take particular note of Chicago’s relatively low transportation emissions and Austin’s relatively low energy & buildings emissions.

Table 2. Percentage of total emissions by sector for Atlanta, GA, Austin, TX, and Chicago, IL


Energy & Buildings



Industrial Processes

Atlanta (2019)





Austin (2016)





Chicago (2015)





As shown in Table 2, Chicago has significantly lower transportation emissions than Austin and Atlanta. Figure 5 shows the public transit map of the City of Chicago, while Figure 6 provides the comparison of Atlanta’s heavy rail lines. While Atlanta has several public transit options in addition to the Metropolitan Atlanta Rapid Transit Authority (MARTA), citizens take the overwhelming majority of their trips on MARTA.

Map of Chicago with different transportation routes stretching from downtown far into the suburbs
Figure 5. Chicago's public transit system

Atlanta's heavy rail is limited within the I-285 perimeter
Figure 6. Atlanta's heavy-rail system

In comparison to Atlanta, Chicago’s public transit options are expansive. The impact is apparent in the modal splits for the two cities. Only 68% of commuters travel alone in their cars in Chicago, while 76% use this mode of transportation in Atlanta. Moreover, Chicago sends 13% of its commuters on public transit while only 3% use the same mode in Atlanta. As Atlanta focuses on reducing its transportation-related emissions, it should look to Chicago for an example of effective transit diversification.

Meanwhile, both Austin and Chicago have made commitments to expand their renewable energy portfolios. Chicago plans to power all city-owned buildings with renewables by 2025 and Austin Energy, a public nonprofit which provides energy to the City of Austin, promises carbon-free generation by 2035. Southern Company, which provides power to the City of Atlanta through its affiliate Georgia Power is committed to net zero power generation by 2050. By contrast, Atlanta has set the goal of providing 100% clean energy to 100% of its residents by 2035. These two goals are at odds with each other.

As an investor-owned utility, Georgia Power has a near complete monopoly on energy generation supplied to the state of Georgia. Georgia Power is regulated by the Public Service Commission, a body of elected officials. The structure of electrical utilities has a direct impact on the ease with which renewables are adopted. Austin’s public nonprofit utility gives citizens more direct control over the trajectory of their energy. Both their current share of energy-related emissions as well as the trend of those emissions shows that the utility has been quick to transition. Chicago’s electrical utility, ComEd, operates in a similar manner as Georgia Power. Yet, Chicago still manages to set lofty goals for its energy-related emission reductions. Although Chicago attributes a large portion of its emissions to energy and buildings, it is unclear how much of that stems from the physical infrastructure of the city rather than the energy sources themselves.

While Chicago and Austin provide a good baseline comparison to Atlanta, each city is unique and therefore requires specific and unique solutions to reduce its emissions; a few of the key recommendations for Energy & Buildings and Transportation are provided below. The students on the Energy & Buildings team suggest that the City of Atlanta form a working group with Georgia Power to develop a 100% clean energy plan, expand access to energy-related data through new and existing programs, and enforce higher mechanical and fuel standards for new buildings. Meanwhile, the Transportation team recommends that the City install more bike, electric vehicle, heavy rail, and bus infrastructure as well as remove minimum parking requirements from new developments.

This brief comparison of GHG inventories for Atlanta, Chicago, and Austin identifies two areas of improvement for the City of Atlanta: transportation and energy & buildings. Key to this analysis is the fact that each inventory applied the GPC standards, allowing for a more direct comparison. That is the value of having global protocols for greenhouse gas inventories. The 2019 GHG Inventory compiled by students in the Fall 2021 Sustainable Cities Studio at the Georgia Institute of Technology delves into these and other comparisons with more detail. In doing so, the report provides a number of specific recommendations for greenhouse gas emission reductions in the City of Atlanta. As more cities conduct GHG inventories, progress can be made towards a sustainable future. Does your city have an up-to-date GHG inventory? Search their website and find out!


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