A community that has set an ambitious carbon-reduction target and mapped its sources and uses of energy is ready to determine how to carve up its reduction strategies using a Carbon Wedge Analysis, a process that Princeton University’s Carbon Mitigation Initiative pioneered in 2004.
The way carbon wedge analyses work is that you plot both a growth scenario of carbon emissions based on projected population growth and the reduced emissions that the community-wide target would require. Visually, the large triangular area between the growth line and the target line represents the emission reductions that the community would need to make to meet the target in a scenario of emissions growth.
The graphic below in Figure 3 shows the City of Issaquah’s carbon reduction challenge to achieve 80% of its 2007 GHG levels by 2050. You see the triangle that is created when you compare where the city’s unchecked emissions will rise to (somewhere north of 650,000 metric tons of C02 equivalent) by 2050.
The green line shows the target level in 2050. The red-dotted baseline shows where the city started in 2007. We divide the large triangle into two smaller triangles: additional emissions growth beyond the base year ("GHG emissions to be avoided"), and emissions reductions to be made in order to meet the target ("GHG emissions to be reduced below base year").
The next step is to set sectoral target emissions reductions to add up to the overall community-wide target. For this we look at 2030 instead of 2050 because it is midway to 2050; within planning timelines that that cities and counties work with; and more realistic to when choosing strategies that are available in 2017, given how dynamic clean energy solutions are and how likely they will change in the coming decades.
We recommend cities adopt a 50% reduction from their base year by 2030 (50X30), which is more aggressive than the 80% by 2050 (80X50) targets most have set. We are well aware coming out of the Paris Climate Agreement that the United States must be on a path to bend its emissions curves downward by 2020, so getting cities focused on achieving significant reductions by 2030 is critically important.
Work with the King County-Cities Climate Collaboration (K4C), a voluntary partnership of 13 cities in King County (Bellevue, Burien, Issaquah, Kirkland, Mercer Island, Normandy Park, Redmond, Renton, Sammamish, Seattle, Shoreline, Snoqualmie, and Tukwila) that share expertise and best practices on carbon reduction, offered a unique opportunity for the County and partner cities to jointly map individual and shared carbon reduction strategies.
To create the wedge reductions for the K4C, we first estimated the combined order-of-magnitude carbon emissions reduction by the year 2030 associated with the following three existing federal and state policies and laws:
In the K4C's case, if these laws were complied with to the full extent, these reductions would bring reductions to approximately its baseline level as Figure 4 below shows:
Once the remaining reductions are known, we calculate the carbon reduction associated with strategies in transportation, building energy efficiency, and renewable energy supply, consistent with national best practices. In the case of the K4C, we looked at achieving the 50x30 target through three different pathway scenarios—one with greater emphasis on reducing vehicle miles traveled (VMT); one with greater emphasis on increased building energy efficiency; and one with a balance of the two. Figure 5 depicts what the following targets would produce in reductions:
An important caveat about this work is that we portray these findings by order of magnitude—not a ton-by-ton inventory. This work provides a framework for cities with limited resources to help city management determine their overall carbon-reduction opportunities and offer guidance for where project-level engineering analyses might be commissioned.
The targets depicted in the carbon wedge analysis are mathematically derived goals to achieve. Determining the actual strategies that the city must pursue to achieve those carbon emissions reductions is where the hard work lies. We will look at how two cities attacked this challenge: City of Shoreline and City of Olympia.
In 2013, the Shoreline City Council adopted the 50x30 and 80x50 goals and Shoreline's Climate Action Plan had outlined efforts that would begin to make reductions in the areas of energy and water conservation, materials and waste management, transportation, and urban greenery.
As discussed when we compared Shoreline to Issaquah’s Energy Maps, Shoreline's electricity comes from Seattle City Light’s nearly carbon-free supply, which means the focus has to be on reducing petroleum use in cars and natural gas use in buildings to meet its carbon reduction goals. The city chose the following broad carbon reduction targets as depicted in Figure 6:
To develop the specific strategies to meet these targets, Shoreline engaged all city staff responsible for transportation and buildings in charrettes to assess the city’s readiness to tackle specific initiatives, and the barriers they needed to overcome. The findings resulted in an extensive spreadsheet of specific actions by sector presented to the City Council on October 13, 2014. Noted actions include:
The City of Olympia, like Issaquah, has Puget Sound Energy as its electricity provider, so its Energy Map has more carbon emissions in its electricity supply than in transport as we see in Figure 7.
Olympia also chose the 50X30 target and its Carbon Wedge Analysis is depicted in Figure 8. The wide array of carbon-reduction strategies are delineated by color.
Transportation Targets and Strategies: These are the assumptions reflected in the Olympia carbon wedge analysis for transportation:
The City is considering the reducing transportation carbon emissions by 2030 through: 1) community-wide vehicle miles traveled (VMT) reduction, 2) electric vehicle adoption, and 3) clean fuels/technologies. We modeled several mechanisms that could reduce carbon emissions from transportation, including reducing the greenhouse gas intensity of transportation fuels and increasing vehicle efficiency for internal combustion engine vehicles.
We also considered a reduction in vehicle-miles traveled and an increase in the percentage of electric vehicles in Olympia’s vehicle fleet, the latter modeled on the ambitious electric vehicle adoption target enacted by the City of Seattle.
Building Energy Use: These are the assumptions reflected in the carbon wedge analysis for building energy emission reduction:
The City is looking at how to reduce energy use for existing buildings community-wide by 2030 by switching residential heating from natural gas to high-efficiency electric heat pumps and by performing more general commercial and residential building energy efficiency retrofits. Pursuing this target will involve deep partnership with building retrofit organizations.
Decarbonized Electricity: These are the assumptions reflected in the carbon wedge analysis for decarbonizing electricity:
The City is examining how to reduce its reliance on coal and increase its electricity generation from renewable sources. We modeled a scenario in which electricity consumption from coal sources is reduced and replaced by electricity from renewable sources. Pursuing this target will involve partnering with Puget Sound Energy.
Net zero emissions from new buildings community-wide by 2030: Although we did not explicitly model a net-zero emissions new buildings scenario as a standalone wedge, our analysis comments on the importance of reducing energy consumption and emissions in new construction as a way to meet community emissions reduction goals, help the Puget Sound region move toward a low-carbon, low-cost fuel mix, and show regional leadership.
When cities set ambitious but achievable greenhouse gas emission reduction targets, understand the carbon intensity of energy sources in their communities, do the carbon math to understand the delta between their projected emissions and their reduction goals, and break down the strategies by target to reduce their emissions, they have the roadmap they need to set them on a path to the rapid and meaningful carbon emissions reductions that the climate crisis requires.