Carving Up Carbon Reduction Strategies

Elizabeth Willmott presenting at K4C Climate Summit | December 2014

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").

Figure 3. City of Issaquah Carbon Wedge

Creating the Wedges

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:

  • Federal Corporate Average Fuel Economy (CAFE) standard: 2011-model year vehicles are to achieve 24.1 miles per gallon on average, with the assumption that all vehicles on the road in 2030 will have 2011-model year fuel economy. This law is being implemented, but is under threat by the Trump Administration, which is attempting to roll it back.
  • State Renewable Energy Standard: 15 percent renewable energy and all cost-effective conservation by 2020 are to be achieved by Washington’s largest utilities, including Puget Sound Energy. This law is implemented but is under consideration for revision as 2020 approaches. The law is implemented in this analysis assuming that growth in electricity demand between 2014 and 2030 is met first by renewables, which is sufficient to meet the renewable energy standard.  
  • State of Washington Energy Code: By state law, new residential and commercial buildings must achieve a 70 percent reduction in annual net energy consumption below 2006 levels by 2031.

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:

Figure 4. K4C Wedge Analysis of Existing Policies

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:

  • 15 percent reduction in GHG emissions intensity of cars and light trucks, 10 percent of which could be achieved through passage of a statewide clean fuels standard. The remaining five percent could be met through partnerships to expand the use of electric vehicles.  
  • 20 percent reduction in vehicle miles traveled (VMT). Achieving this target requires not only typical mode-shift and land use strategies, but also financial signals such as pay-as-you-drive insurance, which is available in Washington.  
  • 25 percent energy use reduction from existing buildings (including heating) and no net carbon emissions from new buildings. This would entail a much more robust regional retrofit economy than we currently have, and an energy code on par with Massachusetts, California, New York City’s recent mandates to require fossil fuel caps to apply to all buildings over 25,000 square feet.   (Vancouver, WA-based New Buildings Institute, which has advised both MA and CA, is a good source for information on this, as is the Regional Code Collaboration in King County.)  
  • 20 percent increase in renewable electricity countywide, with no coal in the energy portfolio and limited new natural gas-based electricity sources. As Seattle City Light is already virtually carbon-neutral, King County and the cities not on Seattle's electricity must collaborate with Puget Sound Energy to get all fossil fuels out of the electricity supply.

Figure 5. K4C Wedge Analysis of Required New Policies

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.

Developing the Carbon Reduction Strategies

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:

  • 35 percent reduction in passenger vehicle miles traveled  
  • 25 percent reduction in the fuel carbon intensity of passenger vehicles  
  • 40 percent reduction in natural gas use for heating in existing buildings

Figure 6. Shoreline Carbon Wedge Analysis

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:

  • Planning for two Sound Transit light rail stations in 2023, with changes in development rules to accommodate future growth and redevelopment.  
  • Policies that put a price on single-occupant vehicle travel.  
  • Building codes that pave the way for greater adoption of electric vehicles (EVs).  
  • Reducing natural gas consumption in buildings will be challenging, as many homeowners have recently opted for cheaper natural gas over electric resistance heating, but it is required to meet carbon emission reduction targets.  
  • Partnering with community energy efficiency programs to insulate existing buildings and install electric heat pumps and support for a stricter state energy code that requires deeper energy efficiency in new construction.  
  • Statewide carbon pricing that incentivize the switch to low-carbon energy sources and possibly provide revenue to support local clean energy strategies

City of Olympia Carbon Reduction Strategies

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.

Figure 7. Olympia Energy Map

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.

Figure 8. Olympia Carbon Wedge Analysis

Transportation Targets and Strategies: These are the assumptions reflected in the Olympia carbon wedge analysis for transportation:

  • 10% reduction in transport fuel GHG intensity  
  • 30% of all vehicles in Olympia are electric by 2030  
  • Gas-powered cars are 10% more fuel efficient in 2030 than CAFE standards  
  • 5% reduction in vehicle miles traveled in 2030 relative to baseline projections.

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:

  • 20% of residential buildings with natural gas heat switch to high-efficiency electric heat pumps  
  • 25% reduction in building energy use beyond 2030 state energy code

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:

  • Shift from 27% (baseline projection) to 2% of electricity demand met by coal in 2030  
  • Renewable energy (not natural gas) substitution for reduced coal generation

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.

Eileen V. Quigley

Founder & Executive Director
Eileen V. Quigley is Founder and Executive Director of the Clean Energy Transition Institute. Eileen spent seven years at Climate Solutions identifying the transition pathways off oil and coal to a low-carbon future in Washington and Oregon. She built and led the New Energy Cities program, which partnered with 22 Northwest cities and counties to reduce carbon emissions. As Director of Strategic Innovations, she oversaw New Energy Cities, as well as Sustainable Advanced Fuels.
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