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  • Writer's pictureDr. Lee Anne Willson

Carbon Budgets for cities

What they are and why they matter

In the end, what matters is not what year we pick for zero, or what reduction we pick for 2030, but the total amount of CO2 and other greenhouse gases we add to the atmosphere, the total emissions. This total is also called the “carbon budget.” Three ways to reduce emissions faster are to start sooner, to decrease more rapidly, or to get to zero sooner. Upper left, Upper right, and Lower left show those three approaches. Lower right shows that starting out soon with a rapid decline adds less carbon than starting slowly, even with the same zero-carbon date.

Figure 1. The blue area is the total amount of carbon added to the atmosphere on the faster approach, and the red shows how much more carbon the slower approach adds to the atmosphere – that is, the total for the slower case is the sum of what shows in red and what shows in blue. Using a carbon budget approach:

When one of the possible plans is displayed in what I like to call a “rainbow diagram,” the various colors show how much the emissions are reduced from specific actions, and then the grey area left is the carbon budget – the total emission between now and when we reach zero. Here is an illustration from the presentation that SSG made to the Supplemental Input Committee:

Figure 2: The grey area is the total carbon added to the atmosphere after all the actions have been done to reduce emissions. The various plans that the Council has been invited to consider have been presented in a slightly different way, using the reduction in emissions by a certain date (2030) and a zero-by date (2050). The bigger the reduction by 2030, the smaller the net emission – the smaller the carbon budget.

Figure 3: For a fixed starting date of 2018, how three plans compare to “business as usual” (BAU aka Business as Planned, since it includes only projects that have started or already been funded.) In Figure 3, the purple line (“Fair share approach”) has the smallest carbon budget, and the “Business as usual” line has a much larger carbon budget. In fact, the purple line (fair share) has half the carbon budget of the two lines in the middle, and those two have half the carbon budget of the BAU line. One way to choose a plan for Ames is to agree on a carbon budget, and then figure out what the curve will look like. This is the idea behind “fair share” approaches – what is our budget for future carbon emission? OK, how will we live within that budget? A simple example: Here is an illustration of how that can be done. Take a simple situation with four different sources of emission – perhaps it is natural gas, electricity, buildings and transportation. We might break our budget down into pieces – 20% for natural gas, 30% for electricity, 10 percent for buildings and 40% for transportation. To visualize this, I made 20 blocks, each one colored for the source – yellow for natural gas, light blue for electricity, dark grey for buildings and red for transportation. To develop my CAP from this, I figure out how to stack the blocks over the next ten years. Then my plan might look like one of these:

These two examples have the same total emission. Example A has an earlier zero-by date, and Example B has a lower % reduction in the middle (years 4 and 5), but from the point of view of total emissions, they are equal. This says that if we can make rapid strides in some of the easier actions to reduce emissions, we can take longer to find ways to reduce those sources that are most difficult to reduce. The C40 Cities: You may hear some discussion about whether Ames could become a C40 city. The “Fair share/carbon budget” option presented by the consultants, SSG, follows the leadership example set by the C40 program. The C40 program ( is an alliance of cities that are making a commitment to meeting fair share commitments for carbon emissions. Their summary of the principles: CARBON BUDGETS AND FAIR SHARE EMISSIONS Equity is a consideration in all recommended methodologies when calculating a city's carbon budget. Carbon budgets are a simplified measurement of the additional emissions that a city or country can still emit, if the world is to limit global heating to 1.5 °C. The carbon budget of a city or country will vary based on the following factors:

  1. Responsibility: GHG emissions, particularly CO2 emissions, accumulate in the atmosphere over time. Many industrialized countries have been the source of dangerous carbon emissions for the past 200 years. These past emissions are termed historical emissions. Other countries are still developing their economies and are permitted to peak their emissions later. These are called late emissions. Carbon budgets take into account historical emissions and late emissions, tasking those countries and cities who are most responsible for global CO2 accumulation with reducing their emissions.

  2. Capacity*: it is acknowledged that different cities and countries have varied capacities to respond to the challenge of climate change based on their respective levels of socio-economic development.

  3. Inter-generational justice: present generations have certain duties towards future generations, in terms of decreasing climate change risks, increasing the availability of natural resources and the health of the planet's ecosystems.

*When they speak of capacity, they are referring to resources that a community has, such as people, capital, natural resources or being in an area with strong wind or solar energy programs.


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