Integrating and evaluating

Many cities have experience in setting carbon reduction targets[1] for either their own operations, or in-boundary emissions. The same approaches can be used to set targets for reducing consumption emissions. Consumption-based targets differ, however, because they include emissions that occur both inside and outside the city boundaries. This means that cities may get credit (or criticism) for shifts in emissions in the production chain that occur outside the city. For this reason, consumption-based targets should be viewed as a supplement to—rather than replacement for—traditional territorial emissions reduction targets.

Regardless of the approach taken, cities should stick to the same principles[2] recommended for territorial emissions targets: include a baseline and a timeframe for achievement, and ideally set both long-term and interim targets.

Set targets based on expectations of what can feasibly be achieved: Cities can set targets by estimating how much they can reduce emissions using existing or available policies, technology, and behavior shifts. This involves constructing long-term scenarios[3], to estimate how much emissions may drop over time, and setting targets accordingly. Because consumption emissions occur both inside and outside city boundaries, these assessments need to take into account local, national, and global trajectories.

The benefit of using a scenario-based approach is that it provides a good framework for planning and tracking emissions reductions. However, this doesn’t necessarily mean that targets generated from this approach will be sufficient. A scenario analysis[4] conducted in the City of Seattle estimated that an aggressive set of measures would only reduce emissions to 15 tCO2e per capita—far from what is necessary to limit dangerous warming.

Set aspirational targets: Another approach is to set highly ambitious targets, without reference to what is presently viewed as feasible. The aim in setting aspirational targets is to inspire transformational change. This is a simpler approach to target setting than the scenario-based approach, however, the risk is that targets may be dismissed as unattainable if the gap between aspiration and reality is too large.

One model of aspirational consumption emission target setting is the City of Stockholm’s “One Tonne Life” experiment.The City—in partnership with researchers and businesses—set out to try to reduce one household’s emission to under a tonne of CO2e per year (while this wasn’t a city-wide target, this number could easily be scaled up to encompass all households). Though the goal wasn’t quite achieved, this experiment lead to the development of new technologies, and highlighted where policy supports might be needed for household emissions reduction. It also provided a useful estimate for future target setting of how much households in the city could potentially reduce emissions (in this case, 1.5–2.8 tCO2e, depending on comfort levels required).

Set targets based on a city's "fair share" of global emissions: Targets can also be set by estimating the contribution a city should make to reducing global emissions. One approach—known as a “science-based target”—is to set targets against the global carbon budget that researchers have estimated can be utilized while still keeping temperature rise below 1.5 or 2°C. This approach involves calculating the city’s share of the remaining global carbon budget (based on population size, and potentially other factors such as responsibility for historical emissions), and then estimating the emissions reductions trajectory needed to stay within this budget and setting targets accordingly.

The benefit of this “science-based” approach is that it focuses on what is needed to avert dangerous climate change. A key challenge, however, is a city's "fair share" of global mitigation can depend on numerous factors, including equity considerations (e.g. richer cities in developed countries may need to be more ambitious than the global average to contribute equitably to global efforts). Furthermore, "science-based" targets defined at a city level can sometimes ignore the bigger picture, e.g. what a city's best strategy would be in a world where governments at all levels are working together.

In general terms, the implication from a science-based approach is clear: for most industrialized cities, a rapid decarbonisation of the economy is needed to avert the most dangerous effects of climate change.[5] The City of Eugene, Oregon, for example, adopted a target consistent with a 1.5°C goal that equates to an annual average emission reduction of 7.6%.[6]