New York city skyscrapiers
New York city skyscrapers. Photo: Kevin Jarrett / Flickr.

The issue of inflated baselines has long plagued researchers focused on climate change mitigation policy.  From the early days of the Kyoto Protocol to the recent commitments under the Paris Agreement, the tendency for actors to overstate what they might otherwise emit – therefore making their pledges to reduce greenhouse gas emissions appear all that much more ambitious – has persisted.

So, it is not surprising that entities newly welcomed into the UNFCCC process – cities, corporations, and other non-state actors – may now at times present similar “hot air” claims. A common technique is to forecast future baseline emissions above business-as-usual, then claim credit for reductions below that inflated level.

In one recent example, a study by C40 Cities claimed that the world’s cities could achieve 800 Gt CO2e of global GHG abatement cumulatively through 2050. But they used a “worst case” baseline that assumed that the GHG-intensity of energy would not improve but for the action of city governments, even as the IEA and others have forecast continued decline based on national policies alone.

In addition, that study appears to have overestimated economic activity (and therefore energy use) in cities by failing to correct their nominal GDP forecasts for inflation. This error alone – compounded at over 2% annually – means they overestimated business-as-usual urban GHG emissions in 2050 (and corresponding abatement potential) by a factor of two.

As a result, C40’s forecasts of urban emissions grow to a level about four times higher in 2050 than in the IEA’s baseline, “New Policies Scenario” (see top panelin Figure). This growth in baseline estimates also greatly exceeds those created by C40’s own member cities, which (like the IEA) take into account the national-level policies recently put in place. (See bottom panel in Figure).

SEI 2017 blog city baselinesfiga
SEI 2017 blog city baselinesfiga

Figure. C40’s Deadline 2020 report forecasts growth in baseline urban emissions to be substantially higher than other researchers (for the entire globe, top panel) and substantially higher than their member cities’ own assessments (bottom panel) in their climate action plans

Source for individual city baselines: City and County of San Francisco (figure 5); SEI for City of Seattle (figure 5); Greater London Authority (figure 2.6); City of New York (page 17).

The C40 report’s authors are not the only ones to calculate urban abatement potential using an exaggerated baseline. For example, the authors of a recent article in Nature Climate Change , “Urban infrastructure choices structure climate solutions”, used a baseline that assumed that new infrastructure built through 2050 would be no better than “current standards and technology”.

Using a “worst case” baseline to understand mitigation potential is not wrong per se. For example, study authors may argue that using such a baseline helps bring greater attention to the potential scale of urban emissions, which can be important to gaining political support from key constituencies, such as city mayors. Still, a danger arises when these abatement estimates are introduced in broader (national or international) policy discussions, suggesting that certain actors – for example city governments – can contribute more to global abatement than is realistic in the presence of other policy, technology, and economic drivers.

Avoiding such confusion requires transparency. Researchers should publish – as the IEA does – what their baseline assumptions are, and why those assumptions are appropriate for the questions they are investigating. For example, the limitations and errors in the C40 study could have been much clearer (and perhaps addressed earlier) if they had published the business-as-usual population, GDP, energy-intensity, and carbon-intensity forecasts underlying their analysis, at least at the global level (if forecasts for individual urban areas were not possible).

From a policy perspective, it helps enormously if analysts can start from a similar understanding of business-as-usual emissions. UNEP, for example, has taken great care to do so in its annual Emissions Gap Report. That report attempts to address what “gap” exists between current policies and a global emissions pathway consistent with a 2˚C limit, as well as identify what new policies and measures (including by cities) may help “bridge” the gap. Efforts to understand how city-scale actions can contribute should be set in a similar context, so that all actors can better understand their respective roles and responsibilities.

There is little doubt that cities have critical roles to play in achieving a low-carbon future. Indeed, numerous researchers have clearly articulated the important and largely underappreciated role of cities in addressing climate change. Still, we must remain vigilant in our methods and clear on their implications to avoid over-promising what can be achieved. Doing so will facilitate productive conversations about what actions different institutions and levels of government can take in the pivotal next few years of climate policy-making.

To learn more about SEI’s recent research into the potential for urban actions to reduce global GHG emissions, see D. Broekhoff, P. Erickson and C. M. Lee, What Cities do Best: Piecing Together an Efficient Global Climate Governance .  A follow-up study focusing on national-scale policy levers to support urban GHG emissions abatement is forthcoming in summer 2017, in partnership with the New Climate Economy .