Burgess and 3 co-authors, reported in late 2020 that scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are central to climate science and policy. Recent studies find that observed trends and International Energy Agency (IEA) projections of global CO2 emissions have diverged from emission scenario outlooks widely employed in climate research. The authors quantified the bases for this divergence, focusing on Kaya Identity factors: population, per-capita gross domestic product (GDP), energy intensity (energy consumption/GDP), and carbon intensity (CO2 emissions/energy consumption).
The authors compared 2005–2017 observations and IEA projections to 2040 of these variables, to ‘baseline’ scenario projections from the IPCC’s Fifth Assessment Report (AR5), and from the shared socioeconomic pathways (SSPs) used in the upcoming Sixth Assessment Report (AR6). The authors found that the historical divergence of observed CO2 emissions from baseline scenario projections can be explained largely by slower-than-projected per-capita GDP growth—predating the COVID-19 crisis. They also found carbon intensity divergence from baselines in IEA’s projections to 2040. IEA projects less coal energy expansion than the baseline scenarios, with divergence expected to continue to 2100.
Future economic growth is uncertain, but the authors showed that past divergence from observations makes it unlikely that per-capita GDP growth will catch up to baselines before mid-century. Some experts hypothesize high enough economic growth rates to allow per-capita GDP growth to catch up to or exceed baseline scenarios by 2100. However, the authors argued that this magnitude of catch-up may be unlikely, in light of: headwinds such as aging and debt, the likelihood of unanticipated economic crises, the fact that past economic forecasts have tended to over-project, the aftermath of the current pandemic, and economic impacts of climate change unaccounted-for in the baseline scenarios. Their analyses inform the rapidly evolving discussions on climate and development futures, and on uses of scenarios in climate science and policy.
The authors concluded that recent (post-2005) trends and energy outlook projections (to 2040) of global CO2 emissions are substantially lower than projected by baseline scenarios used in the IPCC’s Fifth (AR5) and Sixth (AR6) Assessment Reports, and are well off-track from widely-cited high-emission marker scenarios such as RCP8.5. We show that this divergence owes largely to per-capita GDP and carbon intensity growth slower than projected in baseline scenarios. The gap between observed and projected carbon intensity is very likely to continue to increase throughout the 21st century due to
the implausible assumptions high-emission scenarios make about future fossil-fuel expansion (Ritchie and Dowlatabadi 2017). The gap between observed and projected per-capita GDP is unlikely to close by 2040 due to inertia but is more uncertain to 2100.
The authors concluded with three immediate implications of our analysis. First, if scenarios underpin research used to inform policy, such scenarios should be kept up-to-date and corrected when divergences from real-world trends or improved projections are identified. It is unsurprising that scenarios used by the IPCC—many developed over a decade ago—are diverging from the real world. They should be updated more regularly.
Second, policy-relevant climate research, including evaluations of near-term policy options, should use or consider near-term energy scenarios (including but not limited to the IEA’s), which are updated on an annual basis. These provide a more reliable guide to the next several decades than century-long scenarios, which are appropriate in exploratory climate research. Finally, in a world where climate policy is now BAU, the historical distinction of policy and no-policy scenarios may be obsolete, and a new approach to scenario planning may be needed (e.g. Pielke and Ritchie 2020).
The authors analysis supports the conclusions drawn by previous studies (Hausfather and Peters 2020, Pielke and Ritchie 2020) that high-emission AR5 (RCP8.5) and high-emission AR6 (i.e. SSP3-7.0 and SSP5-8.5) baselines should not be utilized as reference scenarios in climate research. The authors' findings also suggest that future scenario development efforts should consider wider ranges of assumptions regarding economic growth due to high uncertainty in growth futures, and the possibility that the current pandemic will bring lasting changes to the economic system.
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