Emissions scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are central to climate change research and policy. Here, we identify subsets of scenarios of the IPCC’s 5th (AR5) and forthcoming 6th (AR6) Assessment Reports, including the Shared Socioeconomic Pathway (SSP) scenarios, that project 2005-2050 fossil-fuel-and-industry (FFI) CO2 emissions growth rates most consistent with observations from 2005-2020 and International Energy Agency (IEA) projections to 2050. These scenarios project between 2 and 3 degrees C of warming by 2100, with a median of 2.2 degrees C. The subset of plausible IPCC scenarios does not represent all possible trajectories of future emissions and warming. Collectively, they project continued mitigation progress and suggest the world is presently on a lower emissions trajectory than is often assumed. However, these scenarios also indicate that the world is still off track from limiting 21st-century warming to 1.5 or below 2 degrees C.

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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. Here, we quantify 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). We compare 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). We find 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. We also find 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 we show 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, we argue 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. Our analyses inform the rapidly evolving discussions on climate and development futures, and on uses of scenarios in climate science and policy.

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Scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are central to climate science and policy. A recent Nature commentary found observed trends and International Energy Agency (IEA) projections of global CO2 emissions substantially diverging from high-emission scenarios such as RCP8.5, which are often treated as equivalent to ‘business as usual’ in climate research and assessment. Here, we quantify the bases for this divergence by comparing “baseline” (or “no policy”) scenario projections of key fossil-fuel CO2 emission drivers to observations from 2005-2017, and also to projections through 2040 from world energy outlooks. We find most of the baseline scenarios used in the Fifth (AR5) and designed for the forthcoming Sixth (AR6) IPCC Assessment Reports have over-projected per-capita GDP growth—severely in most developing regions, and slightly in other regions—and have slightly over-projected carbon intensity (CO2 emissions/primary energy) in most regions. These baseline scenarios will likely continue to over-project carbon intensity through 2040 and beyond, in part due to unrealistic assumptions about fossil-fuel expansion. Long-term economic growth outlooks lack consensus among economists, but we argue that most of these baseline scenarios will likely continue over-projecting per-capita GDP to at least 2040 due to inertia. Our results inform the rapidly evolving discussion on uses of scenarios in climate science and policy.