IPCC baseline scenarios over-project CO2 emissions and economic growth

Scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are central to climate science and policy. Recent studies have found that observed trends and International Energy Agency (IEA) projections of global CO2 emissions are diverging below the emission scenario outlooks widely employed in climate research. Here, we quantify the bases for this divergence, focusing on the Kaya Identity factors: population, per-capita GDP, energy intensity (energy consumption/GDP), and carbon intensity (CO2 emissions/energy consumption). We compare 2005-2017 observations, and IEA projections of these variables to 2040, to “baseline” (or “no policy”) scenario projections from the IPCC’s Fifth Assessment Report (AR5), and from the Shared Socioeconomic Pathways (SSPs) used for the upcoming Sixth Assessment Report (AR6). We find that much of the historical divergence of observed CO2 emissions from these baseline scenario projections can be explained by slower-than-projected per-capita GDP growth—a divergence that predates the COVID-19 crisis. We find that carbon intensity divergence is a key factor in IEA’s projections to 2040 in all scenarios—the IEA projects less favorable conditions for coal energy expansion than the baseline scenarios anticipated—a trend expected to continue to 2100. Future economic growth is uncertain, but we show that per-capita GDP growth is unlikely to catch up to the baselines before mid-century. Some experts hypothesize economic growth rates that could allow per-capita GDP growth to catch up to or even exceed the baseline scenarios by 2100. However, this magnitude of catch-up requires idealized social conditions that could face headwinds from presently unanticipated future economic crises, where the economic aftermath of the current pandemic could be an analog to the impacts from climate change. Our results inform the rapidly evolving discussions on climate and development futures, and on uses of scenarios in climate science and policy.