IPCC baseline scenarios have over-projected CO2 emissions and economic growth

Abstract

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.

Revisiting the global hydrological cycle: is it intensifying?

“…The Aral Sea has dramatically shrunk in volume since 1950 (Gaybullaev et al, 2012;Cretaux et al, 2019) and has thus contributed to a negative water balance in the land (and positive in the sea, corresponding to the rise in sea level), but its stabilization is a likely possibility for the present and future. Reservoir impoundment has also affected the water balance after the construction of reservoirs (Chao et al, 2008). However, given that the number of new reservoirs has diminished after 2000, while a reservoir has zero further effect on the long-term water balance after its first fill, we do not expect further substantial effects.…”

RCP8.5 tracks cumulative CO 2 emissions

“…A widely used scenario and the most aggressive in assumed fossil fuel use, RCP8.5, by design has an additional 8.5 W/m 2 radiative forcing by 2100. Recent comments in the scientific community ( 1 , 2 ) as well as in magazine-style pieces and the gray literature argue that contemporary emissions forecasts from the International Energy Agency (IEA) make it increasingly unlikely that RCP8.5 describes a plausible future climate outcome. RCP8.5 is characterized as extreme, alarmist, and “misleading” ( 1 ), with some commentators going so far as to dismiss any study using RCP8.5.…”

“…RCP8.5 uses emissions at the 90th percentile level of baseline scenarios then available ( 3 ) and depicts “a relatively conservative business as usual case with low income, high population and high energy demand” ( 4 ). The “business as usual” descriptor for RCP8.5 has been used repeatedly, if somewhat inconsistently and controversially ( 1 , 2 , 5 ). Our discussion here, as well as in the broader literature on the usefulness of RCP8.5, centers on these chosen emissions trajectories.…”

“…It is important to note that no RCP was designed to project existing trends forward—the common assumption of what a “business as usual” scenario would entail. Relative to historical and anticipated trends the stylized facts underpinning RCP8.5 show faster economic growth, overestimates in carbon intensity, overaggressive coal use, and overpricing renewables relative to fossil fuels ( 1 , 2 ). However, these issues are not yet at the threshold to substantially degrade the similarity between total cumulative CO 2 emissions and current policies to midcentury.…”

Optimistically biased economic growth forecasts and negatively skewed annual variation

“…The economic effects of these headwinds-in addition to the possibility of economic damages from climate change-could affect future greenhouse-gas emissions, and both the pace of climate change and societies' adaptive capacities (14; 7). The Shared Socioeconomic Pathway (SSP) scenarios (15; 16; 17)-used widely in climate change research-all project declining economic growth rates throughout the 21st century (18), and recent trends (even before the COVID-19 pandemic) have tracked near or below the lowest growth rates in the SSP range in all world regions (7).…”