Schwalm et al.; PNAS August 18, 2020 

Figure: Total cumulative CO₂ emissions since 2005 through 2020, 2030, and 2050. Data sources: Historical data from Global Carbon Project; emissions consistent with RCPs are from RCP Database Version 2.0.5 (https://tntcat.iiasa.ac.at/RcpDb/); “business as usual” and “business as intended” are from IEA Current Policies and Stated Policies scenarios, respectively (9). IEA data (fossil fuel from energy use only) was combined with future land use and industrial emissions to estimate total CO₂ emissions. Future land use emissions estimated from linear trend fit to 2005 to 2019 Global Carbon Project land use emissions data. Industrial emissions estimated as 10% of total emissions. Final IEA data use historical values through 2020 and scenario values thereafter. Biotic feedbacks are not included in any IEA-based estimate. Note that RCP forcing levels are intended to represent the sum of biotic feedbacks and human emissions.

Climate simulation-based scenarios are routinely used to characterize a range of plausible climate futures. Despite some recent progress on bending the emissions curve, RCP8.5, the most aggressive scenario in assumed fossil fuel use for global climate models, will continue to serve as a useful tool for quantifying physical climate risk, especially over near- to midterm policy-relevant time horizons. Not only are the emissions consistent with RCP8.5 in close agreement with historical total cumulative CO₂ emissions (within 1%), but RCP8.5 is also the best match out to midcentury under current and stated policies with still highly plausible levels of CO₂ emissions in 2100.

Because future climate depends on future human behavior, which is inherently unpredictable, scenarios are used to characterize a range of plausible climate futures and to illustrate the consequences of policy choices. For the most recent Intergovernmental Panel on Climate Change (IPCC) Assessment Report (AR5) four scenarios known as Representative Concentration Pathways (RCPs) were developed. The RCP scenarios as used in global climate models use historical greenhouse gas emissions until 2005, and projected emissions subsequently. A widely used scenario and the most aggressive in assumed fossil fuel use, RCP8.5, by design has an additional 8.5 W/m² radiative forcing by 2100. Recent comments in the scientific community 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. This line of argumentation is not only regrettable, it is skewed.

See https://www.pnas.org/content/117/33/19656