The Intergovernmental Panel on Climate Change (IPCC) is the United Nations body for assessing the science related to climate change. The IPCC just came put with its 2021 Summary for Policymakers, which “provides a high-level summary of the understanding of the current state of the climate, including how it is changing and the role of human influence, and the state of knowledge about possible climate futures, climate information relevant to regions and sectors, and limiting human-induced climate change”. As part of its discussion of possible climate futures, the report assesses the climate response to five illustrative scenarios called shared socio-economic pathways, or SSPs. The SSPs differ in their estimated trajectories of emissions and consequent global warming over the next eight decades, which vary depending on socio-economic assumptions, levels of climate change mitigation and air pollution controls for aerosols and other substances. The report does not address the feasibility or likelihood of individual scenarios but notes that alternative scenarios may arrive at the same climate outcomes.
As is clear from the following charts, the five SSPs represent a broad range of possible climate futures. First, CO2 emissions:
Next, the estimated rise in global surface temperature over the course of this century:
Per these SSPs, global surface temperature is very likely to rise between 1.0°C to 5.7°C by 2100, compared to global temperatures during the period of 1850-1900, with a best estimate range of between 1.4°C and 4.4°C. Note that the best estimate for rise in global surface temperature from 1850-1900 to 2019 is 1.07°C. Thus, compared to 2019, the SSP-based best estimate increase in temperature this century would be between .37°C to 3.33°C (1.4-1.07 and 4.4-1.07). For those who think in Fahrenheit, here are the SSP best estimates converted to Fahrenheit:
Reference:
IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press. https://www.ipcc.ch/report/ar6/wg1/