Quantifying the probability distribution function of the transient climate response to cumulative CO2 emissions

• Published in: Environmental research letters Vol. 15; no. 3; pp. 034044 - 34055
• Main Authors: Spafford, Lynsay, MacDougall, Andrew H
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 2020
• Subjects: Carbon; carbon budgets; Carbon dioxide; Carbon dioxide emissions; carbon-climate feedback; climate sensitivity; Diffusivity; Distribution functions; Emissions; land-borne fraction of carbon; Mathematical models; Monte Carlo simulation; Normal distribution; Ocean dynamics; Parameter sensitivity; Probability density functions; Probability distribution; Probability distribution functions; Radiative forcing; Skewed distributions; Statistical analysis; Surface temperature; transient climate response to cumulative CO; transient climate response to cumulative CO2 emissions; Uncertainty
• ISSN: 1748-9326
• DOI: 10.1088/1748-9326/ab6d7b

The Transient Climate Response to Cumulative CO2 Emissions (TCRE) is the proportionality between global temperature change and cumulative CO2 emissions. The TCRE implies a finite quantity of CO2 emissions, or carbon budget, consistent with a given temperature change limit. The uncertainty of the TCRE is often assumed be normally distributed, but this assumption has yet to be validated. We calculated the TCRE using a zero-dimensional ocean diffusive model and a Monte-Carlo error propagation (n = 10 000 000) randomly drawing from probability density functions of the climate feedback parameter, the land-borne fraction of carbon, radiative forcing from an e-fold increase in CO2 concentration, effective ocean diffusivity, and the ratio of sea to global surface temperature change. The calculated TCRE has a positively skewed distribution, ranging from 1.1 to 2.9 K EgC−1 (5%-95% confidence), with a mean and median value of 1.9 and 1.8 K EgC−1. The calculated distribution of the TCRE is well described by a log-normal distribution. The CO2-only carbon budget compatible with 2 °C warming is 1100 PgC, ranging from 700 to 1800 PgC (5%-95% confidence) estimated using a simplified model of ocean dynamics. Climate sensitivity is the most influential Earth System parameter on the TCRE, followed by the land-borne fraction of carbon, radiative forcing from an e-fold increase in CO2, effective ocean diffusivity, and the ratio of sea to global surface temperature change. While the uncertainty of the TCRE is considerable, the use of a log-normal distribution may improve estimations of the TCRE and associated carbon budgets.