Abstract
Abstract. Information on the relationship between cumulative fossil CO2 emissions and multiple climate targets is essential to design emission mitigation and climate adaptation strategies. In this study, the transient response of a climate or environmental variable per trillion tonnes of CO2 emissions, termed TRE, is quantified for a set of impact-relevant climate variables and from a large set of multi-forcing scenarios extended to year 2300 towards stabilization. An ∼ 1000-member ensemble of the Bern3D-LPJ carbon–climate model is applied and model outcomes are constrained by 26 physical and biogeochemical observational data sets in a Bayesian, Monte Carlo-type framework. Uncertainties in TRE estimates include both scenario uncertainty and model response uncertainty. Cumulative fossil emissions of 1000 Gt C result in a global mean surface air temperature change of 1.9 °C (68 % confidence interval (c.i.): 1.3 to 2.7 °C), a decrease in surface ocean pH of 0.19 (0.18 to 0.22), and a steric sea level rise of 20 cm (13 to 27 cm until 2300). Linearity between cumulative emissions and transient response is high for pH and reasonably high for surface air and sea surface temperatures, but less pronounced for changes in Atlantic meridional overturning, Southern Ocean and tropical surface water saturation with respect to biogenic structures of calcium carbonate, and carbon stocks in soils. The constrained model ensemble is also applied to determine the response to a pulse-like emission and in idealized CO2-only simulations. The transient climate response is constrained, primarily by long-term ocean heat observations, to 1.7 °C (68 % c.i.: 1.3 to 2.2 °C) and the equilibrium climate sensitivity to 2.9 °C (2.0 to 4.2 °C). This is consistent with results by CMIP5 models but inconsistent with recent studies that relied on short-term air temperature data affected by natural climate variability.
Highlights
How multiple climate targets are related to allowable CO2 emissions provides basic information to design policies aimed to minimize severe or irreversible damage from anthropogenic climate change (Steinacher et al, 2013)
We explore how different climatic variables respond to a pulse-like input of carbon into the atmosphere (Fig. 2) and determine the so-called impulse response function (IRF) for the different climate variables
We have quantified the transient response to cumulative CO2 emissions, TREX, for multiple Earth system variables, the responses to a CO2 emission pulse defining the IRF, and three other important climate metrics, the equilibrium climate sensitivity (ECS), the transient climate response (TCR), and the TCRE
Summary
How multiple climate targets are related to allowable CO2 emissions provides basic information to design policies aimed to minimize severe or irreversible damage from anthropogenic climate change (Steinacher et al, 2013). The emission of carbon dioxide from burning of fossil fuels is by far the most dominant driver of the ongoing anthropogenic climate change and of ocean acidification (IPCC, 2013; Gattuso et al, 2015). The increase in a broad set of climate variables such as atmospheric carbon dioxide (CO2), CO2 radiative forcing, global air surface temperature, or ocean acidification depends on cumulative CO2 emissions (Allen et al, 2009; IPCC, 1995). It is informative to quantify the link between cumulative, total CO2 emissions, and different climate variables. It is advantageous to represent a climate target, such as the United Nations’ 2 ◦C global mean surface air temperature target, in terms of allowable total CO2 emissions because this is an communicable emission mitigation goal. While the link between cumulative CO2 emissions and global mean surface air temperature has been extensively studied (IPCC, 2013), relatively little attention has been paid to the relationship between cumulative CO2 emissions and other impact-relevant variables such as ocean acid-
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