Hysteresis of the Earth system under positive and negative CO2 emissions

Aurich Jeltsch-Thömmes,Fortunat Joos,Thomas F Stocker

doi:10.1088/1748-9326/abc4af

Abstract

Carbon dioxide removal (CDR) from the atmosphere is part of all emission scenarios of the IPCC that limit global warming to below 1.5 °C. Here, we investigate hysteresis characteristics in 4× pre-industrial atmospheric CO2 concentration scenarios with exponentially increasing and decreasing CO2 using the Bern3D-LPX Earth system model of intermediate complexity. The equilibrium climate sensitivity (ECS) and the rate of CDR are systematically varied. Hysteresis is quantified as the difference in a variable between the up and down pathway at identical cumulative carbon emissions. Typically, hysteresis increases non-linearly with increasing ECS, while its dependency on the CDR rate varies across variables. Large hysteresis is found for global surface air temperature (SAT), upper ocean heat content, ocean deoxygenation, and acidification. We find distinct spatial patterns of hysteresis: SAT exhibits strong polar amplification, hysteresis in O2 is both positive and negative depending on the interplay between changes in remineralization of organic matter and ventilation. Due to hysteresis, sustained negative emissions are required to return to and keep a CO2 and warming target, particularly for high climate sensitivities and the large overshoot scenario considered here. Our results suggest, that not emitting carbon in the first place is preferable over carbon dioxide removal, even if technologies would exist to efficiently remove CO2 from the atmosphere and store it away safely.

Highlights

The future trajectory in human-caused carbon dioxide (CO2) emissions and, global warming, is uncertain
Diagnosed cumulative emissions depend on equilibrium climate sensitivity (ECS) and the carbon dioxide removal (CDR) rate
ECS and are in the range of 2 640 (ECS = 5 ◦C) to 2 885 GtC (ECS = 2 ◦C) in year 140 (figure 1(b)) when atmospheric CO2 peaks at 4 × preindustrial (figure 1(a))

Summary

Introduction

The future trajectory in human-caused carbon dioxide (CO2) emissions and, global warming, is uncertain. In the Earth system, it is caused by system time lags and non-linear processes (Stocker 2000, Boucher et al 2012). Irreversibility arises when a perturbation in climate or the environment does not return to its unperturbed state within a time frame relevant for humans. A range of different forcing scenarios and models has been applied to investigate hysteresis and irreversibility in Earth system responses (Maier-Reimer and Hasselmann 1987, Enting et al 1994, Stocker and Schmittner 1997, Plattner et al 2008, Lowe et al 2009, Samanta et al 2010, Frölicher and Joos 2010, Boucher et al 2012, Joos et al 2013, Tokarska and Zickfeld 2015, Zickfeld et al 2016, Jones et al 2016, Tokarska et al 2020)

Methods

Results

Conclusion