Abstract

We have simulated a large set of idealized simulations with the Norwegian Earth System Model (NorESM2) to explore the Earth system response to net-zero and net-negative CO₂ emissions. Our simulations reach net-zero emissions after different levels of prior emissions and are complemented by phases of negative emissions such that we obtain scenarios of temperature overshoots of different magnitude and duration. We will present results from these simulations focusing on two aspects.

First, we show that in our model, which has a strong AMOC decline in response to global warming, high northern latitudes cool substantially relative to peak warming after emissions are phased out. Interestingly, such cooling can be amplified under deliberate CO₂ removal (CDR) and results in a temporary undershoot of a targeted temperature level, depending on the timing of CDR relative to the recovery of AMOC. We present evidence that such behavior might be generally found in Earth system models that show a strong AMOC decline in response to anthropogenic forcing.

Second, we investigate the centennial scale reversibility of the Earth system after an overshoot. In our model, increased carbon loss from land during an overshoot is compensated by increased ocean uptake such that atmospheric CO₂ concentrations are remarkably similar in the overshoot simulations compared to the reference simulation without overshoot. We find that some aspects of the Earth system (e.g., surface temperature, surface pH) are indeed reversible except under very large overshoots, which are unfeasible in view of limited CDR capacity. In line with previous studies, we find irreversibility for permafrost carbon loss and deep ocean properties.

About Jörg Schwinger

Jörg is a Senior Researcher at NORCE and Bjerknes Centre for Climate Research, in Norway. He obtained a PhD in Geophysics from University of Cologne, Germany. After moving to Bergen, Norway, he has worked at the Bjerknes Centre for Climate Research on modeling the global carbon cycle (and other biogeophysical cycles) and their relation to global climate.

Using Earth system models, he has worked on quantifying contemporary ocean carbon fluxes, on carbon cycle feedbacks, and future projection. His recent research focus is the response of the carbon cycle and the Earth system to net-zero and net-negative emissions.