Zero Emissions Commitment
What are the key processes in the Earth system that determine its response to halting CO₂ emissions?

The zero emissions commitment (ZEC) describes the committed temperature change after anthropogenic emissions cease. It is an intrinsic feature of the Earth’s climate system due to slow and fast equilibration processes and has been shown to depend on the cumulative amount of CO₂ emitted prior to zero (carbon emissions budget) and the state of the Earth system at the point of zero emissions. This state dependency can be described as the balance between the biogeochemical and thermal equilibration processes of the Earth system.

There remains substantial uncertainty and disagreement about the sign and size of temperature change after a cessation of CO₂ emissions between different models. The inter-model range of ZEC 50 years after emissions cease (ZEC₅₀) for a scenario with a cumulative carbon budget of 1000 PgC was found to be -0.36^oC to 0.29^oC with a model ensemble mean of -0.06^oC. This magnitude of uncertainty implies that ZEC may be the largest contributor to the uncertainty on the remaining emissions budget for 1.5^oC.

If ZEC were positive, it could reduce the budget by over a third and imply that net-negative emissions might be needed for temperature stabilisation. However, if ZEC were negative, that might even mean that sustained CO₂ emissions could be allowable for a temperature stabilisation goal.

Ambitious mitigation scenarios
Ambitious climate mitigation scenarios present current Earth system models (ESMs) with new challenges. In such scenarios with reduced anthropogenic CO₂ emissions, the relevance of smaller climate signals of previously secondary processes become more important, since every tenth of a degree of warming counts. This means accurate representation of the climate-carbon interactions and feedback uncertainties including
explicit Carbon Dioxide Removal (CDR) measures is needed. ESMs in an emission-driven configuration must be able to reproduce anthropogenic carbon emissions from fossil fuels and land use change and management in the context of a closed and stable carbon cycle, with accurate representations of natural and anthropogenic carbon sinks.

Carbon Dioxide Removal

Even if ambitious climate mitigation measures aiming for a net-zero greenhouse gas (GHG) target are implemented, it is expected that humanity will continue to emit residual amounts of carbon dioxide and other GHGs, which will further drive global warming.

Options for carbon dioxide removal (CDR) – such as bioenergy combined with carbon capture and storage in geological formations, increasing the alkalinity of the oceans, or managing and expanding ecosystems – provide opportunities to support emission reduction efforts and offset remaining GHG emissions, making net-zero targets achievable. Furthermore, CDR is crucial for achieving net-negative emissions, which can reduce atmospheric CO₂ concentrations and neutralize any overshoot of the carbon emissions budget or global temperature thresholds.

Methods

Our group aims to deepen our understanding of the Earth system's response to net-zero and overshoot scenarios, as well as to human activities like carbon dioxide removal (CDR). To achieve this, we utilize Earth system models of varying complexity (FOCI, UVic ESCM 2.9 & 2.10, along with multi-model ensembles from CMIP (CDRMIP, ZECMIP). Additionally, we contribute to comprehensive assessments of CDR options through interdisciplinary collaboration.

Our main research focuses include:

• Evaluating uncertainties and process understanding of net-zero and overshoot scenarios in relation to the carbon cycle and climate dynamics (FOOTPRINTS, RESCUE)
• Conducting model studies to understand the potential and risks of (marine) CDR options, including increasing ocean alkalinity, managing, and expanding blue carbon ecosystems (RESCUE)
• Performing comprehensive, transdisciplinary assessments of (marine) CDR options (CDRmare consortium ASMASYS, GESAMP WG41)