Non-CO2 effects of carbon dioxide removal methods influence temperature response in overshoot scenarios

Exceedance of the long-term goal of the Paris agreement to limit warming to 1.5 degree Celsius above pre-industrial levels has become inevitable due to insufficient past and present climate action. Therefore, any future scenario consistent with meeting this goal will involve some level of temperature overshoot. Thus, it is essential to understand how the Earth system responds to overshoot scenarios, how reversible these changes may be compared to non-overshoot scenarios, and what the implications could be for future generations.

Overshoot scenarios are commonly derived from Integrated Assessment Models (IAMs). These scenarios describe possible pathways of greenhouse gas and aerosol emissions, along with changes in land use. To reach a specified climate goal, each scenario relies on the deployment of various types and amounts of carbon dioxide removal (CDR), such as reforestation, bio-energy with carbon capture and sequestration (BECCS) and direct air capture (DAC). In addition to removing CO₂ from the atmosphere, each of these methods is associated with distinct non-CO₂ related climate effects (e.g. biogeophysical effects, emissions of non-CO₂ gases).

However, most Earth system modelling studies rely on idealized CDR implementation only modelling carbon dioxide emissions or concentrations for a given scenario. This neglects the non-CO₂ climate effects and feedbacks that are associated with each scenario’s CDR methods. Therefore, the objective of this research is to investigate the

To study the Earth response to overshoot scenarios, two sets of scenarios were generated using the REMIND-MAgPIE IAM, with scenarios within each set designed to meet the same cumulative CO₂ emissions by 2100 (450 GtCO₂ and 650 GtCO₂). Each set includes corresponding pairs of low and high carbon budget overshoot. These scenarios achieve the defined carbon budget through different CO₂ emission trajectories and portfolios of CDR methods, different policy choices affecting land-use and available CDR methods, and different levels of overshoot. The Earth system response to these scenarios is then modelled via emission driven runs using the University of Victoria Earth System Climate Model, an Earth system model of intermediate complexity.

We find that high overshoot pathways have slightly different global temperature outcomes compared to low-overshoot pathways at the time the carbon budget converges. Global mean temperature differences across scenarios range from 0.00–0.04 °C for the 450 Gt CO₂ set and 0.00–0.05 °C for the 650 Gt CO₂ set. Regionally, differences are larger and range from -0.15–0.15 °C and -0.14–0.16 °C, respectively. Cancellation of positive and negative regional temperature differences results in small differences in the global mean. Differences in temperature response across scenarios are attributed to lags in the thermal and carbon cycle response to net-negative CO₂ emissions, and non-CO₂ effects associated with the unique CDR portfolio within each scenario Our results highlight the importance of considering non-CO₂ effects of CDR methods in Earth system models to capture the full range of Earth system responses in overshoot scenarios, particularly at regional scales.