Indirect Effects of Non-CO2 Forcings on Carbon Budgets in Overshoot pathways

Overshoot pathways involve exceeding a specific temperature target temporarily and returning to it using deliberate carbon dioxide removal methods. Quantifying the overshoot carbon budgets is becoming increasingly significant as the global mean surface air temperature approaches the 1.5°C target considered in the Paris Agreement. Contribution from non-CO₂ forcings is a key component of estimating the carbon budgets. Non-CO₂ forcings affect global mean temperature in two ways: i) by altering the energy balance at the top of the atmosphere (direct effect) and ii) by affecting the carbon cycle (indirect effect; for example, the effect of non-CO₂ forcings on temperature causes changes in soil respiration which is a strong function of temperature). Current frameworks quantify the impact of non-CO₂forcings on carbon budgets separately from CO₂ forcing using emulators. Therefore, the effects of the interaction between non-CO₂ forcings and carbon cycle (indirect effects) are not captured. Pre- and post-overshoot carbon budgets refer to the total anthropogenic emissions when the temperature exceeds and subsequently falls below the intended target, respectively. Here, we investigate how the indirect effects of non-CO₂ forcings on global mean temperatures affect pre- and post-overshoot carbon budgets using an Earth system model of intermediate complexity.

Three sets of simulations are performed to isolate the direct and indirect effects of non-CO₂ forcings on global mean surface air temperatures. The reference set involves prescribing fossil fuel emissions following historical data and Shared Socio-economic Pathways (SSP) scenarios, while excluding non-CO₂ forcings.  The second set (total set) involves simulations with both fossil fuel emissions and non-CO₂ forcings prescribed following historical data and SSP scenarios, which simulates the total effect of non-CO₂ forcings on global mean temperature. In the third set (direct set), the same non-CO₂ forcings as in the total set is applied, but the atmospheric CO₂ concentration is prescribed from the reference simulation. Prescribing atmospheric CO₂ concentration isolates the direct effects due to non-CO₂ forcings by preventing the carbon cycle feedbacks from influencing temperature. The indirect effects are calculated as the difference between total and direct sets. We find that direct warming due to non-CO₂ forcing is larger at both pre-and post-overshoots compared to indirect warming. However, the relative contribution of indirect warming increases during the post-overshoot relative to the pre-overshoot because of two reasons: i) non-CO₂ forcings are smaller during the post-overshoot and ii) indirect warming increases from pre- to post-overshoot because of the slow carbon cycle response to non-CO₂ warming. Further, we estimate the associated reductions in pre- and post-overshoot carbon budgets due to indirect effects of non-CO₂ forcings. Our results suggest that frameworks quantifying overshoot carbon budgets should assess the contributions from CO₂ and non-CO₂ forcings together to fully capture the effects of the interactions between non-CO₂ forcings and the carbon cycle.