Abstract
CO2 mitigation policy is expected to suppress global warming and its climate impact; however, this study found that CO2 mitigation could cause an abrupt shortening of El Niño periodicity, which enhances the frequency of El Niño and potential influence on our society. The abrupt shift is responsible for the southward shift of the Intertropical Convergence Zone (ITCZ) due to the CO2 reduction, implying the potential constraint of El Niño property on the global energy balance. Even though there is a high uncertainty in projected El Niño periodicity under global warming, abrupt shifts in CO2 mitigation scenarios can be consistently projected across state-of-the-art climate models. Removing CO2 from the atmosphere is emerging as a viable strategy to mitigate global warming, yet the responses of the climate system to CO2 reduction remain uncertain. One of the most uncertain aspects of El Niño behavior is the change in periodicity in response to CO2 forcing [O. Alizadeh, Earth-Sci. Rev. 235, 104246 (2022)]. In this study, we show that climate models consistently project an abrupt shortening of El Niño periodicity once CO2 reductions commence in ramp-up and ramp-down CO2 experiments. Besides the contribution of slow mean state changes, this phenomenon is shown to be driven by a southward shift of the Intertropical Convergence Zone (ITCZ) [J.-S. Kug, et al., Nat. Clim. Chang. 12, 47–53 (2022)] and the consequent narrowing of El Niño’s spatial pattern, which enhances the effectiveness of ocean heat recharge/discharge processes, thereby shortening its periodicity. This suggests that the abrupt shift in El Niño periodicity results from a cascading reaction involving ITCZ dynamics and El Niño’s spatial configuration. These findings highlight the critical role of the global energy balance in shaping El Niño characteristics.
