Stratospheric geoengineering impacts on El Niño/Southern Oscillation

Abstract. To examine the impact of proposed stratospheric geoengineering schemes on the amplitude and frequency of El Niño/Southern Oscillation (ENSO) variations we examine climate model simulations from the Geoengineering Model Intercomparison Project (GeoMIP) G1–G4 experiments. Here we compare tropical Pacific behavior under anthropogenic global warming (AGW) using several scenarios: an instantaneous quadrupling of the atmosphere's CO₂ concentration, a 1 % annual increase in CO₂ concentration, and the representative concentration pathway resulting in 4.5 W m⁻² radiative forcing at the end of the 21st century, the Representative Concentration Pathway 4.5 scenario, with that under G1–G4 and under historical model simulations. Climate models under AGW project relatively uniform warming across the tropical Pacific over the next several decades. We find no statistically significant change in ENSO frequency or amplitude under stratospheric geoengineering as compared with those that would occur under ongoing AGW, although the relative brevity of the G1–G4 simulations may have limited detectability of such changes. We also find that the amplitude and frequency of ENSO events do not vary significantly under either AGW scenarios or G1–G4 from the variability found within historical simulations or observations going back to the mid-19th century. Finally, while warming of the Niño3.4 region in the tropical Pacific is fully offset in G1 and G2 during the 40-year simulations, the region continues to warm significantly in G3 and G4, which both start from a present-day climate.