The destructive environmental and socio-economic impacts of the El Niño/Southern Oscillation1,2 (ENSO) demand an improved understanding of how ENSO will change under future greenhouse warming. Robust projected changes in certain aspects of ENSO have been recently established3,4,5. However, there is as yet no consensus on the change in the magnitude of the associated sea surface temperature (SST) variability6,7,8, commonly used to represent ENSO amplitude1,6, despite its strong effects on marine ecosystems and rainfall worldwide1,2,3,4,9. Here we show that the response of ENSO SST amplitude is time-varying, with an increasing trend in ENSO amplitude before 2040, followed by a decreasing trend thereafter. We attribute the previous lack of consensus to an expectation that the trend in ENSO amplitude over the entire twenty-first century is unidirectional, and to unrealistic model dynamics of tropical Pacific SST variability. We examine these complex processes across 22 models in the Coupled Model Intercomparison Project phase 5 (CMIP5) database10, forced under historical and greenhouse warming conditions. The nine most realistic models identified show a strong consensus on the time-varying response and reveal that the non-unidirectional behaviour is linked to a longitudinal difference in the surface warming rate across the Indo-Pacific basin. Our results carry important implications for climate projections and climate adaptation pathways.
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We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. S.T.K. is supported by CSIRO Office of Chief Executive and Wealth from Oceans Flagship, and W.C. is supported by the Australian Climate Change Science Program, and a CSIRO Office of Chief Executive Science Leader award. A.S. is supported by the Australian Research Council. S-I.A. is supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2009-C1AAA001-2009-0093042).