Continued increase of extreme El Niño frequency long after 1.5 °C warming stabilization

Abstract

CMIP5 simulations reveal that the frequency of extreme El Nino events doubles under the 1.5 °C Paris target, and continues to increase long after global temperatures stabilize due to emission reductions. Extreme La Nina events, however, see little change at either 1.5 °C or 2 °C warming. The Paris Agreement aims to constrain global mean temperature (GMT) increases to 2 °C above pre-industrial levels, with an aspirational target of 1.5 °C. However, the pathway to these targets1,2,3,4,5,6 and the impacts of a 1.5 °C and 2 °C warming on extreme El Nino and La Nina events—which severely influence weather patterns, agriculture, ecosystems, public health and economies7,8,9,10,11,12,13,14,15,16—is little known. Here, by analysing climate models participating in the Climate Model Intercomparison Project’s Phase 5 (CMIP5; ref. 17) under a most likely emission scenario1,2, we demonstrate that extreme El Nino frequency increases linearly with the GMT towards a doubling at 1.5 °C warming. This increasing frequency of extreme El Nino events continues for up to a century after GMT has stabilized, underpinned by an oceanic thermocline deepening that sustains faster warming in the eastern equatorial Pacific than the off-equatorial region. Ultimately, this implies a higher risk of extreme El Nino to future generations after GMT rise has halted. On the other hand, whereas previous research suggests extreme La Nina events may double in frequency under the 4.5 °C warming scenario8, the results presented here indicate little to no change under 1.5 °C or 2 °C warming.