Increasing frequency of extreme El Nino events due to greenhouse warming

El Nino events are a prominent feature of climate variability with global climatic impacts. The 1997/98 episode, often referred to as 'the climate event of the twentieth century'(1,2), and the 1982/83 extreme El Nino(3), featured a pronounced eastward extension of the west Pacific warm pool and development of atmospheric convection, and hence a huge rainfall increase, in the usually cold and dry equatorial eastern Pacific. Such a massive reorganization of atmospheric convection, which we define as an extreme El Nino, severely disrupted global weather patterns, affecting ecosystems(4,5), agriculture(6), tropical cyclones, drought, bushfires, floods and other extreme weather events worldwide(3,7-9). Potential future changes in such extreme El Nino occurrences could have profound socio-economic consequences. Here we present climate modelling evidence for a doubling in the occurrences in the future in response to greenhouse warming. We estimate the change by aggregating results from climate models in the Coupled Model Intercomparison Project phases 3 (CMIP3; ref. 10) and 5 (CMIP5; ref. 11) multi-model databases, and a perturbed physics ensemble(12). The increased frequency arises from a projected surface warming over the eastern equatorial Pacific that occurs faster than in the surrounding ocean waters(13,14), facilitating more occurrences of atmospheric convection in the eastern equatorial region.