Robust twenty-first-century projections of El Nino and related precipitation variability

The El Nino-Southern Oscillation (ENSO) drives substantial variability in rainfall(1-3), severe weather(4,5), agricultural production(3,6), ecosystems(7) and disease(8) in many parts of the world. Given that further human-forced changes in the Earth's climate system seem inevitable(9,10), the possibility exists that the character of ENSO and its impacts might change over the coming century. Although this issue has been investigated many times during the past 20 years, there is very little consensus on future changes in ENSO, apart from an expectation that ENSO will continue to be a dominant source of year-to-year variability(9,11,12). Here we show that there are in fact robust projected changes in the spatial patterns of year-to-year ENSO-driven variability in both surface temperature and precipitation. These changes are evident in the two most recent generations of climate models(13,14), using four different scenarios for CO2 and other radiatively active gases(14-17). By the mid- to late twenty-first century, the projections include an intensification of both El-Nino-driven drying in the western Pacific Ocean and rainfall increases in the central and eastern equatorial Pacific. Experiments with an Atmospheric General Circulation Model reveal that robust projected changes in precipitation anomalies during El Nino years are primarily determined by a nonlinear response to surface global warming. Uncertain projected changes in the amplitude of ENSO-driven surface temperature variability have only a secondary role. Projected changes in key characteristics of ENSO are consequently much clearer than previously realized.