Tipping the ENSO into a permanent El Niño can trigger state transitions in global terrestrial ecosystems

Abstract

Abstract. Some large-scale components of the Earth's climate system have been identified as policy-relevant tipping elements, meaning that anthropogenic forcing and perturbations may push them across a tipping point threshold, with potential global scale impact on ecosystems and concomitant environmental and social phenomena. A pronounced change in the amplitude and/or frequency of the El Niño-Southern Oscillation (ENSO) is among such tipping elements. Here we use the Planet Simulator (PlaSim), an Earth system model of intermediate complexity, to investigate the potential impact on global climate and terrestrial ecosystems of shifting the current dynamics of the ENSO into a permanent El Niño. When forced with sea surface temperature (SST) derived from observations, the PlaSim model yields a realistic representation of large-scale climatological patterns, including realistic estimates of the global energy and water balances, and gross primary production. In a permanent El Niño state, we found significant differences in the global distribution of water and energy fluxes, and associated impacts on gross primary production, indicating that vegetation productivity decreases in the tropics whereas it increases in temperate and boreal regions. We identify regions in which these El Niño-induced changes are consistent with potential state transitions in global terrestrial ecosystems, including potential dieback of the Amazon rainforest, southward expansion of the Sahel, and further aridification of Australia.