Anthropogenic impacts on twentieth-century ENSO variability changes

El Nino/Southern Oscillation (ENSO) sea surface temperature (SST) variability increased after 1960, influenced by more frequent strong El Nino and La Nina events. Whether such changes are linked to anthropogenic warming, however, is largely unknown. In this Perspective, we consider anthropogenic impacts on ENSO variability in several commonly used modelling designs, which collectively suggest a greenhouse warming-related effect on post-1960 ENSO SST variability. Specifically, a comparison of simulated ENSO SST variability between 1901-1960 and 1961-2020 indicates that more than three quarters of climate models produce an amplitude increase in post-1960 ENSO SST variability, translating into more frequent strong El Nino and La Nina events. Multiple large ensemble experiments further confirm that the simulated post-1960 ENSO amplitude increase (approximately 10%) is not solely due to internal variability. Moreover, multicentury-long simulations under a constant pre-industrial CO2 level suggest that the observed post-1960 ENSO variability is high, sitting in the highest 2.5 and 10 percentiles for eastern Pacific and central Pacific ENSO, respectively. Improvement in model ENSO physics, identification of consistent future and historical change in additional ENSO characteristics and single-forcing large-ensemble experiments are further needed to ascertain climate change impacts on the ENSO. Although model projections indicate increased El Nino/Southern Oscillation (ENSO) variability in the future, contemporary impacts of anthropogenic forcing on ENSO variability have been difficult to ascertain. This Perspective discusses these contemporary effects, outlining that an increase in post-1960 ENSO variability is likely related to greenhouse gas forcing.