Imminent Emergence of Permanent Surface Marine Heatwave States Across Global Oceans

Abstract

Marine heatwaves (MHWs) can devastate ocean ecosystems, yet the timing at which anthropogenic changes in MHW characteristics permanently emerge from natural variability at the global scale remains poorly quantified. Here, we present the first global time of emergence analysis of MHW characteristics of the ocean surface using simulations from 21 Earth system models from the sixth Coupled Model Intercomparison Project under moderate (SSP2‐4.5) and high (SSP5‐8.5) greenhouse gas emissions scenarios. Relative to a 1985–2014 baseline, we find that almost 48% of the global ocean has already experienced significant trends in both annual MHW days and frequency. Across tropical oceans, MHW conditions permanently emerge between 2020 and 2040 regardless of emission pathway, indicating that near‐term transformation is unavoidable. By mid‐century, half the global ocean is projected to experience MHW conditions for at least 50% of the year, increasing to 90% by 2100. Regions experiencing the strongest warming show the earliest emergence, with uncertainty in emergence timing comparatively low in the tropics (10–15 years) but expanding substantially at higher latitudes (30 years). These findings demonstrate that persistent, large‐scale marine thermal stress is imminent, demanding transformation from reactive crisis management to anticipatory ocean governance within the next two decades.