Impacts of marine heatwaves on tropical western and central Pacific Island nations and their communities

Marine heatwaves can have devastating impacts on marine species and habitats, often with flow-on effects to human communities and livelihoods. This is of particular importance to Pacific Island countries that rely heavily on coastal and ocean resources, and for which projected increases in future marine heatwave (MHW) frequency, intensity, and duration could be detrimental across the Pacific Island region. In this study, we investigate MHWs in the tropical western and central Pacific Ocean region, focusing on observed MHWs, their associated impacts, and future projections using Coupled Model Intercomparison Project phase 6 (CMIP6) simulations under a low (SSP1–2.6) and a high (SSP5–8.5) greenhouse gas emissions scenario. Documented impacts from “Moderate” mean intensity MHW events in Fiji, Samoa, and Palau, that were categorised as “Strong” at their peak, included fish and invertebrate mortality and coral bleaching. Based on CMIP6 multi-model mean estimates, and relative to current baselines, “Moderate” intensity MHWs are projected to increase from recent historical (1995–2014) values of 10–50 days per year (dpy) across the region to the equivalent of >100 dpy by the year 2050 under the low emissions scenario, and > 200 dpy nearer the equator. Under the high emissions scenario, 200 dpy of Moderate MHW intensities are projected across most of the region by 2050, with >300 dpy nearer the equator. For the most intense “Extreme” category of MHW, estimates range from <1 dpy under the current climate to >50 dpy projected under the high emissions scenario by 2050. In contrast, “Extreme” MHWs are projected to increase to <5 dpy by 2050 under the low emissions scenario, highlighting the importance for Pacific Island nations that global emissions more closely follow the low emissions scenario trajectory.