The Northwestern Pacific Warming Record in August 2020 Occurred Under Anthropogenic Forcing.

In August 2020, the northwestern Pacific (NWPac; 120°E‐180°E, 20°N‐35°N) experienced its highest sea surface temperature (SST). Although the NWPac Ocean has warmed due to human‐induced greenhouse forcing since the mid‐20th century, the extent to which historical anthropogenic effects increase the occurrence probability of such regionally unprecedented warm temperatures is unclear. In this study, using state‐of‐the‐art climate models, we estimate that the August 2020 record high SST in the NWPac likely occurred once per 12–18 years in 2001–2020 owing to historical anthropogenic forcing, and this record SST unlikely occurred without human influences. In the 20th century, anthropogenic effects on the NWPac SST were not distinguishable from internal variability since warming caused by greenhouse gases was largely canceled by aerosol cooling. Furthermore, the 2020 warm SST is projected to be the new normal by 2031–2050, or even if the 2.0°C goal of the Paris Agreement is achieved. Plain Language Summary: Beyond the seasonal warmth of sea surface temperatures (SSTs) in boreal summer, the SST in the northwestern Pacific (NWPac; 120°E‐180°E, 20°N‐35°N) was unprecedentedly high in August 2020. This anomalous condition potentially intensified tropical cyclones, causing severe damage to the Korean Peninsula and Japan. Although human‐induced greenhouse gas emissions have gradually warmed the NWPac Ocean since the mid‐20th century, it remains unclear how much human activity increases the occurrence likelihood of such regionally unprecedented warm temperatures. Here, we analyzed the observed NWPac August SSTs, as well as those in numerical simulations of state‐of‐the‐art climate models under human‐induced and natural solar and volcanic forcings from the 20th century to the present. Our results show that, owing to human‐induced warming in the past, the August 2020 record high SST in the NWPac likely occurred once per 12–18 years in 2001–2020 as frequently as we observed. We also analyzed the future projection simulations of the climate models, indicating that the 2020 warm SST in the NWPac is expected to be the new normal climate condition of August by 2031–2050, or even if we achieve the Paris Agreement's goal of limiting global temperature rise well below 2.0°C relative to preindustrial levels. Key Points: The northwestern Pacific (NWPac; 120°E‐180°E, 20°N‐35°N) experienced its record high sea surface temperature (SST) in August 2020The occurrence likelihood of such warm SST event in 2001–2020 is increased from 0.0%–0.1% to 5.5%–8.5% because of anthropogenic forcingThe record high August SST is projected to be the new normal by 2031–2050, or even if the 2.0°C goal of the Paris Agreement is achieved [ABSTRACT FROM AUTHOR]