Detecting anthropogenic effects on the record-warm northwestern Pacific sea surface temperature in August 2020

August 2020 set a new record high sea surface temperature (SST) in the northwestern Pacific (NWPac; 120°E–180°E, 20°N–35°N). This anomalous condition potentially intensified tropical cyclones such as Typhoon Haishen, causing severe damage to the Korean Peninsula and Japan. Although the NWPac Ocean has gradually warmed due to human-induced greenhouse gas emissions since the mid‐20th century, the extent to which anthropogenic climate changes increase the occurrence likelihood of such regionally unprecedented warm SSTs is unclear yet. Here we analyzed the historical and SSP2-4.5 scenario simulations of CMIP6 and DAMIP as well as observational datasets. Our results show that owing to historical anthropogenic forcing, the occurrence probability of the 2020 record-warm NWPac SST is increased from once-in-1000 years to about once-in-15 years in 2001-2020. As warming caused by greenhouse gases was largely canceled by aerosol cooling, anthropogenic effects on the NWPac SST were not distinguishable from internal variability in the 20th century. The SSP2-4.5 scenario simulations also indicate that the 2020 record-warm SST is becoming a new normal climate condition of August by 2031–2050, or once the global air temperature above preindustrial level exceeds 1.5°C.