Consensus is emerging that anthropogenic climate change has significantly increased the likelihood of recent marine heat waves   . A widespread MHW occurred in the northeast Pacific in 2013-2015, with upper ocean temperature anomalies of up to 6.2°C relative to 2002-2012 . This event, termed the 'Blob', enhanced surface water stratification, decreasing nutrient supply, primary and community production and leading to widespread changes to open ocean and coastal ecosystems, with geographical shifts of key species across trophic levels, mass strandings of marine mammals, seabird mortalities and the closure of commercially important fisheries . The MHW reappeared in 2019  , with similarly high temperature anomalies extending from Alaska to California, but the ecological effects of this event are expected to differ because the Blob originated in winter, and Blob2.0 intensified in summer . Modelling suggests rapid shifts in the geographic distributions of important fish species in response to MHWs , with projected decreased biomass and distributional shifts of fish at least four times faster and larger than the effects of decadal-scale mean changes throughout the 21st century under RCP8.5  . MHWs can also dramatically increase CH4 emissions from oceans, a significant positive feedback to global warming  .
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report