Abstract:In this report,the IPCC Working Group Ⅱ's contribution to the Sixth Assessment Report Chapter 3 presents an assessment of the impacts of climate change on the ocean and coastal ecosystems,along with their vulnerability and the potential adaptation solutions.This report clarifies the fact that the anthropogenic greenhouse gases have and will continue to cause fundamental changes to the physical and chemical properties of our centrally important oceans,including warming,sea level rise (SLR),acidification,deoxygenation,and change in nutrients.The occurrences of the marine heatwaves have doubled since the 1980s,and will become 4-8 times more frequent in 2081—2100 compared to 1995—2014.In addition,ocean warming since the 1950s has shifted marine species poleward,at an average of 59.2±15.5 km per decade.This has resulted in reduced biomass in tropical waters,a remarkable tropicalization of mid-latitude ocean,and an earlier phytoplankton growth period in polar and sub-polar areas.Marine heatwaves are exposing species,and ecosystems approach or even exceed their tolerance,causing mass coral bleaching and mortality,and loss of seagrasses and kelp forests.Warming,acidification and hypoxia have collectively resulted in changes in the community structure and increases in the occurrence of harmful algal blooms in estuaries,and have contributed to an overall decline in phytoplankton biomass and primary production.In the meantime,phytoplankton biomass in the polar ocean will increase.Accelerating SLR has led to the degradation of coastal wetland ecosystems (e.g.,mangroves,saltmarshes and seagrass beds).Ocean conditions are projected to continue and increase risk of regional extirpations and global extinctions of marine species,especially near the Equator and in the Arctic.Coral reefs are projected to decline by a further 70%—90% at 1.5 ℃ warming with greater losses (>99%) at 2 ℃,as early as the 2040s under SSP5-8.5.The current available adaptation options (e.g.,marine protected areas and mangrove restoration) are unable to offset the growing climate-change risk,thus exemplifying the need for transformative climate adaptation and ambitious mitigation measures.