World Ocean Assessment II, Chapter 09, Pressures from changes in climate and atmosphere

KEYNOTE POINTS 1. Extreme climate events. Marine heatwaves and tropical cyclones are shown to be increasing in severity owing to human activities and are having an impact on nature and human societies. Extreme El Niño events have been observed but, because they occur infrequently, a human influence has not been detected. All three phenomena are projected to increase in the future, with the severity of impacts also increasing, but such increases can be reduced by climate change mitigation efforts. 2. Sea level rise. The alarming observed pace of sea level rise, combined with increasing storminess and coastal urbanization, has resulted in the amplified susceptibility of coastal cities to erosion and flooding and increased the need for substantial investments in hard infrastructure and the restoration of natural barriers, such as reefs. 3. Ocean acidification and deoxygenating. The accelerated increase of anthropogenic CO2 in the atmosphere is creating an increase in the acidification and deoxygenation of the ocean. Under such conditions, both in nature and in the laboratory, marine organisms that support ecosystems and human livelihoods and nutrition typically respond poorly. Marine habitats experience a loss of diversity, many long-lived organisms die and a few resilient species proliferate. Less serious damage to life-supporting ecosystems would be possible under lower-emission scenarios. 4. Other physical and chemical properties. Changes in ocean temperature and salinity induced by climate change and human activities are affecting marine ecosystems by changing the distribution of marine species, decreasing the ecological value of coastal ecosystems and changing marine primary production. Human well-being and the economy are consequently affected.