2.2.3 Impacts of heating on ocean life
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"Unless we take evasive action, our future oceans will have fewer fish, fewer whales and frequent dramatic shifts in ecological structure will occur, with concerning implications for humans who depend on the ocean."
*Dr Éva Plagányi at the Commonwealth Scientific and Industrial Research Organisation, Australia
Scientists have warned that marine heatwaves are sweeping oceans "like wildfires", with extreme temperatures killing swathes of sea-life and destroying crucial species that provide shelter and food to many others – such as seagrass, kelp and corals. In fact, due to the slow response of the oceans to atmospheric heating, we have already locked in a large future increase in marine heatwaves.
Repeated heat stress has now caused nearly half of the world’s corals to bleach and then die. Tropical coral reefs are some of the most important and diverse ecosystems on the planet, which support up to one million other species and provide food, protection from storms and livelihoods for nearly one billion people.
As Michael Mann, Professor of Atmospheric Science at Pennsylvania State University, put it: "Our generation is going to be responsible for the loss of one of the most majestic ecosystems on the face of the Earth. We’re literally watching the death of this natural wonder."
Ocean warming not only damages sea life directly, but also causes the oxygen that is usually dissolved in seawater to become less soluble. This leads to areas of water with depleted levels of oxygen, which can lead to suffocation of the sea creatures living within them. The runoff of nutrients from agriculture and sewage further exacerbates oxygen decline in coastal waters through causing the overgrowth of algae. This ‘algal bloom’ blocks sunlight from reaching submerged plants, which die and are fed off by bacteria that use up the oxygen in the water – a process known as eutrophication.
Indeed, in the last 70 years, low-oxygen ocean zones have grown by more than 4.5 million square km - an area roughly as large as the entire European Union - whilst the number of ocean ‘dead zones’ (areas with exceedingly low oxygen) has increased by a factor of 10.