OCEAN ACIDIFICATION CO2 and other greenhouse gases that humans are generating persist not only in the Earth’s atmosphere, but are also absorbed by the oceans. In fact, about 30% of the CO2 is absorbed by the oceans. This added CO2 causes the water to become more acidic – between 25-30% more acidic since the beginning of the industrial revolution (the mid-1750s). Acidification refers to the decrease in the ph level of the upper levels of the ocean. For more on the chemistry of this phenomenon see this pdf. The rapid rate of ocean acidification will have a negative impact on many species and ocean ecosystems. Several species will struggle to grow or survive in more acidic conditions. An analysis of the studies to date concludes that “heavily calcified organisms, including calcified algae, corals, mollusks, and the larval stages of echinoderms, are the most negatively impacted, whereas crustaceans, fish, fleshy algae, seagrasses, and diatoms are less affected or even benefit from acidification” (source).
CORAL BLEACHING Rising temperatures (among additional types of stress) cause many types of coral to bleach or whiten. Bleaching is caused when the coral, under stress of warmer waters, expels the organisms (dinoflagellate algae) living on them. While this does not always directly kill the coral, it can slow its growth and make it more susceptible to death. Large areas of coral have died. Coral reefs protect coastlines from the severity of waves and storms. In the US alone, researchers estimate that coral reefs are worth over $3.4 billion in the services they provide in reducing coastal flooding, fostering the health of fisheries, and providing tourism revenue. Coral reefs also host a quarter of all ocean species.
As more CO2 and other GHGs are emitted, the seas have become warmer and more mass bleaching events have occurred globally. The map below shows the percentage of coral that experienced bleaching between 1998 and 2017. The key aspect of the second figure below is the red line with an upward slope that shows that the probability of coral bleaching around the world has increased since 2002.
Remember that human impacts are not limited to climate change. Coral reefs are also threatened by pollution, development, and destructive fishing practices. However, even in areas protected from pollution and fishing, warming is having a devastating effect. If warming continues on a “business as usual” pathway, tropical corals reefs could disappear (source). The article below is from March of 2020.