As climate change accelerates, the ocean is rising while becoming warmer, more acidic and less hospitable to coral reefs, marine life and the people that depend on the sea for food and livelihoods.

Warming Waters: As global atmospheric temperatures rise so, too, does the temperature of the ocean. Warmer waters are already driving marine life toward the poles. An estimated 82 percent of all the planet’s marine species are moving to colder climes to escape rising temperatures. Bony fish, for instance, are expected to migrate toward the poles a rate of around 172 miles (277km) per decade. That could disrupt commercial fisheries as sardines, mackerel, anchovies and other fish move to new habitats. Rising ocean temperatures can also spawn algal blooms that release domoic acid, which is toxic to humans and marine life. In 2015, enormous algal blooms shut down commercial fisheries across the West Coast of the United States and killed thousands of seals and sea lions.

Deoxygenation: As temperatures rise, the surface of the ocean heats up and absorbs less oxygen. Warmer water, lighter than the underlying colder water, forms a barrier that reduces the amount of oxygen that sinks to the depths. That can create oxygen-deprived “dead zones” in which marine life cannot thrive. (Naturally variable weather patterns also cause deoxygenation.) Dead zones have been detected in the Indian Ocean, the eastern Pacific Ocean and the Atlantic Ocean. Computer modeling predicts widespread detection of deoxygenation between 2030 and 2040. The United Nations Intergovernmental Panel on Climate Change (IPCC) estimates that oxygen concentrations in the ocean will decrease by 3–6 percent this century.

Acidification: The ocean absorbs about half of the carbon dioxide emitted from the burning of fossil fuels. Carbon dioxide combines with water to form carbonic acid and as the pH of the ocean drops, it becomes more difficult for corals, sea urchins, mussels and other marine organisms to build calcium shells crucial to their survival. Acidification also harms some species of tiny plankton that form the basis of the ocean food chain.

Damaged coral at the Great Barrier Reef. (Greg Torda)

Rising Seas: As the ocean warms and expands and glaciers and ice sheets melt, sea levels have risen an average of 3in (8cm) since 1992, and continue to rise 0.125in (3.2mm) annually. The IPCC estimates that sea levels will rise 1–3ft (0.3–0.9m) by 2100, a scenario rapidly being outstripped by accelerating glacier melt. For example, a worst-case scenario for the U.S. released by the National Oceanic and Atmospheric Administration in January 2017 predicted a 10–12ft (3–3.7m) rise. Even by conservative estimates, sea level rise would flood major coastal cities, trigger a wave of climate refugees and cause trillions of dollars in property damage.

(U.S. 2014 U.S. National Climate Assessment)