Ocean life and rising water temperatures

Oceans, which cover 71 percent of the Earth’s surface, respond more slowly than land to climate change. However, a global warming trend may cause damage to Arctic and Antarctic ecosystems due to melting sea ice, bleaching and die-off of reef corals, disruption of ocean currents, and shifting predator-prey relationships favoring reduced biodiversity. Long-term effects could include massive extinctions due to changes in seawater chemistry.

Background

Until quite recently, scientists and the general public considered Earth’s oceans to be impervious to anthropogenic degradation. Oceans cover 71 percent of Earth’s surface and account for a little less than half of its primary production, that is, the photosynthetic conversion of carbon dioxide (CO₂) into the organic compounds that make up the bodies of living organisms.

The ocean is far from being a uniform habitat; however, with the exception of some near-shore environments, ecological niches cover wide areas and intergrade, meaning that species can readily adapt by shifting their ranges. In consequence, environmental pressures producing elevated extinction rates on land have a less dramatic effect in the open ocean.

Nonetheless, human activity has had an adverse effect on marine life, from phytoplankton to top marine predators such as sharks. While overfishing, pollution, and damming of rivers that serve as spawning grounds for marine fish have all taken their toll, these are only indirectly related to global warming.

Present effects attributable to elevated land temperatures include displacement of currents and upwelling zones and increased runoff in major river systems. Effects attributable to elevated sea surface temperatures include melting in the Arctic and Antarctic, reducing habitat for polar bears, penguins, and the many humbler species of plants and animals that thrive at the margins of the polar ice caps. In the tropics, higher sea surface temperatures alter coral metabolism, causing corals to bleach when they lose symbiotic (zooxanthellae) critical to their growth. Degradation of coral reefs profoundly affects the many organisms restricted to this habitat.

As atmospheric CO₂ has continued to increase, altered seawater chemistry has become a growing concern. An estimated one-quarter of the CO₂ generated by burning fossil fuels does not remain in the atmosphere but rather is dissolved in the oceans, increasing seawater acidity. According to the US National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory (PMEL), since the start of the Industrial Revolution, ocean acidity has increased by approximately 30 percent. One of the effects of ocean acidification is the stunting of shell production. Shells and similar structures found in marine organisms are formed from calcium carbonate, a process known as calcification; as the acidity of the ocean increases, the calcium carbonate in these structures becomes more vulnerable to dissolution. Coelenterates such as corals, whose skeletons are made up of aragonite (one form of calcium carbonate), are more susceptible than are mollusks, which have shells of calcite (another form of calcium carbonate), but both remain vulnerable. The threat posed to marine calcifying organisms by ocean acidification was largely theoretical until 2012, when an article published in the journal Nature Geoscience reported the "extensive dissolution" of the shells of pteropods found in the Antarctic Ocean.

Climate Change and Marine Life in the Geologic Record

Scientists recognize at least five major global mass extinction events, of which the Permian extinction, which took place 251 million years ago at the Permian-Triassic boundary, was the most devastating. At that time, approximately 95 percent of marine species and 70 percent of land species became extinct in three distinct pulses over a period of about eighty thousand years.

While several theories have been proposed to account for the Permian extinction, among the best supported of these theories is one that attributes it to drastic climate change triggered by massive volcanic eruptions in Siberia. According to this theory, each eruption caused brief cooling episodes due to volcanic dust and sulfuric acid aerosols in the atmosphere; once the dust and aerosols dispersed, this cooling was followed by a period of warming caused by the huge amounts of carbon dioxide, sulfur dioxide, and other greenhouse gases (GHGs) that were released along with the lava. Over the course of a million years, repeated eruptions, dwarfing anything humans have experienced in their brief tenure on Earth, eventually overwhelmed the planet’s capacity to self-correct.

Another theory postulates that the ocean depths became increasingly oxygen depleted, favoring the growth of bacteria that produce hydrogen sulfide. High pressures and cold temperatures in the abyss allowed the hydrogen sulfide to build up, only to be released in a gigantic “burp” of highly toxic fumes. Still another theory points to the storage of large quantities of methane in the form of clathrates in deep-sea sediments, suggesting that this methane was abruptly released when warming raised the temperature of the deeper regions of the ocean by 5 degrees Celsius. In addition to being toxic and a powerful GHG, methane is explosive at concentrations as low as 5 percent. Both of these events—deoxygenation of the oceans and the release of trapped methane—could have similarly been triggered by the Siberian eruptions and the significant global warming that resulted.

Whatever the cause, the Permian extinction, which devastated every group of plants and animals, was extremely abrupt by geological standards. Sedimentary rocks dating from after the cataclysm are nearly bare of fossils for the first ten million years of the Triassic period.

The Present and Near Future

Unless the most carefully researched models are far off the mark, nothing resembling the devastating geochemical upheavals of the Permian-Triassic period looms in the foreseeable future, even if present levels of fossil fuel consumption persist. These models presuppose that volcanic activity will continue at levels typical of the Holocene and that no asteroids are headed in Earth’s direction.

Possible changes due to increasing ocean acidity are being closely monitored, with the discovery of shell dissolution in Antarctic pteropods presenting new cause for concern. Acidity alone is not expected to reach lethal levels in the near future, but temperatures are rising rapidly, and marine organisms in the Antarctic cannot adjust their ranges southward. A wide variety of fish and birds depend on this snail for continued survival.

Polar Regions

In both the Arctic and the Antarctic, chilled surface seawater sinks, allowing nutrient-rich waters to well up from below and support high phytoplankton productivity. The polar seas teem with life. The lower surfaces of ice sheets also support dense growth of attached algae. Global warming near the North Pole causes the most productive zone to retreat northward and contract in extent. This restricts the number of both herbivores and carnivores the system can support. Most polar animals are unable to extend their ranges into temperate seas, because their unique adaptations to frigid temperatures make them poor competitors and susceptible to disease in warmer climates. The situation in the Southern Hemisphere is even more acute, as species migrating southward encounter the continental margin.

The plight of polar bears has received considerable attention. These huge carnivores prey almost entirely on seals that they hunt on sea ice. As the seals are declining in numbers and retreating farther from shore in response to the shrinking of the ice cap, bears are starving and failing to reproduce. Whale populations that had begun to recover from overexploitation by the whaling industry are also declining again as a result of low food supplies. Antarctic penguins also face declining food supplies and an influx of predators, including sharks, which are extending their ranges southward.

Coral Reefs

Reef-building corals, and the numerous species that depend on them, have a narrow temperature range for optimum growth. They are also vulnerable to changes in sea level due to either global warming or global cooling. During the last Pleistocene glaciation, the resulting drop in sea level exposed much of Australia’s Great Barrier Reef, restricting this unique ecosystem to isolated pockets. A rapid rise in sea level would damage existing reefs by reducing light levels below those needed by symbiotic algae.

A 2 degree Celsius rise in surface temperature is sufficient to cause bleaching in corals as the individual polyps eject symbiotic algae. Bleaching initially causes growth to cease and eventually kills the coral colony. In recent years, there have been massive die-offs of corals. Notably, coral bleaching occurred on the Great Barrier Reef in Australia between 2016 and 2024 because of high sea surface temperatures. By 2024, scientists, using drone imagery, discovered that 97 percent of corals on a reef in the Great Barrier Reef's north had died. A previous die-off of corals in the South Pacific was associated with a severe El Niño–Southern Oscillation (ENSO) event to which global warming may have contributed. Near-shore pollution also devastates coral reefs in populated areas.

Dead Zones

In a number of parts of the world, extensive areas of ocean have become depleted in oxygen, turning once-productive fisheries into wastelands. Most of these dead zones are associated with rivers that drain populated areas; one of the largest lies offshore of the mouth of the Mississippi River. This dead zone owes its existence to influxes of nutrient-laden freshwater to the Gulf of Mexico. These nutrients stimulate massive algal blooms. There is an indirect connection to global warming, in that warming generally causes increased precipitation and therefore increased runoff. Dead zones off the west coasts of the United States and South Africa result from disruption of cold currents and associated upwelling zones and thus are believed to be directly related to global warming.

Productivity

While global warming due to elevated CO₂ levels can cause local drops in productivity due to drought on land and disruption of thermohaline cycles in the ocean, the long-term predicted effect of such warming on a global scale is a net increase in photosynthesis, with an upper limit that far exceeds any projections based on realistic economic indicators. In the long term, if Earth begins producing more food, both the numbers and the diversity of herbivores and predators can be expected to increase.

In the short term, however, such changes lead to the proliferation of weedy species with high reproductive rates and broad ecological ranges, loss of diversity, and generally unstable conditions. Species with specialized ecological requirements become extinct, and natural ecosystems increasingly resemble intentional agriculture or aquaculture. A glimpse of the future may be gleaned from the formerly rich fisheries off the West Coast of North America. These have been in decline for several decades, mainly because of pollution and overfishing. Warmer waters coupled with a persistent dead zone off the coast of California and Oregon have further reduced stocks of commercial and sport fishes, but they have favored proliferation of the Humboldt giant squid, an aggressive predator adapted to warm temperatures and low oxygen levels.

Rising temperatures can be expected to reduce areas of high planktonic productivity near the poles while expanding them near the equator and at continental margins, threatening polar species with starvation and extinction while increasing in numbers in warmer climates without a corresponding increase in diversity.

There may be reef-building organisms ready to replace corals should the seas become inhospitable. During the very warm late Cretaceous, rudists, a group of bivalve mollusks related to clams, were the main reef builders. Several types of algae also have limestone skeletons. If any of these groups were to replace corals, the structural integrity of reefs would be preserved, but the beauty and diversity of the ecosystem would be sadly compromised on any conceivable human time scale.

Context

The main threats to the abundance and diversity of marine life derive from Earth’s human population explosion and its concomitant overexploitation and pollution of coastal waters. The exploding population is also a major factor in global warming. In terms of direct threats posed to marine life from rising temperatures, dwindling sea ice in the Arctic and especially the Antarctic is probably the most clear-cut. While anthropogenic climate change is undoubtedly a factor in coral-reef destruction and the decline of fisheries, it is likely not the only one. As scientists learn more about long-term cycles involving ENSO and analogous oscillating pressure and current systems in other oceans, a better understanding of the relationship of current extreme events to long-term trends should emerge.

Key Concepts

• dead zones: areas of deepwater oxygen depletion due to surface algal blooms or disruption of thermohaline circulation
• El Niño–Southern Oscillation (ENSO): periodic fluctuation of temperatures and currents in the Pacific Ocean on a four-, ten-, and ninety-year cycle
• primary production: production of fixed carbon through photosynthesis
• thermohaline circulation: the rising and sinking of water caused by differences in water density due to differences in temperature and salinity

Bibliography

Bednaršek, Nina, et al. "Extensive Dissolution of Live Pteropods in the Southern Ocean." Nature Geoscience, vol. 5, no. 12, 2012, pp. 881–85.

Benton, Michael J., and Richard J. Twitchett. “How to Kill (Almost) All Life: The End-Permian Extinction Event.” Trends in Ecology and Evolution, vol. 18, no. 7, 2003, pp. 358–65.

Peters, Robert L., and Thomas E. Lovejoy, editors. Global Warming and Biological Diversity. Yale UP, 1992.

Readfearn, Graham. "'Most of It Was Dead': Scientists Discover One of the Worst Coral Bleaching Events." The Guardian, 20 June 2024, www.theguardian.com/environment/article/2024/jun/26/most-of-it-was-dead-scientists-discovers-one-of-great-barrier-reefs-worst-coral-bleaching-events. Accessed 9 Dec. 2024.

Reynolds, Colin S. Ecology of Phytoplankton. Cambridge UP, 2006.

Saltzman, Barry. Dynamical Paleoclimatology: Generalized Theory of Global Climate Change. Academic Press, 2002.

"What Is Ocean Acidification?" National Ocean Service, National Oceanic and Atmospheric Administration, 16 June 2024, https://oceanservice.noaa.gov/facts/acidification.html. Accessed 9

Dec. 2024,