A Discordant Sea: Global Warming and its Effect on Marine Populations

Coral Reef
Colorful Coral Reef, Similan Islands, Thailand. kampee patisena/Moment/Getty Images

Global warming, an increase in the Earth's average atmospheric temperature that causes corresponding changes in climate, is a growing environmental concern caused by industry and agriculture in the mid-20th century to the present.

As greenhouse gasses such as carbon dioxide and methane are released into the atmosphere, a shield forms around the Earth, trapping heat and, therefore, creating a general warming effect. Oceans are one of the areas most affected by this warming.

Rising air temperatures affect the physical nature of the oceans. As air temperatures rise, water becomes less dense and separates from a nutrient-filled cold layer below. This is the basis for a chain effect that impacts all marine life that counts on these nutrients for survival.

There are two general physical effects of ocean warming on marine populations that are crucial to consider:

  • Changes in natural habitats and food supply
  • Changing ocean chemistry/acidification

Changes in Natural Habitats and Food Supply

Phytoplankton, one-celled plants that live at the ocean's surface and algae use photosynthesis for nutrients. Photosynthesis is a process that removes carbon dioxide from the atmosphere and converts it into organic carbon and oxygen, which feed almost every ecosystem. 

According to a NASA study, phytoplankton is more likely to thrive in cooler oceans. Similarly, algae, a plant that produces food for other marine life through photosynthesis, is vanishing due to ocean warming. Since oceans are warmer, nutrients can't travel upward to these suppliers, which survive only in ocean's small surface layer. Without those nutrients, phytoplankton and algae cannot supplement marine life with necessary organic carbon and oxygen.

Yearly Growth Cycles

Various plants and animals in the oceans need both a temperature and light balance in order to thrive. Temperature-driven creatures, such as phytoplankton, have started their yearly growth cycle earlier in the season due to warming oceans. Light-driven creatures start their yearly growth cycle around the same time. Since phytoplankton thrive in earlier seasons, the entire food chain is affected. Animals that once traveled to the surface for food are now finding an area void of nutrients, and light-driven creatures are starting their growth cycles at different times. This creates a non-synchronous natural environment.


The warming of oceans may also lead to migration of organisms along the coasts. Heat-tolerant species, such as shrimp, expand northward, while heat-intolerant species, such as clams and flounder, retreat northward. This migration leads to a new mix of organisms in an entirely new environment, ultimately causing changes in predatory habits. If some organisms cannot adapt to their new marine environment, they will not flourish and will die off.

Changing Ocean Chemistry/Acidification

As carbon dioxide is released into the oceans, the ocean chemistry drastically changes. Greater carbon dioxide concentrations released into the oceans create increased ocean acidity. As ocean acidity increases, phytoplankton is reduced. This results in fewer ocean plants able to convert greenhouse gasses. Increased ocean acidity also threatens marine life, such as corals and shellfish, which may become extinct later this century from the chemical effects of carbon dioxide.

Acidification's Effect on Coral Reefs

Coral, one of the leading sources for the ocean's food and livelihood, is also changing with global warming. Naturally, coral secretes tiny shells of calcium carbonate in order to form its skeleton. Yet, as carbon dioxide from global warming is released into the atmosphere, acidification increases and the carbonate ions vanish. This results in lower extension rates or weaker skeletons in most corals.

Coral Bleaching

Coral bleaching, the breakdown in the symbiotic relationship between coral and algae, is also occurring with warmer ocean temperatures. Since zooxanthellae, or algae, give coral its particular coloration, increased carbon dioxide in the planet's oceans causes coral stress and a release of this algae. This leads to a lighter appearance. When this relationship that is so important for our ecosystem to survive vanishes, corals begin to weaken. Consequently, food and habitats for a great number of marine life are also destroyed.

Holocene Climatic Optimum

The drastic climate change known as Holocene Climatic Optimum (HCO) and its effect on surrounding wildlife is not new. The HCO, a general warming period displayed in fossil records from 9,000 to 5,000 BP, proves that climate change can directly impact nature's inhabitants. In 10,500 BP, younger dryas, a plant that once spread throughout the world in various cold climates, became nearly extinct due to this warming period.

Toward the end of the warming period, this plant that so much of nature had depended on was only found in the few areas that remained cold. Just as younger dryas became scarce in the past, phytoplankton, coral reefs, and the marine life that depend on them are becoming scarce today. Earth's environment is continuing on a circular path that may soon lead to chaos within a once naturally balanced environment.

Future Outlook and Human Effects

The warming of the oceans and its effect on marine life has a direct impact on human life. As coral reefs die, the world loses an entire ecological habitat of fish. According to the World Wildlife Fund, a small increase of 2 degrees Celsius would destroy almost all existing coral reefs. Additionally, ocean circulation changes due to warming would have a disastrous effect on marine fisheries.

This drastic outlook is often hard to imagine. It can only be related to a similar historical event. Fifty-five million years ago, ocean acidification led to a mass extinction of ocean creatures. According to fossil records, it took more than 100,000 years for the oceans to recover. Eliminating the use of greenhouse gases and protecting the oceans can prevent this from occurring again.

Nicole Lindell writes about global warming for ThoughtCo.

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Lindell, Nicole. "A Discordant Sea: Global Warming and its Effect on Marine Populations." ThoughtCo, Oct. 29, 2020, thoughtco.com/global-warming-effect-on-marine-populations-1434916. Lindell, Nicole. (2020, October 29). A Discordant Sea: Global Warming and its Effect on Marine Populations. Retrieved from https://www.thoughtco.com/global-warming-effect-on-marine-populations-1434916 Lindell, Nicole. "A Discordant Sea: Global Warming and its Effect on Marine Populations." ThoughtCo. https://www.thoughtco.com/global-warming-effect-on-marine-populations-1434916 (accessed June 19, 2021).