9.6 - Ocean Warming

Learning Objectives

I. Causes of Ocean Warming

• Two-Way Relationship: The relationship between atmospheric and oceanic warming goes both directions. The atmosphere warms the ocean, and the ocean can pass some of that heat back to the atmosphere.

• Greenhouse Gas Effect: As the planet warms because greenhouse gases radiate heat back to Earth, the ocean absorbs a lot of that heat.

• High Specific Heat: The ocean absorbs so much because of water’s high specific heat, which lets a water molecule absorb a lot of heat compared to other molecules.

• Storage and Distribution: The ocean has absorbed up to 90% of the heating that’s happened on Earth. That heat then gets mixed throughout the ocean by the thermohaline circulation.

• Decadal Release: The ocean can hold heat for a very long time and slowly release it back into the atmosphere for decades to come. Scientists estimate that even if greenhouse gas concentrations were stabilized immediately, the atmosphere would keep warming for decades because of the heat the ocean releases.

II. Effects on Marine Life

• Reduced Dissolved Oxygen: Warmer water holds less oxygen, which can cause respiratory stress or suffocation for species like fish and large mammals that need high oxygen levels.

• Altered Cycles: Higher ocean temperature can alter migratory routes and breeding seasons. Changes in the timing of seasonal ocean warming can make fish migrate earlier or later, which can disrupt their reproduction or cause them to arrive after their food source is gone.

• Metabolic Changes: Most marine animals are ectotherms (cold-blooded), so warmer water speeds up their metabolism. They need more oxygen and food just to stay alive, at the same time that warm water holds less oxygen. Many fish respond by moving toward the poles or into deeper, colder water, and some species grow to smaller adult sizes.

• Reproductive Changes: Warming can throw off spawning times and kill eggs and larvae that need cool water. In sea turtles, the temperature of the sand decides a hatchling's sex: warmer sand makes more females. On the northern Great Barrier Reef, a 2018 study found about 99% of young green sea turtles were female.

• Real Example (Gulf of Maine): from 2004 to 2013 the Gulf of Maine warmed faster than 99% of the world's ocean, which hurt the recovery of Atlantic cod by cutting the survival of young fish (Pershing and others, 2015). Lobsters have shifted north toward Canada as waters off southern New England got too warm.

• Habitat Loss: Melting polar and glacial ice, plus thermal expansion, make shallow coastal waters deeper. That threatens areas critical for the algae growth that helps coral thrive.

III. Algae Blooms

• Toxic blue green algae prefer warmer waters, and warming drives their growth.

• These algae can be toxic, and they form a thick coating that blocks sunlight, choking out aquatic plant growth beneath the surface.

• When the algae die, bacteria break them down and consume oxygen, which leads to hypoxia, meaning decreased oxygen levels.

• Algae also decrease the albedo of the water surface, so it absorbs more heat and warms even further.

IV. Coral Bleaching

• Coral reefs rely on a symbiotic mutualistic relationship between tiny animals (coral polyps) and algae called zooxanthellae. The algae provide sugar, which is the coral’s primary energy source, and they give the reef its color.

• Bleaching is the loss of these algae. The algae have a narrow temperature range, so when the ocean warms, they get expelled or leave the coral, and the reef turns white.

• How Heat Triggers It: when water stays about 1°C above the usual summer maximum for several weeks, the heat damages the algae's photosynthesis and they start making toxic oxygen compounds. The coral expels the algae to protect itself.

• Bleaching can also be caused by pollutants like pesticides or sunscreen.

• Bleached coral aren’t dead, but they’re severely stressed, vulnerable, and immunocompromised, because their main food source is gone. The algae can come back if ocean conditions improve.

• Real Example (2023 to 2025): during the fourth global coral bleaching event, bleaching-level heat stress hit about 84% of the world's coral reef area, in at least 83 countries and territories (NOAA). NOAA later concluded that the event likely ended in mid-2025. El Niño years bring extra ocean heat, so the strong El Niño building in late 2026 raises the risk of new bleaching.

V. General Threats to Coral Reefs

• Sedimentation: Rising sea levels cause sediment to smother the coral or block sunlight for the algae.

• Storm Patterns: More severe storms physically damage the reef structure.

• Salinity Changes: Increased fresh water runoff can dilute the salinity of the ocean water, which disrupts the habitat.

• Ocean Acidification: A decrease in pH, making the ocean more acidic, is a major threat to the coral's ability to build its structure.

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