Ocean Warming (College Board AP® Environmental Science) : Study Guide

What causes ocean warming?

• Ocean warming refers to the increase in average sea surface temperatures due to human activities

• The oceans absorb over 90% of the excess heat trapped by greenhouse gases in the atmosphere

  • The term 'sea surface temperature' (SST) is used specifically because:

    1. The uppermost layer of the ocean (about 100-200 meters) directly absorbs most of the heat from the sun and from the atmosphere

    2. Changes in sea surface temperatures significantly influence weather patterns and climate systems, such as El Niño and La Niña—warmer surface waters can enhance storm formation and alter ocean currents

    3. SST is more accessible and widely measured by satellites and buoys compared to deeper ocean temperatures

  • By focusing on SST, scientists can better track and understand the immediate effects of greenhouse gases and heat absorption on the oceans

Main causes of ocean warming

Increased greenhouse gas emissions

• Burning fossil fuels (coal, oil, natural gas) releases carbon dioxide, methane, and nitrous oxide

• These gases trap heat in the atmosphere, causing both air and ocean temperatures to rise

Deforestation and land-use changes

• Clearing forests reduces carbon sequestration, increasing CO₂ levels

Loss of sea ice and albedo effect

• Melting polar ice reduces the albedo effect (reflection of sunlight)

• Dark oceans absorb more heat, which increases evaporation into the atmosphere—a positive feedback loop

  • The Arctic Ocean has warmed rapidly due to declining sea ice cover

Increased atmospheric water vapor

• Warmer air holds more water vapor, a potent greenhouse gas, which amplifies warming

  • This creates a positive feedback loop that accelerates ocean warming

The effects of ocean warming

• Ocean warming affects marine species, habitats, and food webs by changing

  • temperature

  • oxygen levels

  • nutrient availability

• Warmer oceans can disrupt metabolic rates, reproduction, and migration patterns of marine species

Main effects of ocean warming

Loss of habitat

• Warmer waters cause coral bleaching and loss of kelp forests, reducing habitats for numerous marine species

  • Example: The Great Barrier Reef has experienced mass bleaching events due to elevated sea temperatures

Metabolic and reproductive changes

• Higher temperatures increase metabolic rates in marine species

  • This can raise their oxygen and food requirements

• Disrupted temperature cues can interfere with breeding cycles and spawning

  • Example: Pacific salmon struggle to migrate and spawn successfully as river and ocean temperatures rise

Shifts in species distribution

• Marine species migrate poleward to find cooler waters

  • This can alter local food webs and predator-prey relationships

  • Example: Atlantic mackerel have shifted northward, impacting traditional fisheries in southern areas

Ocean deoxygenation

• Warmer water holds less dissolved oxygen, creating hypoxic zones that are uninhabitable for many species

  • Example: Dead zones in the Gulf of Mexico result from nutrient run-off and warming, depleting oxygen levels

Increased disease outbreaks

• Higher sea temperatures favor pathogens and weaken immune responses in marine organisms

  • Example: Sea star wasting disease has been linked to warmer waters, causing mass die-offs along the U.S. Pacific coast

Ocean warming & coral bleaching

What is coral bleaching?

• Coral bleaching occurs when corals expel the symbiotic algae (zooxanthellae) living in their tissues

  • This occurs due to stress from higher sea temperatures

• Corals are technically still alive after expelling zooxanthellae during bleaching but are in a weakened state

• Without these algae, corals lose their color and their primary source of energy

  • This makes them more vulnerable to disease and death

• Sustained temperature increases of just 1-2°C above average can trigger bleaching

  • Example: In 2016, 93% of the Great Barrier Reef was affected by a severe bleaching event due to record-high sea temperatures

Consequences of coral bleaching

Loss of biodiversity

• 25% of marine species depend on coral reefs for food and shelter

• Extensive bleaching leads to collapse of reef ecosystems, reducing biodiversity

  • If high temperatures persist for an extended period, bleached corals cannot recover and regain their zooxanthellae and eventually die completely

Impacts on coastal communities

• Coral reefs act as natural breakwaters, reducing wave energy and protecting coastlines from erosion

  • Example: The Maldives are facing higher erosion risks due to declining coral health

Economic impacts

• Coral reefs support tourism and fisheries

  • Bleaching events therefore threaten livelihoods in regions reliant on reef-based tourism and fishing

Awaiting image: Coral bleaching
Image caption: Without zooxanthellae, corals lose their main energy source (from photosynthesis) and cannot grow or reproduce effectively—if they reacquire zooxanthellae soon enough, they can recover; if not, they starve and eventually die