9.6 Ocean Warming

Oceans absorb heat and CO2

The ocean absorbs over 90% of excess heat trapped by greenhouse gases, making it a major buffer against atmospheric warming. It also absorbs about 30% of anthropogenic CO2, which drives ocean acidification as CO2 forms carbonic acid, lowering pH and threatening calcifying organisms like corals and shellfish.

Thermal expansion and sea level rise

As ocean water warms, it undergoes thermal expansion, increasing in volume and raising sea level. Combined with melting glaciers and ice sheets, this contributes to rising sea levels, threatening coastal ecosystems, infrastructure, and low-lying island nations through flooding and erosion.

Ecosystem and circulation impacts

Warmer waters cause coral bleaching, as corals expel symbiotic algae (zooxanthellae) under thermal stress, weakening reefs. Warming also disrupts ocean currents and thermohaline circulation, altering nutrient distribution, marine species migration, and weather patterns like El Niño, with cascading effects on global climate and fisheries.

Key terms

Ocean heat absorption

The ocean has absorbed over 90% of the excess heat caused by greenhouse gas emissions.

Thermal expansion

Increase in water volume as it warms, a major contributor to sea level rise.

Ocean acidification

Lowering of ocean pH caused by CO2 dissolving in seawater and forming carbonic acid.

Coral bleaching

Loss of color in corals when they expel symbiotic algae (zooxanthellae) due to thermal stress.

Thermohaline circulation

Global ocean circulation driven by temperature and salinity differences; disrupted by warming, affecting climate and nutrient cycling.

Try an AP-style question

What happens to corals during prolonged ocean warming?

1. They bleach by expelling their algae
2. They move to colder water quickly
3. They produce more oxygen
4. They grow faster and brighter