AP® Environmental Science · Unit 9 · Topic 9.6
Ocean Warming
Water has an enormous heat capacity, so the ocean has soaked up over 90 percent of the extra heat from the greenhouse effect while its average temperature has risen only slightly. That absorbed heat still disrupts coral, fish ranges, storm intensity, and deep ocean circulation.
Water heats up slowly. It takes far more energy to raise the temperature of a kilogram of water than a kilogram of air, and the ocean has roughly a thousand times the mass of the atmosphere. So when extra infrared energy gets trapped by greenhouse gases, the ocean can absorb the vast majority of it while its temperature barely budges. That absorbed energy does not just sit at the surface, either. Wind and waves mix heat into the surface mixed layer, the top tens to hundreds of meters, and from there slower circulation and sinking water carry some of it into the deep ocean, spreading the warming across an enormous volume instead of concentrating it where you would feel it.
Even a small rise in surface temperature matters because marine life is tuned to narrow temperature ranges. Corals expel the symbiotic algae that feed them when water gets too warm, a stress response called bleaching that can kill the reef if heat persists. Fish and other mobile species swim toward cooler water, shifting entire fisheries poleward or into deeper water and away from the communities that depend on them. Warmer surface water also feeds more energy into hurricanes, strengthening the storms that form over it. None of this is a simple upgrade for ocean life; it is a disruption of systems built around stable conditions.
Ocean warming also reaches into global circulation. The thermohaline conveyor belt depends on cold, salty water near the poles sinking because it is dense. Warming surface water, plus freshwater pouring in from melting ice, makes that polar water lighter and less willing to sink, slowing the whole conveyor. That connects ocean warming directly to the next layer of global change: a weaker conveyor alters heat distribution and weather patterns far from the ocean itself.
Key terms
- Heat capacity
- The amount of energy needed to raise a substance's temperature, which is much higher for water than for air.
- Mixed layer
- The upper layer of the ocean, mixed by wind and waves, where most absorbed heat first accumulates.
- Coral bleaching
- The loss of a coral's food-providing algae under heat stress, turning it white and risking death.
- Thermohaline conveyor belt
- The global ocean circulation driven by differences in water temperature and salinity that sink cold, salty water at the poles.
Watch it explained
- How it worksWalk me through heat moving from the atmosphere into the mixed layer and then deeper.
- Going furtherWhat happens to coral, fish ranges, and hurricane strength as the surface ocean warms?
- Common mistakeWhy isn't a warmer ocean simply better for the life living in it?
- Ties togetherHow does ocean warming connect to the thermohaline conveyor belt slowing down?
Check yourself
End of topic.