Gyres are large systems of rotating ocean currents driven by wind and by differences in temperature and salinity. There are five major gyres: the North and South Pacific, the North and South Atlantic, and the Indian Ocean subtropical gyres.

The Atlantic Meridional Overturning Circulation, or AMOC, carries warm surface water north and cooler water south at depth, and the Gulf Stream is part of it, which moderates climate worldwide.

Tides depend on more than the Moon and Sun, because continents, the shape of the Earth and the depth of the ocean in different places change when and how high the tide gets in a given spot.

The edges of the ocean's depth zones are conventions, and sources draw them differently. WHOI puts the abyssal zone at 3,000 to 6,500 meters, while NOAA gives 4,000 to 6,000 meters.

As of November 2026, not every country agrees on the Southern Ocean's northern boundary. The U.S. Board on Geographic Names sets it at 60 degrees South, the proposal to the International Hydrographic Organization in 2000 has not been ratified, and National Geographic recognized the Southern Ocean as a fifth ocean in 2021.

Many models predict that phytoplankton production will fall as the ocean surface warms, but scientists are not sure. Shifts between larger and smaller species have already been seen, and whether total production will change remains uncertain.

Marine snow is a mix of dead plankton, waste and mucus that slowly sinks from the surface, and it carries carbon to the deep sea, and scientists still do not fully understand how much.

Ocean acidification does not hurt every species the same way. In one lab study some shell-builders such as conchs deteriorated in high carbon dioxide while some others thrived, and some fish detect predators less well in more acidic water.

When stressed by changes in temperature, light or nutrients, corals expel the algae that live in their tissues and turn white. One idea about why is that stressed corals make too much of a chemical called superoxide.

As of November 2026, the ocean has absorbed about 90 percent of the extra heat that people's greenhouse gas emissions have trapped on Earth since 1971.

The ocean has taken in a large share of the carbon dioxide people release, and without that, the amount in the air would be even higher. How well the ocean can keep doing this is one of the big unknowns about the greenhouse effect.

Sea level rise is not the same everywhere, because local factors such as sinking land, ocean currents, and land still rebounding after the weight of Ice Age glaciers can make a place's sea level rise faster or slower than the global average.

Words to know

• gyres: rotating currents
• salinity: saltiness
• abyssal: very deep
• conventions: agreed rules
• superoxide: a reactive chemical