🧪 Ocean chemistry and physics

When carbon dioxide from the air dissolves into seawater, it triggers a chemical reaction that gradually makes ocean water slightly more acidic than it used to be, a slow but measurable shift scientists call ocean acidification. This changing chemistry makes it harder for many ocean creatures, like corals, oysters, and certain plankton, to build and maintain their hard shells or skeletons, since the chemical building blocks they need become less available in more acidic water. Because so many ocean food chains depend on shell-building creatures near the bottom, from tiny plankton to coral reefs, ocean acidification can ripple outward and affect an enormous number of other species further up the chain.

Thermohaline circulation is a massive, slow-moving system of ocean currents driven by differences in water temperature and salt content, since colder, saltier water is denser and sinks below warmer, less salty water. In certain polar regions, surface water becomes extremely cold and salty enough to sink dramatically down toward the deep ocean floor, pulling more water in behind it and helping drive currents across the entire globe. This deep water eventually travels enormous distances along the ocean floor before slowly rising back toward the surface elsewhere, completing a journey that can take a single drop of water over a thousand years to finish.

Carbon dioxide gas naturally dissolves into water, similar to how it dissolves into a fizzy drink, and because the ocean covers such an enormous surface area, it continuously absorbs a significant portion of the carbon dioxide humans release into the atmosphere. Ocean plants and plankton also actively pull carbon dioxide out of the water to grow, using it much like land plants use it during photosynthesis, which helps pull even more carbon dioxide out of the air above the water. When ocean creatures die, some of that carbon-containing material sinks down and eventually gets buried in seafloor sediment, locking that carbon away for an extremely long time rather than returning quickly to the atmosphere.

Scientists use a combination of tide gauges, physical instruments fixed along coastlines that have measured water height at the very same spots for decades, and satellites that precisely measure the height of the entire ocean surface from space. By comparing measurements taken at the same locations over many years, scientists can detect even small, gradual changes in sea level that would be nearly impossible to notice just by looking at the ocean on any single day. Sea levels rise mainly because warmer ocean water physically expands to take up more space, combined with melting ice from glaciers and ice sheets adding extra water into the ocean over time.