ocean warming

ocean warming, the absorption of surplus heat attributed to global warming by water in the oceans. Ocean warming is measured as the change in thermal energy in a set volume of water (which is called ocean heat content, OHC). OHC can be calculated by measuring using floating sensors on buoys (or Argo floats) and ships or from probes dropped from aircraft to measure sea surface temperatures (SST) or by using satellite-borne sensors to measure the thermal expansion of large parcels of water.

Moreover, SST measurements are taken directly from instruments deployed from the air-water interface to selected depths. These in situ measurements are integrated to produce OHC values for different locations. These values are later checked with a set of other OHC values calculated from data produced by radar altimeters aboard satellites which track changes in the height of the ocean. In other words, heat causes water to expand, and changes in sea surface height in several locations are then used as proxies to estimate OHC. OHC considers the top 2,000 meters (6,562 feet) of ocean water across the globe. Ongoing efforts have revealed that heat is continuing to rise in the oceans at increasing rates, and there is evidence that this increase in available heat energy is affecting marine ecosystems and human-dominated environments.

Role and pace of ocean warming

The world’s oceans serve as heat reservoirs by absorbing and storing heat from the atmosphere, and ocean temperatures have risen and fallen many times throughout Earth’s history, with regional ocean temperatures also changing as heat is redistributed from one location to another through ocean currents and other forms of circulation. Greenhouse gas emissions, which are generated primarily by the burning of fossil fuels, trap solar energy in the atmosphere, which helps to increases air temperatures at Earth’s surface. Much of this trapped heat energy—more than 90 percent since the 1970s—is then transferred to the oceans.

Heat added to the oceans does not dissipate uniformly, with deep-sea regions warming at a slower rate than those near the surface and the Atlantic and Southern Oceans warming faster than the Pacific. Scientists note that OHC has increased by about 360 zettajoules (ZJ; 1 zettajoule = 5 quintillion joules) from 1955 to 2024. Studies note that the annual rate at which heat increases has risen from 5 ZJ per year in the 1950s to about 10 ZJ per year by 2019, which has prompted scientists to predict that OHC would increase by an additional 1,030 to 1,874 ZJ by 2100, depending upon the pace of global carbon dioxide emissions. Scientists also predict that in some places ocean warming will cause SST increases of between 1 and 4 ˚C (1.8 and 7.2 ˚F) between 2005 and 2100, depending on global carbon emission rates.

Effects on ocean currents and weather

Ocean warming has been shown to cause a number of physical and ecological effects. One of the most noticeable is sea level rise, which increases at roughly 3.4 mm (0.13 inch) per year and is driven by a combination of water’s ability to expand as temperature rises and the release of liquid water to the oceans from melting glaciers and ice sheets. In addition to being responsible for one-third to one-half of this total in the form of thermal expansion, ocean warming also contributes to the melting of ice near the poles.

There is evidence that it alters ocean currents by reducing the temperature gradient between colder and warmer regions, thereby inhibiting the ability of water at the surface to sink and reducing the current’s speed. Ocean warming also affects weather patterns by accelerating the evaporation of water over the ocean. This rise in the rate of evaporation leads to heavier snows and rainfall when the moisture is released. It also increases the frequency and strength of tropical cyclones and accelerates the pace of their intensification (see also Hurricane Helene), while also making flooding and storm surges associated with tropical cyclones more severe.

Scientists are also concerned that ocean warming could melt frozen methane hydrate stored on the sea floor. This liberated methane would float to the surface and enter the atmosphere, adding large quantities of this powerful greenhouse gas that would enhance global warming.

Biological effects

Biologically, ocean warming is causing changes in organisms’ seasonal cycles, geographic distribution, biodiversity, and ecosystem structure. These impacts are being felt in food chains and in the populations of different organisms that make them up, including microorganisms, plankton, fish, seaweed, invertebrates, mammals, reptiles, and birds. Changes in phytoplankton abundance and distribution have altered fisheries—as the ranges of plankton, fish, and other organisms have shifted closer to the poles—threatening food security. Such changes also promote the growth algal blooms, including red tides and pathogenic vibrio bacteria, increasing the risk of humans and other animals coming into contact with them.

There is also strong evidence of ocean warming disrupting the symbiotic relationship between corals and algae leading to bleaching events that kill coral (see coral bleaching). This is important because coral bleaching contributes to biodiversity loss more broadly, because many organisms depend on these ecosystems during at least one of their life stages. In addition, higher concentrations of dissolved carbon dioxide in the water contributes to the creation of carbonic acid (H₂CO₃) associated with ocean acidification. In contrast, as the ocean warms, its ability to store dissolved atmospheric carbon dioxide declines, so, as this carbon sink weakens, the pace of the buildup of carbon dioxide in Earth’s atmosphere will accelerate.

Warming ocean water can stimulate the metabolism of resilient ocean-dwelling organisms, requiring them to consume more food to keep up with their increased energy demands. For less-resilient organisms, warm water can disrupt cellular respiration, which inhibits the organism’s development at different life stages. In addition, dissolved oxygen concentration in the water declines as temperature rises, and some species must either flee (if they are able) or succumb to low-oxygen conditions that they cannot tolerate.