The ocean is warming primarily because the Earth system has accumulated additional heat as a result of human caused increases in greenhouse gas concentrations, with the ocean absorbing most of the excess heat that has entered the climate system. Greenhouse gases such as carbon dioxide, methane and nitrous oxide alter the balance between incoming solar energy and outgoing infrared radiation, producing a positive energy imbalance at the top of the atmosphere.
The ocean absorbs a very large share of this excess energy because water has a high heat capacity and because ocean circulation transports heat vertically and horizontally. Warming is therefore not confined to the sea surface. Heat has penetrated into substantial portions of the upper ocean and, to a lesser extent, deeper layers. The rate and geographical pattern of warming vary because currents, winds, mixing, sea ice, atmospheric circulation and basin geometry redistribute heat. Ocean warming is consequently a global energy balance phenomenon expressed through a highly variable three dimensional fluid system.
The greenhouse effect explains why changes in atmospheric composition can warm the ocean as well as the land. Sunlight passes through the atmosphere and warms Earth's surface, while the surface emits thermal infrared radiation. Greenhouse gases absorb some of this outgoing infrared energy and re emit radiation in different directions, including back towards the surface.
Increasing concentrations change the radiative balance, so the climate system must ultimately warm until outgoing energy once again balances absorbed energy at the top of the atmosphere. The ocean is the dominant reservoir receiving this excess heat because it covers most of the planet and has a much greater heat capacity than the atmosphere. Water also circulates through the climate system, allowing heat absorbed near the surface to be transported into deeper layers. This thermal inertia means that ocean warming responds more slowly to forcing than the temperature of some atmospheric components but stores energy over much longer periods.
Ocean circulation plays a major role in determining where warming occurs. Surface winds, density differences, Earth's rotation and the shape of ocean basins drive large scale currents that transport heat from lower latitudes towards higher latitudes and from the surface into the ocean interior. Warm surface water can be subducted into deeper layers in regions where density increases through cooling and changes in salinity. Deep ocean circulation operates over much longer timescales and can redistribute heat accumulated at the surface.
Upwelling can bring relatively cool deep water towards the surface, temporarily moderating surface warming in some regions while transporting heat and dissolved substances upward. Ocean eddies and boundary currents also move heat on regional scales. These processes explain why ocean warming is spatially uneven and why local measurements can differ substantially from the global mean.
The ocean does not warm uniformly with depth. The upper ocean responds more directly to exchanges with the atmosphere, while deeper layers receive heat through mixing and circulation. The upper few hundred metres have experienced substantial warming, and measurable warming has also penetrated below this depth. The penetration of heat into deeper water can temporarily reduce the rate of surface temperature increase without eliminating the underlying energy imbalance.
Oceanographers therefore examine temperature changes at different depths using ships, moored instruments, satellites and autonomous observing systems such as Argo floats. Argo profiling floats measure temperature and salinity through much of the upper and intermediate ocean and have greatly improved global coverage. Deep ocean measurements remain more challenging and spatially sparse, but long term observations increasingly show warming at depth as well.
Ocean warming is influenced by interactions with sea ice and snow. Ice and snow reflect a large fraction of incoming solar radiation, whereas open seawater absorbs much more solar energy. When warming reduces sea ice coverage, more dark ocean surface becomes exposed, increasing solar absorption and creating a positive feedback known as the ice albedo feedback.
This does not mean that melting sea ice itself directly creates all of the ocean warming; rather, changes in surface reflectivity can amplify warming initiated by other processes. Sea ice also acts as an insulating layer between ocean and atmosphere, so changes in its extent and thickness alter heat and moisture exchanges. In polar regions, these interactions are particularly important because sea ice, ocean temperature, atmospheric circulation and freshwater input are tightly coupled.
Ocean warming causes several physical changes with broad consequences. Warmer seawater expands, contributing to sea level rise through thermal expansion. Warming also affects oxygen solubility, generally reducing the amount of oxygen that water can hold at a given pressure and salinity, although circulation and biological processes determine the actual regional response. Temperature influences marine metabolism, growth, reproduction, species distributions and the timing of ecological events.
Coral bleaching becomes more likely when marine heat stress pushes corals beyond their thermal tolerance for sufficiently long periods. Warming can also alter fisheries by shifting the geographical distribution of species and changing productivity. These biological effects occur alongside chemical changes such as acidification, meaning marine ecosystems can face multiple simultaneous pressures driven by the same underlying increase in atmospheric greenhouse gases.
Natural processes can influence ocean temperatures on seasonal, interannual and longer timescales. El Nino and La Nina events redistribute heat between the ocean and atmosphere and can temporarily accelerate or reduce global surface warming rates. Volcanic eruptions can inject particles and gases into the atmosphere that alter incoming and outgoing radiation, producing temporary cooling that affects ocean heat uptake.
Changes in solar activity also influence Earth's energy balance, although recent long term ocean warming cannot be explained by variations in solar output alone. Ocean circulation changes can redistribute existing heat without changing the total heat content of the climate system. Distinguishing these internal variations from externally forced long term warming requires long records, global observations and climate models that account for both natural variability and anthropogenic forcing.
The ocean is therefore warming because the climate system has a persistent positive energy imbalance, while ocean circulation determines how and where that heat is stored. Measurements of ocean heat content are particularly important because surface temperature alone does not capture the majority of the heat accumulated by the climate system. Ocean heat content integrates temperature changes over depth and provides a physically meaningful measure of energy accumulation.
Rising ocean heat content, increasing sea surface temperatures in many regions, widespread marine heat events and associated physical changes together provide multiple lines of evidence for sustained ocean warming. The warming is not instantaneous or spatially uniform because the ocean has enormous thermal inertia and complex circulation, but this same thermal capacity means that changes occurring today can influence ocean conditions for decades to centuries.
