The ocean is the main component of the global climate system, and therefore is a key factor in climate change with major effects on the marine environment and ocean related human activities. This is due to the amount of water the ocean contains, its heat capacity, its dynamical inertia (the energy needed to drive the ocean circulation) and its storage capacity for chemical compounds.

The ocean and atmosphere exchange water (through precipitation and evaporation), heat, momentum, carbon dioxide and other chemical compounds. The ocean also receives the inflow of fresh water and chemical compounds such as carbon from rivers, plus the inflow of freshwater from the melting of polar ice caps.

The large-scale currents circulating near the surface are mainly driven by the winds, in patterns determined by the ocean basin shapes and the Earth’s rotation. This results in the formation of ocean gyres which are constrained in great part by deep ocean topography. The circulation in the Northern Hemisphere consists of anticlockwise subpolar gyres, clockwise subtropical gyres and mainly zonal equatorial current systems. In the Southern Hemisphere, the ocean circulation is dominated by anticlockwise subtropical gyres and a strong Antarctic circumpolar eastward current.

The deep ocean circulation is described via the concept of "Meridional Overturning Circulation", the main pattern for the worldwide distribution of climate-related physical properties and chemical and biological compounds. Those properties and compounds are also transported vertically via turbulent eddies near the surface and movements driven at various scales by buoyancy, with a dominant role of density stratification and the presence of a well mixed layer near the surface in most locations. The mixed layer base acts as a kind of semi impermeable frontier between near surface waters and the deep ocean and prevents some of the downward transport. The oceanic circulation varies with time and a number of identified modes of variability in the ocean basins over years and decades are important drivers of climate variations.

Systematic ocean observations, initiated in the second half of the 19th century, have developed as part of major observation campaigns organised in the last 40 years and are coordinated by the Global Ocean Observing System (GOOS). Two major conferences, OceanObs99 (in Saint Raphael, France, 1999) and OceanObs09 (in Venice, Italy, 2009), reaffirmed and improved the international strategy for ocean observation.

There is now a more complete description of the ocean within the climate system due to improvements in remote sensing of sea and continental ice and parameters of the water cycle and of lower atmosphere. Since the 1970s, remote sensing from space has allowed a progressive global mapping of a number of surface parameters. These include temperature measured by infrared and micro-wave radiometry, surface wind from radar type remote sensing, phytoplankton from ocean colour observations. More recently sea level from high precision satellite positioning and radar altimetry , and salinity from micro-wave observation have been mapped.

In addition to remote sensing, satellites have progressively allowed the development of automated observation devices. This is through moored and drifting sensors with increasing capacity for subsurface measurements. However improvements are still needed in the coverage and precision of deep ocean measurements. Advances in observation techniques and in modelling and data assimilation techniques have allowed a progressively more thorough representation of the ocean state and evolution. Recent global re-analyses use all observations available and consist of model reconstitutions of the global climate system over parts or the whole of 20th century. These represent important tools for assessing oceanic changes.

Improvements in observation and reconstitution techniques over time make it difficult to compare present with earlier observation periods. This limits many ocean change assessments to the period since 1971 (advent of satellite remote sensing) or to the last 2-3 decades. Depending on the size of the variations in time and the precision of the estimates, there are therefore limitations on attributing some of the observed trends either to time-scale natural variations over decades or to longer term climate change tendencies.

The increased greenhouse effect resulting from human emissions creates an imbalance between the radiative energy coming from the sun and the energy radiated back to space. The ocean plays a primary role in the absorption of this energy imbalance and its worldwide distribution. According to IPCC AR5, ocean warming dominates the increase in energy stored in the climate system, accounting for approximately 93 percent of the energy accumulated between 1971 and 2010. It is estimated that about 70 percent of this extra heat content is stored in the upper 700 metres of the ocean.

Sea surface salinity evolves in response to changes in precipitation, evaporation and run-off, as well as ocean circulation. Observations available since the 1950s indicate an increase in the geographical contrasts of sea surface salinity. There is increased salinity in the evaporation dominated mid latitudes and a decrease in the rainfall dominated tropical and polar regions. In addition, the Atlantic has become saltier and the Pacific and Southern oceans fresher.

Sea-ice cover is particularly sensitive to climate variations. During the last 35 years where good quality satellite imagery is available, sea-ice cover in the Arctic has undergone a significant decrease, of the order of 30 percent for the seasonal minimum, leaving ice-free areas in the Canadian Arctic Archipelago and North of Siberia. Mean ice thickness has also significantly decreased.

Changes in surface heat flux, wind stress and surface wind waves are more difficult to assess. The changes observed with most confidence are an increase in the mean wind stress in the Southern Ocean, and in the wave height maxima in the Southern Ocean, North Atlantic and North Pacific. Those wave maxima are observed in the open sea but this may lead to increased damages in some coastal areas.

The ocean plays a key role as a major component of the global cycle for most chemical compounds. They represent a major sink of carbon dioxide, itself a main driver for oceanic biological processes. It is estimated that about 30 percent of human emissions are presently retained by the oceans. One of the consequences is an acidification of ocean surface waters of the order 0.1 pH unit since the pre-industrial era. This represents approximately a 30 percent increase in acidity, enough to perturb some of the sensitive ocean biological processes, for example the capacity for organisms to produce and maintain their shell.

Global sea level change has been monitored globally since 1993 by satellite radar altimetry with up to millimetric precision. The sea level change is mostly a consequence of the combined effects of ocean density changes (related to temperature and salinity) and input from glaciers and ice cap melting. The observed global sea level change is 3mm/year, with ice melt playing an increasing role. Important regional effects are observed with sea level variations going from negative values over the Eastern Pacific to about four times the mean global value in the Indonesia-Philippines area. Impacts to populations are mostly felt from extreme sea level changes, superimposed on the mean, and due to high tides and storm surges accompanied by extreme wind waves.

Information on this page is based to a large extent on the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), complemented as appropriate with recent research results.