Oceanography is the scientific study of the global ocean’s physical, chemical, geological, and biological characteristics, including how they interact to shape climate, ecosystems, and the seafloor. According to Encyclopædia Britannica, oceanography’s modern scope spans ocean circulation, seawater chemistry and biogeochemical cycles, marine geology, and marine ecology, integrating observations and theory across disciplines. Britannica

The ocean covers about 71% of Earth’s surface and averages roughly 3,682 meters in depth, with the deepest measured point at the Challenger Deep in the Mariana Trench at approximately 10,935 meters. NOAA Ocean Service; NOAA Ocean Exploration

Historical development

Systematic, global oceanography coalesced during the 1872–1876 HMS Challenger expedition, which circumnavigated the globe, completed hundreds of soundings and dredges, and produced landmark multi-volume reports that established methods and baselines for the field. Britannica

By the mid‑20th century, wartime needs and geophysical discoveries (for example, seafloor spreading and mid‑ocean ridge processes) catalyzed the growth of physical and geological oceanography and the use of acoustics, sonar, and seismic profiling at sea. USGS

Major branches and scope

• –Physical oceanography examines currents, waves, turbulence, and air–sea exchange that govern heat, freshwater, and momentum transport and underlie climate variability. Britannica
• –Chemical oceanography investigates seawater composition, biogeochemical cycles, and processes such as ocean acidification, including pH changes and carbonate chemistry. NOAA PMEL
• –Geological oceanography focuses on seafloor morphology, stratigraphy, plate boundaries, and hydrothermal systems that shape the ocean basins. USGS
• –Biological oceanography (marine ecology) studies marine organisms, primary production, trophic structure, and biodiversity from surface waters to the deep sea. Britannica

Observing the ocean

Modern oceanography relies on complementary platforms spanning surface to abyssal depths and global to local scales. A core system is the Argo array, a global fleet of roughly 4,000 autonomous profiling floats that measure temperature and salinity to 2,000 meters every 10 days, with Deep Argo extending observations to 6,000 meters and BGC‑Argo adding biogeochemical sensors. Argo Program; Argo Program; Argo Program

Satellite remote sensing retrieves sea surface temperature, color, sea level, and wind fields, while new altimetry missions (e.g., Sentinel‑6/Jason‑CS and SWOT) resolve mesoscale to submesoscale sea‑surface height variability linked to currents and eddies. NASA Climate; NASA Climate

Shipboard surveys, moorings, gliders, and seafloor instruments support time‑series and process studies, while multibeam sonar and submersibles map and sample the seabed for bathymetry, geology, and ecology. NOAA Ocean Exploration

Ocean circulation and climate

The ocean stores more than 90% of the excess heat in the climate system since the mid‑20th century, driving thermal expansion and influencing weather and climate extremes. NASA Climate

Global mean sea level rose 3.7 mm per year during 2006–2018, reflecting contributions from ocean warming and land‑ice mass loss; the rise since 1901 totals about 0.20 m. IPCC AR6 Synthesis Report SPM

In the Atlantic, the basin‑scale overturning circulation, of which the surface Gulf Stream is a key western boundary component, has shown observed variations since 2004 and is assessed to very likely weaken over the 21st century in climate model projections. NOAA NESDIS; IPCC AR6 WGI Chapter 4; IPCC AR6 WGI Chapter 2

The El Niño–Southern Oscillation (El Niño–Southern Oscillation) is the dominant interannual mode, involving coupled changes in equatorial Pacific sea‑surface temperature and atmospheric pressure that modulate global rainfall and temperature patterns. NOAA Climate.gov

Seawater chemistry and biogeochemistry

On average, seawater contains about 3.5% dissolved salts (≈35 PSU), dominated by chloride and sodium ions; major sources include rock weathering delivered by rivers and inputs from seafloor hydrothermal and volcanic processes. USGS Water Science School

Since the Industrial Revolution, surface‑ocean pH has decreased by a little over 0.1 units (≈26–30% increase in hydrogen ion concentration), a change widely documented and attributed to uptake of atmospheric CO₂. NOAA PMEL; NOAA PMEL

Oxygen concentrations have declined in the upper ocean over recent decades in many regions, consistent with warming‑driven stratification and biogeochemical changes assessed by multiple lines of evidence. IPCC AR6 WGI Chapter 2

The deep ocean and hydrothermal systems

The “deep ocean” is commonly defined as waters below about 200 meters, characterized by low light, low temperature (near 4 °C), and high pressure, yet supporting diverse life and unique energy pathways. NOAA Ocean Exploration

Hydrothermal vent fields form where seawater circulates through hot oceanic crust at spreading ridges and subduction zones, emerging as mineral‑rich fluids that build sulfide chimneys and sustain chemosynthesis‑based ecosystems independent of sunlight. NOAA Ocean Service; NOAA PMEL Vents

Seafloor, plate boundaries, and marine geology

Ocean basins are sculpted by plate tectonics, with mid‑ocean ridges creating new oceanic crust, transform faults segmenting ridges, and trenches marking subduction of old seafloor; magnetic lineations symmetrically straddling ridges provided key evidence for seafloor spreading. USGS

Hydrothermal circulation at ridges exchanges heat and chemicals between crust and ocean, influences mineral deposition, and supports specialized ecosystems, making ridge systems natural laboratories for geology–biology–chemistry coupling. USGS

Mapping and exploration of the seabed

Although there is one connected global ocean, large portions of the seafloor remain unmapped to modern standards; through the Seabed 2030 initiative, the GEBCO 2025 global grid reports that 27.3% of the ocean floor has been mapped at modern resolution, up from 26.1% in 2024. GEBCO; GEBCO

Seafloor mapping underpins tsunami hazard assessment, navigation, cable routing, habitat characterization, and circulation modeling, and depends on multibeam echo sounders, autonomous vehicles, and data contributions from government, academic, and industry vessels. Seabed 2030

The ocean’s role in Earth’s system

By absorbing over 90% of the excess heat associated with greenhouse forcing, the ocean moderates atmospheric warming, while heat uptake and mass inputs from melting land ice contribute to observed sea‑level rise. NASA Climate; IPCC AR6 Synthesis Report SPM

Primary producers in surface waters, especially phytoplankton, contribute roughly about half of global oxygen production, highlighting the biological significance of planktonic communities for atmospheric composition and carbon cycling. NOAA Ocean Service

Ocean–atmosphere coupling via currents such as the Gulf Stream, large‑scale overturning, and climate modes like the El Niño–Southern Oscillation shapes regional weather, hydroclimate extremes, and ecosystem responses on seasonal to decadal timescales. NOAA NESDIS; NOAA Climate.gov

Fundamental quantities and benchmarks frequently cited in oceanography

The ocean’s global average depth is about 3,682 m; the deepest known point is the Challenger Deep at ~10,935 m; and typical open‑ocean salinity is ~35 PSU. NOAA Ocean Exploration; NOAA Ocean Service; USGS Water Science School

Oceanography synthesizes these metrics with process understanding and observations—from Argo profiles to satellite altimetry—to quantify circulation, biogeochemistry, and seafloor change in support of navigation, hazard assessment, resource stewardship, and climate services. Argo Program; NASA Climate