The ocean temperature varies by depth, geographical location and

season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...

. Both the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

ocean water Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approx ...

differs. Warm surface water is generally saltier than the cooler deep or polar waters; in polar regions, the upper layers of ocean water are cold and fresh. Deep ocean water is cold, salty

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

found deep below the surface of Earth's oceans. This water has a very uniform temperature, around 0-3°C. The ocean temperature also depends on the amount of solar radiation falling on its surface. In the tropics, with the Sun nearly overhead, the temperature of the surface layers can rise to over while near the poles the temperature in equilibrium with the

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's o ...

is about . There is a continuous circulation of water in the oceans. Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. Warm surface currents cool as they move away from the tropics, and the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, before eventually welling up again towards the surface. ''Ocean temperature'' as a term is used either for the temperature in the ocean at any depth, or specifically for the ocean temperatures that are not near the surface (in which case it is synonymous with "deep ocean temperature"). It is clear that the oceans are warming as a result of climate change and this rate of warming is increasing.IPCC, 2019
Summary for Policymakers
In
IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.) Cambridge University Press, Cambridge, UK and New York, NY, USA. https://doi.org/10.1017/9781009157964.001. The upper ocean (above 700 m) is warming fastest, but the warming trend extends throughout the ocean.

Definition and types

Sea surface temperature

Deep ocean temperature

The temperature further below the surface is called "ocean temperature" or "deep ocean temperature". Ocean temperatures (more than 20 metres below the surface) also vary by region and time, and they contribute to variations in

ocean heat content In oceanography and climatology, ocean heat content (OHC) is a term for the energy absorbed by the ocean, where it is stored for indefinite time periods as internal energy or enthalpy. The rise in OHC accounts for over 90% of Earth’s excess t ...

and ocean stratification.Fox-Kemper, B., H.T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S.S. Drijfhout, T.L. Edwards, N.R. Golledge, M. Hemer, R.E. Kopp, G. Krinner, A. Mix, D. Notz, S. Nowicki, I.S. Nurhati, L. Ruiz, J.-B. Sallée, A.B.A. Slangen, and Y. Yu, 2021
Chapter 9: Ocean, Cryosphere and Sea Level Change
I
Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
[Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1211–1362, doi:10.1017/9781009157896.011. The increase of both ocean surface temperature and deeper ocean temperature is an important effect of climate change on oceans. Deep ocean water is the name for cold, salty

found deep below the surface of Earth's oceans. Deep ocean water makes up about 90% of the volume of the oceans. Deep ocean water has a very uniform temperature, around 0-3°C, and a salinity of about 3.5% or 35 ppt (parts per thousand).

Relevance

Ocean temperature and dissolved oxygen concentrations are two key parameters that influence the ocean's primary productivity, the oceanic carbon cycle, nutrient cycles, and marine ecosystems. They work in conjunction with salinity and

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

to control a range of processes such as mixing versus stratification, ocean currents and the thermohaline circulation.

Measurements

There are a variety of techniques for measuring ocean temperature. Away from the immediate sea surface, general temperature measurements are accompanied by a reference to the specific depth of measurement. This is because of significant differences encountered between measurements made at different depths, especially during the daytime when low wind speed and a lot of sunshine may lead to the formation of a warm layer at the ocean's surface and strong vertical temperature gradients (a diurnal thermocline). The basic technique involves lowering a device everybody calls CTD (the abbreviation stands for conductivity, temperature, and depth) which measures temperature and other parameters electronically. It continuously sends the data up to the ship via a conducting cable. This device is usually mounted on a frame which includes water sampling bottles. Since the 2010s there is an increasingly widespread availability of autonomous vehicles - gliders, mini-

submersibles A submersible is a small watercraft designed to operate underwater. The term "submersible" is often used to differentiate from other underwater vessels known as submarines, in that a submarine is a fully self-sufficient craft, capable of ind ...

etc. - which carry the same CTD sensor but which operate independent of a research ship. Sea surface temperature measurements are confined to the top portion of the ocean, known as the near-surface layer. They can be measured with thermometers or from satellites. Weather satellites have been available to determine this parameter since 1967, with the first global composites created during 1970. The Advanced Very High Resolution Radiometer (AVHRR) is "the most commonly used instrument to measure sea surface temperature from space".

Mercury thermometers

The measurement technique used on ships and buoys is usually thermistors and mercury thermometers. Mercury thermometers are widely to measure the temperature of surface waters; in this case they can be placed in buckets dropped over the side of a ship. To measure deeper temperatures they are placed on Nansen bottles.

Argo program

Increasing temperature due to climate change

Trends

Causes

The root cause of these observed changes is the Earth warming due to anthropogenic emissions of greenhouse gases, such as for example

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

and

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...

Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License This leads inevitably to ocean warming, because the ocean is taking up most of the additional heat in the climate system.

Main physical effects

Increased stratification and lower oxygen levels

Warming of the ocean surface due to higher air temperatures leads to increased ocean temperature stratification: The decline in mixing of the ocean layers stabilises warm water near the surface while reducing cold, deep water circulation. The reduced up and down mixing reduces the ability of the ocean to absorb heat, directing a larger fraction of future warming toward the atmosphere and land. Energy available for tropical cyclones and other storms is expected to increase,

nutrients A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excr ...

for fish in the upper ocean layers are set to decrease, as is the capacity of the oceans to store carbon. Warmer water cannot contain as much oxygen as cold water. As a result, the gas exchange equilibrium changes to reduce ocean oxygen levels and increase oxygen in the atmosphere. Increased thermal stratification may lead to reduced supply of oxygen from the surface waters to deeper waters, and therefore further decrease the water's oxygen content. The ocean has already lost oxygen throughout the water column, and oxygen minimum zones are expanding worldwide.Bindoff, N.L., W.W.L. Cheung, J.G. Kairo, J. Arístegui, V.A. Guinder, R. Hallberg, N. Hilmi, N. Jiao, M.S. Karim, L. Levin, S. O’Donoghue, S.R. Purca Cuicapusa, B. Rinkevich, T. Suga, A. Tagliabue, and P. Williamson, 2019
Chapter 5: Changing Ocean, Marine Ecosystems, and Dependent Communities
In
IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.) In press.

Changing ocean currents

Ocean currents An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contou ...

are caused by varying temperatures associated with sunlight and air temperatures at different latitudes, as well as by prevailing winds and the different densities of saline and fresh water. Air tends to be warmed and thus rise near the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...

, then cool and thus sink slightly further poleward. Near the poles, cool air sinks, but is warmed and rises as it then travels along the surface equatorward. Driven by this sinking and the upwelling that occurs in lower latitudes, as well as the driving force of the winds on surface water, the ocean currents act to circulate water throughout the entire sea. When global warming is added into the equation, changes occur, especially in the regions where deep water is formed.

In the geologic past

Temperature reconstructions based on oxygen and silicon isotopes from rock samples have predicted much hotter Precambrian sea temperatures. These predictions suggest ocean temperatures of 55–85 °C during the period of , followed by cooling to more mild temperatures of between 10-40 °C by . Reconstructed proteins from Precambrian organisms have also provided evidence that the ancient world was much warmer than today. The

Cambrian Explosion The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil record. ...

(approximately 538.8 million years ago) was a key event in the evolution of life on Earth. This event took place at a time when sea surface temperatures have been proposed to reach about 60 °C. CC-BY icon

Text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
/ref> Such high temperatures are clearly above the upper thermal limit of 38 °C for modern marine invertebrates and preclude a major biological revolution. During the later portion of the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

, from , average global temperatures reached their highest level during the last ~200 million years. This is likely to be the result of a favorable configuration of the continents during this period that allowed for improved circulation in the oceans and discouraged the formation of large scale ice sheet. Data from an oxygen isotope database indicates that seven global warming events happened during a number of geologic time periods, for example during the Late

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is ...

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

, and Paleocene-Eocene transition. During these warming periods, the sea surface temperatures were about 5–30 °C higher than today.

References

{{Physical oceanography, expanded=other Oceans Coastal and oceanic landforms