Ocean temperature
The ocean temperature plays a crucial role in the global climate system, ocean currents and for marine habitats. It varies depending on depth, geographical location and season. 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 found deep below the surface of Earth's oceans. This water has a uniform temperature of 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 25°C. Near the poles the temperature in equilibrium with the sea ice is about -1.8°C.
There are various ways to measure ocean temperature. Below the sea surface, it is important to refer to the specific depth of measurement as well as measuring the general temperature. At this time low wind speed and a lot of sunshine may lead to the formation of a warm layer at the ocean surface and big changes in temperature as you get deeper. Experts call these strong daytime vertical temperature gradients a diurnal thermocline. The basic technique involves lowering a device to measure temperature and other parameters electronically. This device is called CTD which stands for conductivity, temperature, and depth. It continuously sends the data up to the ship via a conducting cable. Since the 2010s autonomous vehicles such as gliders or mini-submersibles have been increasingly available. They carry the same CTD sensors, but operate independently of a research ship. Weather satellites have been available to determine this parameter since 1967. Scientists created the first global composites during 1970. The Advanced Very High Resolution Radiometer (AVHRR) is widely used to measure sea surface temperature from space. A small test fleet of deep Argo floats aims to extend the measurement capability down to about 6000 meters.
It is clear that the oceans are warming as a result of climate change and this rate of warming is increasing. In 2022, the global ocean was the hottest ever recorded by humans. The cause of recent observed changes is the warming of the Earth due to human-caused emissions of greenhouse gases such as carbon dioxide and methane. Growing concentrations of greenhouse gases increases Earth's energy imbalance, further warming surface temperatures. The ocean takes up most of the added heat in the climate system, raising ocean temperatures. The upper ocean above 700 m is warming fastest, but the warming trend extends throughout the ocean. Experts calculate ocean heat content by using ocean temperatures at different depths. This increase of both ocean surface temperature and deeper ocean temperature is an important effect of climate change on oceans.
Higher air temperatures warm the ocean surface. And this leads to greater ocean stratification. Reduced mixing of the ocean layers stabilises warm water near the surface. At the same time it reduces cold, deep water circulation. The reduced up and down mixing reduces the ability of the ocean to absorb heat. Warmer water cannot contain as much oxygen as cold water. Increased thermal stratification may reduce the supply of oxygen from the surface waters to deeper waters. This would further decrease the water's oxygen content. This process is called ocean deoxygenation. The ocean has already lost oxygen throughout the water column. Oxygen minimum zones are expanding worldwide. Nutrients for fish in the upper ocean layers are set to decrease. This is also like to reduce the capacity of the oceans to store carbon.
Varying temperatures associated with sunlight and air temperatures at different latitudes cause ocean currents. Prevailing winds and the different densities of saline and fresh water are another cause of currents. Air tends to be warmed and thus rise near the equator, 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. There is a continuous large-scale circulation of water in the oceans. One part of it is the thermohaline circulation (THC). It is driven by global density gradients created by surface heat and freshwater fluxes. Warm surface currents cool as they move away from the tropics. This happens as the water becomes denser and sinks. Changes in temperature and density move the cold water back towards the equator as a deep sea current. Then it eventually wells up again towards the surface. Global warming on top of these processes causes changes to currents, especially in the regions where deep water is formed.
Scientists believe the sea temperature was much hotter in the Precambrian period. Such temperature reconstructions derive from oxygen and silicon isotopes from rock samples. These reconstructions suggest the ocean had a temperature of 55, 85°C. It then cooled to milder temperatures of between 10 and 40°C by the Cambrian period. The Cambrian Explosion 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 scientists believe sea surface temperatures reached about 60°C. During the later Cretaceous period, from 145 to 66 million years ago, average global temperatures reached their highest level in the last 200 million years or so. Data from an oxygen isotope database indicate that there have been seven global warming events during the geologic past. These include the Late Cambrian, Early Triassic, Late Cretaceous, and Paleocene-Eocene transition. The surface of the sea was about 5-30º warmer than today in these warming periods.
Common questions
What is the temperature of deep ocean water?
Deep ocean water has a uniform temperature of around 0-3°C. This cold, salty water is found deep below the surface of Earth's oceans.
How do scientists measure ocean temperature today?
Scientists use devices called CTD which stands for conductivity, temperature, and depth to measure parameters electronically. Since the 2010s autonomous vehicles such as gliders or mini-submersibles have been increasingly available to carry these sensors independently.
When was the global ocean hottest ever recorded by humans?
In 2022, the global ocean was the hottest ever recorded by humans. The upper ocean above 700 m is warming fastest but the trend extends throughout the entire ocean.
Why does warmer ocean water contain less oxygen?
Warmer water cannot contain as much oxygen as cold water so increased thermal stratification reduces the supply of oxygen from surface waters to deeper waters. This process is called ocean deoxygenation and oxygen minimum zones are expanding worldwide.
What caused ocean currents in the past and present?
Varying temperatures associated with sunlight and air temperatures at different latitudes cause ocean currents along with prevailing winds and density differences. One part of this circulation is the thermohaline circulation driven by global density gradients created by surface heat and freshwater fluxes.
How hot were oceans during the Precambrian period compared to today?
Scientists believe sea temperature reconstructions suggest the ocean had a temperature of 55-85°C during the Precambrian period. Surface temperatures reached about 60°C during the Cambrian Explosion approximately 538.8 million years ago.