Ocean temperature

Sea surface temperature Deep ocean temperature Experts refer to the temperature further below the surface as ocean temperature or deep ocean temperature. Ocean temperatures more than 20 metres below the surface vary by region and time. They contribute to variations in ocean heat content and ocean stratification. 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 water 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 of around 0-3°C. Its salinity is about 3.5% or 35 ppt (parts per thousand). == Relevance ==

Ocean temperature and dissolved oxygen concentrations have a big influence on many aspects of the ocean. These two key parameters affect the ocean's primary productivity, the oceanic carbon cycle, nutrient cycles, and marine ecosystems. They work in conjunction with salinity and density to control a range of processes. These include mixing versus stratification, ocean currents and the thermohaline circulation. Ocean heat content Experts calculate ocean heat content by using ocean temperatures at different depths. == Measurements ==

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. The reason is there is a lot of variation with depths. This is especially the case during the day. 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. This device is usually mounted on a frame that includes water sampling bottles. 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. Accurate profiling requires sensors with very fast response times. If a sensor with a slow time constant is lowered too quickly, it produces a 'hysteresis' effect - a distortion where measured temperatures appear too high during descent and too low during ascent. Scientists can deploy CTD systems from research ships on moorings gliders and even on seals. With research ships they receive data through the conducting cable. For the other methods they use telemetry. There are other ways of measuring sea surface temperature. At this near-surface layer measurements are possible using thermometers or satellites with spectroscopy. 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 will accurately sample temperature for a majority of the ocean volume once it is in full use. The most frequent measurement technique on ships and buoys is thermistors and mercury thermometers. Scientists often use mercury thermometers to measure the temperature of surface waters. They can put them in buckets dropped over the side of a ship. To measure deeper temperatures they put them on Nansen bottles. == Monitoring through Argo program ==

Trends Causes 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. This process is called ocean deoxygenation. The ocean has already lost oxygen throughout the water column. Oxygen minimum zones are expanding worldwide. Changing ocean currents 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. The sinking and upwelling that occur in lower latitudes, and the driving force of the winds on surface water, mean the ocean currents circulate water throughout the entire sea. Global warming on top of these processes causes changes to currents, especially in the regions where deep water is formed. == Recent trends ==

The current decade has seen a groundbreaking acceleration in ocean warming. The year 2025 has been confirmed as the warmest year ever recorded for the global ocean, extending a streak of nine consecutive years of record-breaking heat. The North Atlantic has emerged as a primary hotspot for intense temperature anomalies, making deep-ocean heat rising not uniform. Several factors have contributed to this spike. Beyond greenhouse gas emissions, scientists are investigating the possibility that declining air pollution has allowed more solar radiation to reach and warm the ocean surface. The consequences of this heat have been immediate and physical; 2024 saw an unexpected jump in sea-level rise linked to extreme thermal expansion, and the heat in the Atlantic increased the energy available for record-breaking hurricane seasons. To track these rapid changes, researchers have moved beyond traditional devices such as buoys, deploying next-generation technologies to monitor regional anomalies with higher precision than before. == In the geologic past ==

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 . Reconstructed proteins from Precambrian organisms also provide evidence that the ancient world was much warmer than today. 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. Such high temperatures are above the upper thermal limit of 38 °C for modern marine invertebrates. They preclude a major biological revolution. During the later Cretaceous period, from , average global temperatures reached their highest level in the last 200 million years or so. This was probably the result of the configuration of the continents during this period. It allowed for improved circulation in the oceans. This discouraged the formation of large scale ice sheet. 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. == See also ==