Ocean temperature facts for kids
The ocean temperature is super important for our planet's climate system, ocean currents, and all the amazing marine life. It changes a lot depending on how deep you go, where you are on Earth, and the season. Not only does the temperature change, but also how salty the seawater is.
Warm water near the surface is usually saltier than the colder water found deep down or near the poles. In cold polar areas, the top layers of ocean water are cold and not very salty. Deep ocean water is cold, salty water found far below the surface of Earth's oceans. This deep water stays at a steady temperature of about 0-3 °C.
Ocean temperature also depends on how much sunlight hits the surface. In tropical areas, where the Sun is almost directly overhead, the surface water can get warmer than 30 °C (86 °F). Near the poles, where there's sea ice, the water temperature is around −2 °C (28 °F).
There's a huge, continuous flow of water in the oceans called ocean currents. One part of this is the thermohaline circulation. This is driven by differences in water density, which are caused by heat from the Sun and fresh water entering the ocean. Warm surface currents cool down as they move away from the tropics. As they cool, the water gets denser and sinks. These changes in temperature and density make the cold water flow back towards the equator as a deep current. Eventually, it rises back to the surface.
When we talk about ocean temperature, it can mean the temperature at any depth. But sometimes, it specifically means the temperature of the water that isn't near the surface. In that case, it's the same as deep ocean temperature.
It's clear that our oceans are getting warmer because of climate change. This warming is happening faster and faster. The upper part of the ocean (down to about 700 meters) is warming the quickest. But the warming trend is happening throughout the entire ocean. In 2022, the global ocean was the hottest it has ever been since we started recording temperatures.
What is Ocean Temperature?
Sea Surface Temperature
Deep Ocean Temperature
Experts call the temperature deeper below the surface ocean temperature or deep ocean temperature. Ocean temperatures more than 20 meters below the surface change depending on the area and time. They affect how much heat the ocean holds and how its layers are formed. The warming of both surface and deeper ocean temperatures is a big effect of climate change on oceans.
Deep ocean water is cold, salty water found far below the surface of Earth's oceans. This deep water makes up about 90% of all the ocean's water. It has a very steady temperature of about 0-3 °C. Its saltiness is about 3.5%, or 35 parts per thousand.
Why Ocean Temperature Matters
Ocean temperature and the amount of dissolved oxygen in the water greatly affect many things in the ocean. These two important factors influence the ocean's primary productivity (how much life it can support), the oceanic carbon cycle (how carbon moves through the ocean), nutrient cycles, and marine ecosystems.
They work with saltiness and density to control many processes. These include how water layers mix or stay separate, how ocean currents flow, and the thermohaline circulation.
Ocean Heat Content
Scientists figure out ocean heat content by using ocean temperatures at different depths.
How We Measure Ocean Temperature
There are many ways to measure ocean temperature. Below the surface, it's important to say how deep the measurement was taken. This is because the temperature can change a lot with depth. This is especially true during the day. If there's little wind and lots of sunshine, a warm layer can form at the surface. This causes big temperature changes as you go deeper. Scientists call these strong daytime temperature changes a diurnal thermocline.
A common way to measure is by lowering a device called a CTD. CTD stands for conductivity, temperature, and depth. This device sends data continuously to a ship through a cable. It's often attached to a frame that also holds bottles to collect water samples. Since the 2010s, self-driving vehicles like gliders or small submersibles have become more common. They carry the same CTD sensors but work without a research ship.
Scientists can use CTD systems from research ships, on moorings (anchored buoys), gliders, and even on seals! With research ships, they get data through a cable. For other methods, they use telemetry (sending data wirelessly).
There are other ways to measure sea surface temperature. At the very top layer, measurements can be taken using thermometers or satellites. Weather satellites have been able to measure this since 1967. The first global maps of ocean temperature were made in 1970.
The Advanced Very High Resolution Radiometer (AVHRR) is widely used to measure sea surface temperature from space.
Various devices measure ocean temperatures at different depths. These include the Nansen bottle, bathythermograph, CTD, or ocean acoustic tomography. Moored (anchored) and drifting buoys also measure sea surface temperatures. Examples are those used by the Global Drifter Program and the National Data Buoy Center. The World Ocean Database Project is the largest database for temperature readings from all the world’s oceans.
A small group of deep Argo floats is being tested to measure temperatures down to about 6000 meters. Once fully used, they will accurately measure temperature for most of the ocean's volume.
Argo Program
Ocean Warming
Trends in Ocean Warming
Why the Ocean is Warming
The recent warming we see is caused by the warming of Earth. This warming is due to human-caused emissions of greenhouse gases like carbon dioxide and methane. More greenhouse gases in the air make Earth's energy balance uneven, which warms surface temperatures even more. The ocean takes in most of this extra heat, causing ocean temperatures to rise.
Main Effects of Ocean Warming
More Layers and Less Oxygen
Higher air temperatures warm the ocean surface. This leads to greater ocean stratification, meaning the ocean forms more distinct layers. When ocean layers mix less, warm water stays near the surface. This also reduces the circulation of cold, deep water.
Less mixing means the ocean can't absorb as much heat. This sends more future warming to the atmosphere and land. The energy available for tropical cyclones and other storms is likely to increase. Nutrients for fish in the upper ocean layers are expected to decrease. This also means the oceans might not be able to store as much carbon.
Warmer water cannot hold as much oxygen as cold water. Increased layering in the ocean might reduce the flow of oxygen from the surface to deeper waters. This would further lower the water's oxygen content. This process is called ocean deoxygenation. The ocean has already lost oxygen throughout its depths. Areas with very low oxygen, called Oxygen minimum zones, are growing worldwide.
Changing Ocean Currents
Ocean currents are caused by different temperatures from sunlight and air at various latitudes. Winds and the different densities of salty and fresh water also cause currents. Air tends to warm and rise near the equator, then cool and sink a bit further towards the poles. Near the poles, cool air sinks, but then warms and rises as it travels along the surface towards the equator.
The sinking and rising of water in lower latitudes, along with the force of winds on surface water, make ocean currents circulate water throughout the entire sea. Global warming on top of these natural processes causes changes to these currents. This is especially true in areas where deep water is formed.
Ocean Temperature in the Past
Scientists believe the sea temperature was much hotter millions of years ago. They figure this out by studying oxygen and silicon from rock samples. These studies suggest the ocean was 55–85 °C between 2000 and 3500 million years ago. It then cooled to milder temperatures of 10 to 40 °C by 1000 million years ago.
The Cambrian explosion happened about 538.8 million years ago. This was a key time when many new forms of life appeared on Earth. Scientists believe sea surface temperatures reached about 60 °C during this event. Such high temperatures are above the limit for modern marine invertebrates (animals without backbones), which is 38 °C. This suggests that a major biological revolution might not have happened with such high temperatures.
During the later Cretaceous period, from 100 to 66 million years ago, average global temperatures were the highest in the last 200 million years. This was probably because of how the continents were arranged during this time. It allowed for better circulation in the oceans, which stopped large ice sheets from forming.
Data from an oxygen isotope database show that there have been seven global warming events in Earth's past. These include the Late Cambrian, Early Triassic, Late Cretaceous, and Paleocene-Eocene transition. The sea surface was about 5-30º warmer than today during these warming periods.
See Also
- Ocean current § Causes, how temperature helps cause ocean currents
- Global surface temperature, the average temperature of the Earth's surface (both land and sea)
- Marine heatwave
- Upwelling
