In Hot Water: Ocean Heat and Our Warming World
The global ocean is heating up, with far-reaching consequences. As the planet has warmed, the ocean has provided a critical buffer, slowing the effects of climate change by absorbing more than 90 percent of the excess heat in the Earth's system.
Because the ocean plays such a critical role in our climate system, it is an important research topic for climate scientists. Subsurface ocean temperatures are a consistent way to track the effects of greenhouse gas emissions because they are only faintly influenced by short-term weather patterns.
The ocean is absorbing heat due to increasing levels of greenhouse gases in the atmosphere. This leads to big problems for our planet such as sea-level rise, increases in severe weather, large-scale coral reef bleaching events, and massive melting of glaciers and ice sheets.
An Unbalanced Energy Budget
The top few meters of the ocean store as much heat as Earth’s entire atmosphere. The total amount of heat energy stored by the oceans is called Ocean Heat Content.
The Earth absorbs radiation from the Sun and emits some of it back into space. If the amount of energy emitted matches the amount it absorbs, the planet’s “ energy budget ” is in balance, and the surface temperature remains steady. If the incoming and outgoing energy are not balanced, the planet is either warming or cooling over time.
Covering more than 70% of Earth’s surface, our global ocean serves as the largest solar energy collector on Earth. Because water has a higher heat capacity than air, it can absorb an immense amount of heat without a large increase in temperature. The ability to store and release heat over long periods of time gives the ocean a central role in stabilizing Earth’s climate system.
However, over the past several decades, increasing levels of greenhouse gases are preventing excess energy from escaping to space, contributing to the warming of the planet.
Accounting for all the energy that enters and leaves the Earth system helps scientists understand why the planet is warming. This accounting of energy is known as Earth’s energy budget. (Source: NOAA NCEI)
Importance of Ocean Heat Content
Ocean Heat Content (OHC) is one of the most important indicators of climate change.
Just as a speeding car can take some time to stop after the driver hits the brakes, the Earth’s climate system may take some time to reflect the change in its energy balance. In other words, there’s a time lag between when the Earth begins to experience an energy imbalance and when the climate fully responds to it. Even if greenhouse gas emissions were completely halted today, heat already stored in the ocean will eventually be released, committing Earth to additional surface warming in the future.
Scientists across the globe are studying OHC to better understand how best to combat climate change on a warming planet.
Collecting the Data
NOAA uses a variety of instruments and techniques to gather ocean heat content data.
To collect long-term information about the ocean’s temperature and currents, satellite instruments take extremely precise measurements of the height of the ocean surface above the center of the Earth, a measurement commonly called "sea level." These data are combined to reveal the ocean surface topography (not to be confused with bathymetry, which is the relief on the bottom of the ocean).
Ocean heat is of interest to scientists because it reveals information about sea level rise. Because warm water is less dense than cold water, it is higher in volume and causes the surface of the ocean to increase in height. It is estimated that roughly 50% of sea level rise is due to thermal expansion.
Taking the Ocean's Temperature with Argo floats. (Source: NOAA)
To get a more complete picture of sea level change and OHC at different depths, scientists also use a range of in situ (examining phenomenon and collecting data in the place where it occurs) temperature-sensing instruments. The temperatures measured by these instruments are then used to calculate the thermosteric component of sea level change, which is the amount that sea levels rise due only to temperature increases. Water increases in volume as it warms, which leads to sea level rise.
A wide array of instruments have been used to collect ocean data since the early 1700s.
Collection Instruments Through Time
Bottles
Late 1800s to Present
Ocean water samples were and still are, collected using Nansen (and later, Niskin) bottles, which included reversing thermometers. These bottles were lowered to preselected depths on wire. A weight (i.e., “messenger”) would slide down the cable and cause the bottle to "trip" and turn upside down, close, and record the water temperature.
Image: Deploying Nansen bottles while a ship is drifting downwind.
Mechanical Bathythermographs
Mid-1900s
Slowly moving ships lowered mechanical bathythermographs (MBTs) into the water using a winch. The MBT recorded pressure and temperature on a smoked or coated glass slide. When the MBT reached a predetermined depth, a brake was applied and the instrument was drawn back to the surface.
Image: Placing smoked glass bathythermograph slide in instrument prior to deployment.
Expendable Bathythermographs
1960s to Present
Expendable bathythermograph (XBT) probes are dropped from vessels and are used only once. They relay water temperature while descending through the water. The drop-rate speed is used to determine the temperature depth.
Image: Deepwater Wonders of Wake Expedition Mapping Watch Lead, Dan Freitas, conducts an XBT cast.
Conductivity, Temperature, and Depth
1960s to Present
Conductivity, Temperature, and Depth (CTDs) are electronic instruments attached to a large metal frame (rosette) that holds water-sampling bottles and other sensors used for observing conductivity, temperature, and pressure (i.e., depth).
Image: After recovering the CTD, a scientist attaches a tube to each bottle and transfers collected water into plastic jugs where it is taken to the on-ship lab to be filtered for analysis.
Buoys
1970s to Present
Buoys range in size and are designed to measure ocean temperature, air temperature, wave energy, wind direction and speed, carrying CTD and other sensors on the platform. Moored buoys are instrument platforms anchored to remain in the same location.
Image: The TELOS protoype mooring interfaces with multiple sensors on the mooring line collecting, storing and transmitting the data in real time.
Argo Floats
Early 2000s to Present
Argo is a network of about 4,000 battery-powered, autonomous profiling floats that drift with the ocean currents. Starting at roughly 1,000 meters (0.6 miles) deep, the floats dive to about 2,000 meters (1.2 miles) to gather salt content and temperature data as they return to the surface. Data are transmitted via satellite and available within hours.
Image: Argo float being deployed.
Ocean Gliders
Early 2000s to Present
Ocean gliders are autonomous, unmanned robotic vessels that can dive up to a half mile below the ocean surface and transmit data to satellites when they surface. Gliders can travel far distances over long periods underwater.
Image: A NOAA ocean glider, seen in waters off the coast of Puerto Rico in July 2018.
More Instruments, More Data
The World Ocean Database contains a variety of instruments since the early 1900s. This graph highlights the change in instruments used over time due to technological advancements. (Source: NOAA NCEI)
After World War II, technology for in situ subsurface ocean temperature measurements led to the development of a global ocean observing system, drastically increasing the amount of data collected. To centralize knowledge of the ocean, scientists at the NOAA National Centers for Environmental Information (NCEI) created the World Ocean Database (WOD). Researchers in oceanography, meteorology, and climatology use and contribute to the WOD in an effort to better study and understand our changing ocean.
The WOD provides access to millions of uniformly formatted and continuously updated records, also called profiles, submitted by about 90 countries. These profiles, archived by NCEI, provide records of temperature, salinity, nutrients, plankton, pH, and oxygen data, as well as several other ocean variables. Each profile provides a snapshot of oceanographic conditions over multiple depths at one time from one location. Combined with other available profiles over discrete time periods, a view of regional and global changes in the ocean can emerge.
WOD forms the basis for the World Ocean Atlas (WOA), which is a collection of global climatologies based on long-term averages. NCEI uses these long-term climatologies to calculate OHC.
Argo float latest locations via data transmitted to the Global Data Assembly Centres (GDAC). Colored circles refer to the operating country (see map legend). Click on the points to see details.
Studying a Changing Ocean
Knowing how much heat energy the ocean absorbs and releases is essential for understanding and modeling global climate. Climate models are often run on monthly intervals and require monthly heat content and temperature data to produce accurate climate forecasts.
Heat content trends in the upper 700 meters (2,300 feet) of the world ocean show where heat was gained or lost between 1993 and 2019. Large parts of most ocean basins are gaining heat (orange), but some areas have lost heat (blue). Gray shaded areas have trends that are not statistically significant. (Source: NOAA Climate.gov )
NCEI uses the most complete set of historical ocean profile data available through the WOD and the WOA as a basis to produce its global ocean heat product suite. NCEI carefully evaluates ocean heat content data to account for differences among measurement techniques and data collection programs.
Scientists also test their ocean heat estimates by looking at corresponding changes in other properties of the ocean. For example, they can check to see whether observed changes in sea level match the amount of sea level rise that would be expected based on the estimated change in ocean heat.
Animation of Ocean Heat Content for the upper 2000 m of the ocean (5-year averages or pentads: 1955-59 to 2016-20). Red areas gained heat compared to the long-term average, and blue areas lost heat. (Source: NOAA NCEI)
Until 2021, OHC and temperature anomalies were only available at yearly, seasonal, and 5-year time-scales. Newer short-term monthly anomalies provide researchers with a tool to understand this critical climate variable.
The January–December 2020 average global and ocean surface temperature was the highest since global records began in 1880, according to the annual Global Climate Report produced by NCEI.
Graph of global ocean heat content change over a pentadal (5-year), yearly, seasonal, and monthly average by year. (Source: NOAA NCEI )
This increase in ocean heat content coincides with increases in global average land and sea surface temperatures.
Warming trends in the ocean are likely to continue even if global average surface temperatures stabilize, according to research published in Advances in Atmospheric Sciences . The five highest years on record for annual ocean heat content are 2015–2019. Not coincidentally, the five warmest years for the globe have also occurred since 2015.
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Science in Society: Using the Data
Reducing errors in hurricane forecast models saves lives and property as the number of severe tropical storms increases on our warming planet. This knowledge of OHC is fundamental to accurately forecast intensity changes of tropical cyclones.
Intensifying Tropical Storms
A warmer ocean can contribute to more intense storms and hurricanes increasing cyclone (hurricane, typhoon) intensity and frequency, as energy in the form of heat for these atmospheric disturbances is drawn from the upper ocean.
Infrared satellite image of Hurricane Ida at 3:21 p.m. EDT August 29, 2021, after making landfall near Port Fourchon, Louisiana. (Source: NOAA)
An example of this effect is the response of 1995’s Hurricane Opal when it crossed a warm eddy. When Opal encountered this deeper, warmer region, the storm unexpectedly and rapidly intensified from a Category 1 hurricane to a Category 4 hurricane in just 14 hours.
More recently, in 2021, Hurricane Ida underwent a similar rapid intensification as it crossed a very warm eddy. Hurricane Ida’s wind speeds increased by more than 35 miles per hour within 24 hours and became the strongest storm to hit Louisiana, tied with Hurricane Laura and the Last Island Hurricane of 1856. For intense storms, the effect of OHC is very significant, making it a key variable in the rapid intensification of tropical cyclones.
The NOAA National Hurricane Center (NHC) issues watches, warnings, forecasts, and analyses of hazardous tropical weather in order to save lives, mitigate property loss, and improve economic efficiency.
The NHC generates daily ocean heat content estimates based in part on the NCEI products. This ocean heat content analysis is used to generate the NHC’s intensity forecasts.
While the NHC uses a variety of models to forecast tropical storm activity, both the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model and the Logistic Growth Equation Model (LGEM) utilize OHC estimates to improve their forecasts. These models are run every six hours and predict intensity changes in tropical cyclones five days out.
A study published in the American Meteorological Society Weather and Forecasting Journal found that using OHC in these models improved the average intensity errors of the forecasts by up to 5% for Category 5 storms and up to 20% for individual storms.
Research Highlights
Take Action
Although climate action and ocean conservation efforts historically have been separated, the ocean cannot be saved without acting on climate change, and the climate crisis cannot be confronted without considering the ocean.
The complexity of Earth’s changing climate, affecting various regions of the globe differently, presents a formidable challenge to society and science. Warming events will continue to threaten the balance of ocean life in the coming decades and the ocean’s ability to serve as a buffer, absorbing excess heat and CO2 from the atmosphere, is not limitless.
To protect our communities, ocean, and planet, a few key actions that must be taken:
- Limit greenhouse gas emissions: Substantially reducing greenhouse gas emissions, including specifically targeting C02 reductions to fight ocean warming and acidification, is the single most important action that can be taken for the ocean.
- Protect and restore marine and coastal ecosystems: Well-managed protected areas can help conserve and protect ecologically and biologically significant marine habitats. Ecosystems that have already experienced damage can be restored. Marine protected areas are a proven conservation tool that, when applied effectively, enhance the size, abundance, and biodiversity of ocean animals.
- Improve human adaptation: Appropriate regional planning and infrastructure can help coastal communities adapt to rising sea levels and the greater likelihood of storms, floods, and extreme weather events. Introducing policies that keep fisheries production within sustainable limits can help prevent overfishing in a fragile ecosystem.
- Strengthen scientific research and innovation: Increased investment in scientific research to measure and monitor ocean warming and its effects will provide more precise data on the scale, nature, and impacts of ocean warming. This will make it possible to design and implement mitigation and adaptation strategies.
The ocean is an important and special place to all who rely on it for their livelihoods, food, and recreation. Even though the challenge of addressing climate change seems immense, solutions are possible to secure a living ocean for a healthy global climate.
Learn More
- World Ocean Atlas (NOAA NCEI)
- World Ocean Database (NOAA NCEI)
- World Ocean Database Profiles the Ocean (NOAA NCEI)
- Satellite Ocean Heat Content (NOAA NESDIS)
- Monthly Ocean Heat Content and Temperature Anomalies Released (NOAA NCEI)
- To Study Earth’s Climate, Look to the Ocean (NOAA NCEI)
- Ocean Heat Content Rises (NOAA NCEI)
- Climate.gov: Ocean Heat Content (NOAA)
