Ocean Heat Content (OHC)

Ocean Heat Content (OHC) refers to the total heat energy stored in the world’s oceans. It is a critical metric for understanding and monitoring the Earth’s energy balance and its implications for climate. Read here to learn more.

According to a new study, the amount of heat stored in the upper 2,000 meters of the global ocean or the ocean heat content reached 286 Zetajoules (ZJ) in 2023 relative to the 1981–2010 average.

The 2023 ocean heat content value was 15 ZJ more than the preceding year. For comparison, the entire globe consumes around half a ZJ of energy to fuel economies. And 15 ZJ is enough energy to boil 2.3 billion Olympic-sized swimming pools of 50 m length, 25 m width, and 2 m depth.

OHC provides insights into the distribution of heat within the ocean, the impact of climate change, and the potential influence on weather patterns.

Ocean heat content (OHC)

The ocean is the largest solar energy collector on Earth. Not only does water cover more than 70 percent of our planet’s surface, it can also absorb large amounts of heat without a large increase in temperature.

This tremendous ability to store and release heat over long periods gives the ocean a central role in stabilizing Earth’s climate system.

• The main source of ocean heat is sunlight. Additionally, clouds, water vapor, and greenhouse gases emit heat that they have absorbed, and some of that heat energy enters the ocean.
• Waves, tides, and currents constantly mix the ocean, moving heat from warmer to cooler latitudes and deeper levels.

Heat absorbed by the ocean is moved from one place to another, but it doesn’t disappear.

• The heat energy eventually re-enters the rest of the Earth’s system by melting ice shelves, evaporating water, or directly reheating the atmosphere.
• Thus, heat energy in the ocean can warm the planet for decades after it is absorbed.
• If the ocean absorbs more heat than it releases over a given time, its heat content increases.
• Knowing how much heat energy the ocean absorbs and releases is essential for understanding and modeling global climate.

How is the heat content measured?

To get global coverage, scientists turned to satellites that measure the height of the ocean’s surface. As water warms, it expands, so estimates for ocean temperature can be deduced from sea surface heights.

To get a more complete picture of ocean heat content at different depths, scientists and engineers also use a range of in situ temperature-sensing instruments.

• Among these are a fleet of more than 3,000 robotic “floats” that measure ocean temperature around the world.
• Known as Argo floats, the sensors drift through the ocean at different depths. Every 10 days or so, according to their programmed instructions, they rise through the water, recording temperature (and salinity) as they ascend.
• When a float reaches the surface, it sends its location and other information to scientists via satellite and then descends again.

Other methods:

• Satellites equipped with infrared and microwave sensors measure sea surface temperatures, providing valuable data for estimating OHC.
• Oceanographic buoys equipped with temperature sensors are strategically deployed to collect data on subsurface temperatures, allowing for a more comprehensive understanding of OHC.
• Research vessels collect ocean temperature data during scientific expeditions, contributing to the understanding of OHC on regional scales.
• Climate models simulate oceanic and atmospheric processes to project future changes in OHC. These models help researchers assess the impact of climate change on the world’s oceans.

Factors Influencing Ocean Heat Content

• Solar Radiation: The sun’s energy is a primary driver of OHC. The ocean absorbs solar radiation, particularly in the upper layers, influencing temperature distributions.
• Atmospheric Heat Exchange: Heat exchange between the ocean and the atmosphere plays a significant role. Factors like air temperature, humidity, and wind patterns influence this exchange.
• Ocean Circulation: Ocean currents redistribute heat globally. Warm water is transported from the equator towards the poles, influencing regional climate patterns.
• Greenhouse Gas Effect: The absorption of infrared radiation by greenhouse gases, such as carbon dioxide, contributes to the warming of the ocean. This process is linked to anthropogenic climate change.
• Ocean Mixing: Vertical and horizontal mixing within the ocean contribute to the distribution of heat. Processes like upwelling and downwelling influence temperature profiles.

Significance of Ocean Heat Content

• Climate Change Indicator: OHC is a key indicator of climate change. Increasing OHC is consistent with the warming of the Earth’s surface due to elevated greenhouse gas concentrations.
• Sea Level Rise: The expansion of seawater as it warms contributes to sea level rise. Monitoring changes in OHC is crucial for understanding and predicting sea level trends.
• Impact on Weather Patterns: Changes in OHC influence weather patterns and can lead to more intense and frequent extreme weather events, including hurricanes and heat waves.
• Ecosystem Impact: The distribution of marine species is influenced by variations in OHC. Temperature changes can impact the breeding, migration, and survival of marine organisms.
• Thermal Inertia: Oceans have a high thermal inertia, meaning they can store and release heat slowly. These characteristics moderate the Earth’s climate by absorbing and redistributing heat over extended periods.

Global Trends and Concerns

More than 90 percent of the warming that has happened on Earth over the past 50 years has occurred in the ocean.

• Recent studies suggest that warming extends to deeper layers of the ocean, highlighting the importance of monitoring surface temperatures and subsurface layers.
• Changes in OHC can have profound effects on marine ecosystems, including coral reefs, fisheries, and biodiversity.
• OHC influences atmospheric circulation patterns and can contribute to shifts in climate systems, such as El Niño and La Niña events.

Understanding OHC is crucial for developing effective mitigation and adaptation strategies to address the impacts of climate change on coastal communities, ecosystems, and economies.

Conclusion

Ocean Heat Content is a key parameter in the study of climate science, providing essential information about the Earth’s energy budget and the dynamics of the global climate system.

Monitoring OHC trends and understanding the mechanisms driving changes in ocean temperatures are critical for making informed decisions about climate mitigation and adaptation efforts.

Warm waters jeopardize the health of corals, and in turn, the communities of marine life that depend upon them for shelter and food.

Ultimately, people who depend upon marine fisheries for food and jobs may face negative impacts from the warming ocean.

As ongoing research improves our understanding of OHC, it will contribute to more accurate climate models and projections, aiding efforts to address the challenges posed by climate change.

Related articles:

• Marine heatwaves
• Atlantic Meridional Overturning Circulation (AMOC)
• Insolation and heat balance of earth

-Article by Swathi Satish