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How is Ocean Heat Content Measured?

Emily Heaslip

A new report from the Chinese Academy of Sciences found that 2021 was the hottest year on record for our oceans. This is the sixth year in a row that a new high mark has been set, with 2021 data showing that the upper 2,000 meters of the ocean absorbed 14 more Zettajoules of heat energy than in 2020; for context, the total energy use for all humans globally in a single year is about 0.5 Zettajoules.

Understanding the amount of heat that is stored by the ocean — known as ocean heat content — is critical for our understanding of how the climate crisis is threatening both marine ecosystems and coastal communities. Let’s explore how scientists are approaching the task and what rising ocean heat content means for the future of our planet. 

Background: ocean heat absorption

Our oceans have always absorbed a huge amount of solar heat. Oceans cover more than 70% of the Earth’s surface and can absorb heat without a large increase in water temperature. This ability to store heat is what fuels our climate: waves, tides, and currents constantly move heat from warmer to cooler latitudes, as well as to deeper levels of the ocean.

Surface currents are driven by global wind systems that are fueled by energy from the sun. These currents bring heat from the tropics to the polar regions; the Gulf Stream, for instance, brings warm water along the eastern coast of the U.S. up to Northern Europe. Deep currents are driven by a process called thermohaline circulation, which, at the poles, produces colder, denser water that sinks. Surface water flows to replace this sinking water, causing a conveyor belt-like effect of water circulating around the globe. As currents move heat from the equator and around the planet, they affect climate patterns, local weather, the cycling of gases, and the delivery of key nutrients to marine ecosystems.

[Read more: Understanding Surface Currents vs Deep Ocean Currents]

As the ocean heat content rises, however, currents change. This is because warmer water is less dense and when it circulates to the poles, it doesn’t sink like its colder counterpart. This slows down the entire conveyor belt, which could lead to drastic temperature changes and a disruption to the base of the global food chain, with algae, seaweed, etc. unable to access the cool, nutrient-rich water that they need to flourish. 

How is ocean heat content measured? 

In the past, ships would dangle sensors or take samples of ocean water to try to collect ocean heat content data. This approach provided a limited understanding of ocean heat content, offering a small amount of insights from a tiny portion of our vast oceans.

Today, researchers have more advanced technology at their disposal. Temperature-sensing buoys, such as Sofar’s Spotter buoy, measure ocean temperature around the world in real-time. Spotter, along with Smart Mooring, can be equipped with temperature sensors to record ocean temperatures with +/- 0.1 °C accuracy at depths up to 100 meters. When used in an array, these marine sensing devices can capture a wider data set at regular intervals.

The Argo float is another effective temperature-sensing tool. Currently, scientists and engineers are using a fleet of more than 3,000 Argo floats to capture temperature data at different depths. “Every 10 days or so, according to their programmed instructions, [the floats] 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,” reported NOAA.

In addition to in situ buoys like Spotter and the Argo float, researchers are also using satellite data to create ocean heat content maps. As the ocean warms, it also expands, with thermal expansion being one of the primary causes of sea level rise. Satellite imagery can help us understand the relationship between these two variables: by determining how much sea levels have risen over the past few decades, scientists can extrapolate how much heat content has been absorbed

Why does ocean heat content data matter?

“Ocean heat accumulation is the equivalent of five Hiroshima bombs exploding every second since 1990,” wrote researchers at Yale. The level of ocean heat content is at dangerous levels and, forebodingly, is only projected to get worse.

Argo float data measuring heat down to 2,000 meters shows that the excess energy in the ocean is penetrating deeper and further south, a worrisome finding. Recall that as the ocean warms, polar ice melts at a faster rate, and thermohaline circulation slows, a pattern that could have dramatic impacts on our climate. 

“Rapidly warming Arctic waters could worsen summer heat waves in Europe and North America by lowering the temperature differential that drives mid-latitude circulation. And a recent rash of unusually intense cyclones may be linked to changes in the tropical Pacific,” wrote Yale.

As the ocean continues to warm, the implementation and expansion of accurate and real-time ocean heat content measurement systems will be vital. By utilizing marine sensing technology like Spotter buoys and Argo floats, the scientific community can more effectively track the causes and effects of warming waters, and better prepare marine ecosystems and coastal communities for present and future changes to the oceanic climate.

[Read more: How Does Climate Change Affect the Ocean?

To learn more about ocean heat content, check out our blog

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