Why Does the Ocean Store So Much Heat?
To mark World Ocean Day and in connection with the United Nations Ocean Conference (UNOC), the Office for Climate Education (OCE) presents a series of four articles to explore the theme “Ocean and Climate” in the classroom.
After discovering in the first article the regulatory role of the ocean through ocean circulation, stratification, and the El Niño phenomenon, this second article focuses on the ocean as a heat sink—a fundamental mechanism to understand the planet’s thermal balance and anticipate the effects of climate change.
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Thermal Inertia
The ocean is a major heat sink. It is estimated that it absorbs over 90% of the excess heat generated by global warming. Without this ability—made possible by the thermal inertia of water—we would have already exceeded the 1.5°C warming target set by the Paris Agreement.
Seawater has a high resistance to temperature variations: it can absorb a large amount of energy before warming up, and conversely, release a large amount of energy before cooling down. This property helps mitigate local climate variations and explains the significant difference between oceanic and continental climates.
However, this does not prevent the oceans from gradually warming. Since 1993, measurements show that the amount of heat stored by the ocean has more than doubled, affecting both surface and deep waters.
Classroom Activity Ideas:
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Ocean Warming
Episodes of extreme heat affecting the ocean surface, known as marine heatwaves, have doubled in frequency and become longer, more intense, and more widespread over the past 40 years.
By 2100, depending on emission scenarios, marine heatwaves could become 20 to 50 times more frequent.
Moreover, due to the ocean’s strong thermal inertia and massive volume, even if humanity stopped emitting greenhouse gases tomorrow and the ocean absorbed less heat, it would take thousands of years for the ocean to cool down and return to pre-industrial temperatures.
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Sea Level Rise
Like all liquids, water expands when heated. Thus, ocean warming leads to the expansion of seawater, causing it to occupy more volume. Combined with the melting of ice sheets and glaciers, this thermal expansion results in global mean sea level rise. It is estimated that sea level has already risen by 16 cm over the last century, and the rate of rise has been accelerating. The rise from 2006–2015, at 3.6 mm per year, is 2.5 times faster than the rate from 1901–1990.
While a few millimeters per year may seem small, they accumulate over time, leading to more frequent extreme flooding in coastal regions. They also worsen the impacts of storms and coastal erosion.
Due to the thermal inertia of the ocean, sea levels will continue to rise even after global temperatures have stabilized. This trend could be further amplified by the melting of the Antarctic ice sheet.
Educational Resources: