Global Ocean Heat Content

• Context (DTE): In 2023, global ocean heat content doubled or tripled since the late 1980s.

Key Findings

• The amount of heat stored in the upper 2,000 metres of the global ocean or the ocean heat content reached 286 Zetajoules (ZJ) in 2023 relative to the 1981–2010 average.
• The 2023 estimate represents around 4.6 billion Hiroshima nuclear bombs.
• Warming in much of the Atlantic, North Pacific, Western Pacific and Southern oceans is occurring at a faster rate than the global average.
• CO₂ concentration in the atmosphere in 2023 was more than 50% above the preindustrial level.
• The Salinity Contrast index (difference between the salinity averaged over high and low-salinity regions) reached 7.2 mg per kg in 2023, the fourth-highest value since 1958.

What is Ocean Heat Content?

• Ocean heat content (OHC) is the amount of heat energy stored by the oceans.
• It is calculated by measuring the ocean’s temperature at different locations and depths, and then integrating the areal density of ocean heat over the entire ocean.
• OHC is measured in joules, a unit of energy, and compared against the 1971–2000 average, which is set at zero for reference.

Significance of Ocean Heat Content

• Changes in OHC influence ocean currents and weather patterns. E.g, the heat stored deep in the ocean can influence the strength, duration, and frequency of tropical cyclones.
• Indicator of climate change and is used to quantify the rate of global warming as over 90% of the extra heat trapped by humanity’s GHG emissions is stored in the oceans
• Sea level rise: The expansion of seawater as it warms (thermal expansion) contributes to sea level rise. The increase in OHC accounts for 30–40% of global sea-level rise from 1900 to 2020.
• Climate stabilization: The ocean’s ability to store and release heat over long periods gives it a central role in stabilizing Earth’s climate system.
• Marine ecosystems: High OHC can disrupt marine ecosystems, bleach coral, and contribute to extreme weather events such as hurricanes.