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Context

• 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, according to a new study published in Advances in Atmospheric Sciences.

Findings

• The 2023 ocean heat content value was 15 ZJ more than the preceding year.
• 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.
• The ocean covers 70 per cent of the planet and absorbs about 90 per cent of the heat from global warming. When the ocean warms, it releases extra heat and moisture into the atmosphere, making storms more severe with heavier rain, with stronger winds and more significant flooding.
• Ocean heat content also plays an essential role in Earth’s energy, water, and carbon cycles, and significantly affects human society.
• The 2023 ocean heat content is ranked as one of the five hottest years of the world’s ocean since 1955. The upper 2,000 m of the world’s ocean has warmed on average by 6.6 ± 0.3 ZJ per year from 1958-2023.
• Both ocean heat content and sea level rise are robust indicators of climate change.
• Ocean heat content also plays an essential role in Earth’s energy, water, and carbon cycles, and significantly affects human society.

Global Ocean Heat Content (OHC):

• Global Ocean Heat Content (OHC) refers to the total amount of heat stored in the world's oceans.
• It is a crucial indicator of climate change and plays a significant role in understanding the Earth's energy balance.
• The OHC is measured in terms of the amount of heat energy (in joules) absorbed by a unit mass of seawater.

Key Points:

Climate Indicator:

• OHC is a key metric for assessing climate change. Monitoring changes in ocean heat content helps scientists understand how the Earth's climate system is responding to increasing greenhouse gas concentrations.

Warming Trend:

• Over the past few decades, there has been a notable increase in global OHC, indicating a warming trend. The oceans absorb and store much of the excess heat trapped in the atmosphere due to human activities such as the burning of fossil fuels.

Impact on Sea Levels:

• The thermal expansion of seawater due to increased ocean heat content contributes significantly to rising sea levels. This poses a threat to coastal areas and low-lying regions.

Ocean Circulation and Climate Patterns:

• Changes in OHC influence ocean circulation patterns and, consequently, climate variability. Understanding these changes is essential for predicting and adapting to shifts in weather patterns.

Measurement Techniques:

• OHC is measured using a combination of satellite observations, autonomous floats, and ship-based measurements. These technologies provide comprehensive data on temperature variations at different depths and locations.

Deep Ocean Warming:

• Studies indicate that not only the surface waters but also deeper layers of the ocean are experiencing warming. This underscores the significance of OHC as a comprehensive metric for assessing overall oceanic warming.

Thermal Inertia:

• Oceans have a high thermal inertia, meaning they can store and release heat over long periods. This delayed response has implications for long-term climate projections.

Ecosystem Impact:

• Changes in OHC can impact marine ecosystems, affecting the distribution of marine species, coral reefs, and fisheries. Warmer oceans can lead to coral bleaching events and disrupt marine biodiversity.

Global Climate Models:

• OHC data are crucial for validating and improving global climate models. Incorporating accurate information about ocean heat content helps enhance the reliability of climate projections.

International Collaboration:

• Monitoring global OHC requires international collaboration. Organizations such as the Intergovernmental Panel on Climate Change (IPCC) and international research initiatives contribute to the collection and analysis of OHC data.

Reasons behind rising Global Ocean Heat Content (OHC)

The rising Global Ocean Heat Content (OHC) is primarily attributed to human-induced climate change and various factors related to the Earth's energy balance. Here are key reasons behind the increasing OHC:

Greenhouse Gas Emissions:

• The predominant factor driving the rise in OHC is the increased concentration of greenhouse gases in the atmosphere, particularly carbon dioxide (CO2). These gases trap heat, leading to a warming of the Earth's surface, and much of this heat is absorbed by the oceans.

Increased Radiative Forcing:

• The enhanced greenhouse effect results in increased radiative forcing, which is the imbalance between incoming solar radiation and outgoing infrared radiation.
• This imbalance leads to a net gain of heat in the Earth's system, with the oceans absorbing a significant portion.

Ocean Thermal Inertia:

• Oceans have high thermal inertia, meaning they can store and release heat over extended periods. Even if greenhouse gas emissions were to stabilize, the oceans would continue to absorb heat for some time, contributing to the ongoing rise in OHC.

Surface Warming and Deep Ocean Heating:

• While surface warming is evident, studies indicate that deep layers of the ocean are also warming. This vertical mixing of heat throughout the ocean contributes to the overall increase in OHC.

Natural Climate Variability:

• Natural climate variability, such as El Niño and La Niña events, can influence ocean heat content. El Niño tends to increase ocean heat uptake, while La Niña events can contribute to heat release.

Decreased Heat Loss to the Atmosphere:

• Changes in atmospheric circulation patterns and cloud cover can impact the exchange of heat between the ocean and the atmosphere. If there is a reduction in heat loss to the atmosphere, more heat remains stored in the ocean.

Changes in Ocean Circulation:

• Alterations in ocean circulation patterns, which can be influenced by climate change, can impact the distribution of heat within the ocean. Changes in circulation can lead to regional variations in OHC.

Melting Ice and Glaciers:

• The melting of polar ice caps, glaciers, and sea ice contributes freshwater to the oceans. The addition of this freshwater alters ocean salinity and density, affecting circulation patterns and heat distribution.

Long-Term Warming Trends:

• Observations reveal a long-term warming trend in ocean temperatures. This trend is indicative of the ongoing accumulation of heat in the oceans, driven by the factors mentioned above.

Human-Induced Climate Change:

• Ultimately, the overarching reason behind the rising OHC is human-induced climate change. Activities such as burning fossil fuels, deforestation, and industrial processes release greenhouse gases, leading to a warmer Earth and oceans.

Way Ahead

• Decarbonize Energy: Rapidly transition to renewable energy sources to reduce carbon emissions, a primary driver of ocean warming.
• Sustainable Fisheries: Implement and enforce sustainable fisheries management to protect marine ecosystems and mitigate stress on ocean biodiversity.
• International Cooperation: Strengthen global collaboration to address climate change, emphasizing shared responsibilities and coordinated efforts.
• Afforestation: Increase afforestation and conservation of coastal ecosystems to enhance carbon sequestration and protect against rising sea levels.
• Plastic Reduction: Implement measures to reduce plastic pollution in oceans, preventing harm to marine life and ecosystems.
• Renewable Shipping: Promote the use of sustainable fuels and technologies in the maritime industry to minimize its impact on ocean health.
• Climate-Resilient Infrastructure: Develop and implement adaptive strategies, including climate-resilient coastal infrastructure, to protect vulnerable communities.
• Marine Protected Areas: Establish and expand marine protected areas to preserve critical habitats and promote ecosystem recovery.
• Innovation for Carbon Capture: Invest in innovative technologies for ocean-based carbon capture and storage to mitigate ocean acidification.
• Public Awareness: Increase public awareness and education on the importance of oceans, fostering support for sustainable practices and policies.

PRACTICE QUESTION Q. In the context of climate change and its impact on marine ecosystems, consider the following statements regarding the rising Global Ocean Heat Content (OHC):  Statement 1: The primary driver of the increasing OHC is the cooling effect of ocean currents redistributing heat globally. Statement 2: The thermal inertia of the oceans allows them to rapidly release absorbed heat, contributing to the persistent rise in OHC. Choose the correct answer using the codes below: A. Both statements are true. B. Statement 1 is true, but Statement 2 is false. C. Statement 1 is false, but Statement 2 is true. D. Both statements are false. The correct answer is: C. Statement 1 is false, but Statement 2 is true. Explanation: Statement 1 (The primary driver of the increasing OHC is the cooling effect of ocean currents redistributing heat globally): This statement is false. The primary driver of the rising Global Ocean Heat Content (OHC) is not the cooling effect of ocean currents but rather the increased absorption of heat due to human-induced climate change. Greenhouse gas emissions, particularly carbon dioxide, trap heat in the atmosphere, leading to a warming planet. This excess heat is absorbed by the oceans, contributing to the rise in OHC. Statement 2 (The thermal inertia of the oceans allows them to rapidly release absorbed heat, contributing to the persistent rise in OHC): This statement is true. The oceans have high thermal inertia, meaning they can store and release heat over extended periods. Even if greenhouse gas emissions were to stabilize, the oceans would continue to release absorbed heat, contributing to the persistent rise in Global Ocean Heat Content. Therefore, the correct answer is C. Statement 1 is false, but Statement 2 is true.