The story so far: For the fourth year in a row, the world’s oceans recorded extreme heating in 2022 on account of anthropological activities like greenhouse gas emissions. The worrying trend continues even as the time to reverse climate crisis is running out.
More than 90% of the excess heat accumulated in the earth’s climate is deposited in the oceans. A study conducted by researchers from 16 institutes worldwide and published in the Journal of Atmospheric Sciences analyses factors that drive heating trends in the world’s oceans. The study uses in situ data obtained from the World Ocean Database.
How does rising ocean heat content cause a climate crisis?
Ocean heat content (OHC) is the amount of energy absorbed by and stored in the oceans. It is measured in joules, the unit of energy.
When sunlight reaches the earth, oceans absorb this energy and store it as heat. While the heat is first absorbed at the surface of the water body, some of it is eventually disbursed throughout. Water also has a higher heat capacity than air, which means that it can store much larger amounts of heat. This also means that water heats up slower than air.
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OHC is an important indicator of climate change. An increase in greenhouse gas emissions traps more energy from the sun in the atmosphere, not allowing it to escape. Rising ocean temperatures strengthen the exchange of energy from oceans to the atmosphere by increasing the evaporation of water and thus the quantity of atmospheric moisture. This leads to changes in global precipitation patterns as well as temperatures.
Salinity
Salinity is a particularly important characteristic of sea and ocean water. In 2022, the salinity-contrast index, defined as the difference between the salinity averaged over climatologically high-salinity and low-salinity regions, also reached its highest level on record in 2022. Salinity determines water density, which drives the circulation of water in oceans. When evaporation occurs, liquid water that is high in salinity is left behind. Similarly, during precipitation, freshwater is added to saline water. Over time, changes in salinity of oceans include large areas and act as an indicator of a change in the water cycle. The salinity-contrast index, a measure of “salty gets saltier–fresh gets fresher” pattern, is a powerful tool to observe salinity pattern changes.
Vertical stratification
Together, temperature and salinity changes in oceans change the density of water and lead to vertical stratification. This stratification hinders water mixing and consequently the exchange of heat, carbon, oxygen and so on between layers. As a result, stratification, along with OHC and salinity-contrast index, becomes an important element in quantifying climate change.
Findings of the study
Spatial maps of ocean heating in 2022, relative to the mean 1981-2010 conditions, show significant warming in most ocean areas. Atlantic and southern oceans are heating at a faster rate than other ocean basins. The increasing occurrences of heatwaves and droughts in the northern hemisphere are consistent with the intensive ocean warming in the mid-latitude Pacific and Atlantic oceans.
(a) The annual OHC anomaly in 2022 relative to the 1981–2010 baseline for IAP/CAS data. (b) The difference in annual mean OHC in the upper 2000 m between 2022 and 2021. (Unit: 10^9 J m^−2) | Photo Credit: Advances in Atmospheric Sciences, 2023
The datashows that heat has increased in equatorial regions over the past year— both through advection in the ocean and anomalous surface exchanges (advection refers to the exchange of energy, moisture, microbes or solutes carried by the bulk motion of water.)
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Salinity trends for 2022 show that most of the Pacific and East Indian Oceans are currently undergoing a freshening, while mid-latitude Atlantic, the Mediterranean Sea and West Indian oceans are becoming more saline.
The rise in ocean temperatures is a result of the earth’s energy imbalance, primarily associated with an increase in greenhouse gas emissions. These gases trap heat in a blanket around the earth, not allowing it to escape, thus raising the temperature of the earth’s surface and leading to global warming. The study also noted that the global long-term warming trend is so steady that annual records continue to be set with each new year, as noted in the last four years.
