In 2023, global ocean heat content doubled or tripled since the late 1980s, shows study 

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. 

The 2023 ocean heat content value was 15 ZJ more than the preceding year. For comparison, the entire globe consumes around half a ZJ of energy to fuel economies. And 15 ZJ is enough energy to boil 2.3 billion Olympic-sized swimming pools of 50 m length, 25 m width, and 2 m depth.

The 2023 estimate represents around 4.6 billion Hiroshima nuclear bombs, Eliot Jacobson, retired professor of mathematics and computer science and a climate analyst, wrote on  X, formerly Twitter.

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,” researchers from China, the United States, New Zealand and Europe wrote in the study published in the journal Advances in Atmospheric Sciences.

Researchers from China, the United States, New Zealand and Europe arrived at these results by analysing data from the World Ocean Database, a collection of uniformly formatted, quality-controlled, publicly available ocean profile data. 

Two groups — Institute of Atmospheric Physics (IAP) at the Chinese Academy of Sciences and the National Centers for Environmental Information at the National Oceanic and Atmospheric (NOAA) — looked at ocean heat content, sea surface temperature, salinity, stratification to describe ocean changes in 2023.

Both groups observed a long-term warming in the ocean due to global warming. 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,” researchers from China, the United States, New Zealand, and Europe wrote in the study published in the Advances in Atmospheric Sciences.

However, there were some differences. The IAP group estimated that the world ocean heated by 15 ZJ relative to 2022 while NOAA’s data showed the warming was 9 ZJ.

“Regardless of which estimate is used, there has been a two- to three-fold increase in the rate of increase in ocean heat content since the late 1980s,” the researchers explained.

For example, the IAP analysis showed that the ocean heat content trend was 3.1 ± 0.5 ZJ per year between 1958 and 1985. But from 1986 onwards, the trend is 9.2 ± 0.5 ZJ annually.

Further, from 2007-2023, IAP and NOAA observed a significant warming trend of 10.8 ± 1.2 ZJ per year and 10.3 ± 0.8 ZJ per year, respectively.

These trends tend to peak shortly before an El Nino event and then declines after the event. El Nino, a recurring climate pattern that occurs in the Pacific at intervals of 2–7 years, is associated with the release of ocean heat into the atmosphere, mainly through increased evaporation.

The El Nino conditions developed in 2023 and became “strong” towards the end of the year. The carbon dioxide concentration in the atmosphere in 2023 was more than 50 per cent above the preindustrial level.

The global sea surface temperature also reached its record high in 2023. The annual mean was roughly 0.23 degrees Celsius higher than in 2022.

Also, the annual mean sea surface temperature in 2023 was 0.54°C higher than the 1981–2020 average.

September was observed to have the highest sea surface temperature on record. Normally, the hottest month occurs in March, according to the study.

Very high sea surface temperatures in the extratropics, according to the paper, were partly due to the three-year-long La Nina, which ended in 2023.  

This anomalously high value led to a higher global mean surface temperature. The year 2023 was ranked the warmest on record, with surface temperatures reaching 1.48C above the preindustrial era.

The data also showed a robust increase in salt levels in the last 50 years. The Salinity Contrast index -- the difference between the salinity averaged over high and low-salinity regions — reached 7.2 mg per kg in 2023. This was the fourth-highest value since 1958.

Changes in temperature and salinity have affected the ocean density stratification since the late 1950s.  The ocean stratifies when there is less mixing between the less dense, warm, and fresh waters near the surface and the deeper layers.  This reduces the transfer of heat and carbon dioxide to the layers below.

In 2023, the upper 2,000 metres stratification reached record-high levels mainly due to the development of a strong El Nino.

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