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In 2022, 58 per cent of the ocean surface suffered at least one marine heatwave event and 25 per cent of the surface experienced at least one marine cold spell, stated the State of the Global Climate 2022 report released April 21, 2023.
Marine heatwaves and cold spells are prolonged periods of extreme heat or cold conditions arising in the seas and oceans.
The area affected by marine heatwaves in the previous year was similar to the 2021 record of 57 per cent. But it was less than the 2016 record of 65 per cent, according to the report released by World Meteorological Organization.
In 2022, 25 per cent of the ocean surface was affected by marine cold spells, similar to the 2021 record; but much less than the 1985 record of 65 per cent. This is concerning as marine heatwaves are appearing more often, while cold spells have become less frequent in the late 20th and early 21st centuries, the report highlighted.
In the Arctic, severe and extreme marine heatwaves gripped the Laptev and Beaufort seas from spring to autumn of 2022.
The ice edges to the north of Svalbard at the north pole and east of the Ross Sea at the south pole faced notable extreme marine heatwaves for the second year in a row, according to the report.
The equatorial Pacific waters were one of the few ocean places to experience widespread strong cold spells in 2022. This coincides with La Niña, which is associated with cooler temperatures.
Further, limited areas in the Southern Ocean are the only regions in the world to have seen long-term increases in the length of marine cold spells, the report highlighted.
Oceans, which capture 90 per cent of the heat from greenhouse gases, have been bearing the brunt of climate change. Ocean heat content — the total amount of heat stored by the oceans — reached a record high in 2022. The 2022 record exceeded the 2021 value by nearly 17 zettajoules (ZJ).
The rate of warming in the top 2,000 metres was around 0.7 watts per square metre from 1971-2022. But, in more recent periods, during 2006-2022, the rate was nearly 1.2 watts per square metre.
Warming below a depth of 2,000 m was about 0.0725 watts per square metre from 1992-2022, according to estimates.
Not all oceans are warming at the same rate. The warming in the top 2,000 m of the Southern Ocean, North Atlantic and South Atlantic was 2 watts per square metre. The upper 2,000 m of the Southern Ocean accounts for around 36 per cent of the global increase in ocean heat content since 1958.
“The upper 2,000 m of the ocean continued to warm in 2022, and it is expected that it will continue to warm in the future, causing changes that are irreversible on centennial to millennial time scales,” the WMO report read.
The ocean heat content is also linked with rising sea levels. The global mean sea-level rise was estimated to be around 3.4 millimetres per year over the 30 years of the satellite records (1993–2022). The rate during 2013-2022 was 4.62 millimetres per year, which has doubled from 2.27 mm per year from 1993-2002.
The sea-level rise is not the same everywhere. The differences are partly due to local changes in ocean heat content, the report noted.
Some 36 per cent of this rise in sea level is due to ice loss from glaciers in Greenland and Antarctica.
Ocean warming (expansion of warm waters) contributed 55 per cent, while variations in land water storage contributed less than 10 per cent to sea-level rise.
Oceans have been absorbing around 25 per cent of yearly carbon dioxide (CO2) emissions injected into the atmosphere from human activities.
The absorption of CO2 alters the chemical composition of oceans. This greenhouse gas reacts with seawater and acidifies the water (ocean acidification). This impacts marine lives and their ecosystem services.
Moreover, the ocean’s capacity to absorb CO2 from the atmosphere decreases with increasing acidity and temperature. Therefore, the sea loses its capacity to moderate warming.