The global average temperature for the world's ocean surfaces has been between 19.7° and 21° Celsius (67.5° and 69.8° Fahrenheit) since record-keeping began in the early 1980s. However, in late March 2023, the temperature rose above 21° and stayed there for a month, which is an extreme and unprecedented spike. The most recent reading, from April 26, was just below 21°.This is abnormal, as in March, the average sea surface temperatures usually start to decrease as the Southern Hemisphere transitions from summer to autumn. The Southern Hemisphere has more ocean coverage than the Northern Hemisphere, which has more land masses. As the southern oceans cool, they lower the average global sea surface temperature.
The warming of the Atlantic Ocean may be a contributing factor to the extreme heat that has recently been experienced in Spain. This highlights the larger problem caused by high ocean temperatures, which is that what happens in the sea doesn't stay in the sea. The oceans have absorbed about 90% of the excess heat that humans have put into the atmosphere. However, the oceans are capable of transferring that heat back to the atmosphere, which in turn heats up the land.
Warmer ocean water is causing other problems, too. Higher temperatures are rapidly eroding Antarctica's massive ice shelves, and warmer water expands and takes up more space, leading to rising sea levels. Also, warmer ocean water can lead to reduced biological productivity as some organisms reach their thermal limits and are forced to migrate to colder waters, altering the ecosystems they leave behind and the ones they move to. Corals, which are particularly sensitive to heat, are unable to migrate and may become extinct.
It is important to point out that the ocean food chain relies on the natural circulation of water, which is partly influenced by temperature. When cold water in the depths rises to the surface, it brings up nutrients that fertilize phytoplankton. These microscopic plants become a critical food source for tiny animals called zooplankton. However, when surface water heats up, it stratifies and becomes a cap on top of colder waters below, reducing the amount of nutrients that come up.
Some scientists have started asking themselves if the ocean will become more of a desert over time.
Source: Ars Technica