![[Translate to English:] Die Sonne schaut unter den Wolken über dem Atlantik hervor.](/fileadmin/_processed_/9/f/csm_Sonne_Atlantik_Bild_DAM_25ad943a74.jpg "Sonnenuntergang")
The ocean functions like a gigantic heat battery. It absorbs heat on its surface and stores it at increasing water depths. The oceans have absorbed more than 90 per cent of the heat that has been trapped in the atmosphere due to man-made greenhouse gas emissions in the last decades.
In 2022, the ocean swallowed around 100 times more energy than humans used to produce electricity in the same period. But the energy in the ocean is not disappearing - it is changing its environment, resulting in several consequences for the climate and for the environment.
Heat waves in the sea
The constant supply of heat leads to a continuous increase in water temperature. This effect is not uniformly found everywhere. Researchers are observing that the water masses in a particular sea area are warming comparatively quickly to a level that is significantly above the normal average values. If this warming continues for at least five consecutive days, experts refer to it as a marine heatwave.
Both, the continuous increase in water temperature and heat waves in the oceans result in marine organisms being out of their usual temperature comfort zone. This refers to a temperature range, in which organisms metabolise most effectively. For most organisms applies, the warmer the water the more energy they have to use to supply their bodies with sufficient oxygen. If the oxygen supply can no longer be guaranteed, it may be dangerous for their survival and the organisms must migrate to cooler waters, if these areas still exist. However, only mobile species are able to do this.
Additionally, the temperature comfort zone can change over the course of a fish or mussel's life. In early life stages, i.e. as an embryo in the egg or as a larva, cold-blooded animals are usually more sensitive to heat than in later stages of development. Their sensitivity also increases further if the organisms are simultaneously exposed to increasing ocean acidification or oxygen depletion. In this case, the stress factors reinforce each other's effects.
Lack of oxygen in the water
A lack of oxygen is not only due to a poorly functioning metabolism of living organisms. Warmer seawater loses oxygen, and thus one of the most important component of life. In the period from 1960 to 2010, the world's oceans lost more than two per cent (around 77 billion tonnes of O2) of their oxygen content (*).
Researchers suggest that this loss has two main reasons: over-fertilisation of the oceans and increasing water temperatures. This process is enhanced by the physical properties of the water. The colder and saltier the water, the greater its density. This means that it is also found in deeper areas of the ocean. Reversely, there is warmer water at the surface areas containing a lot of oxygen due to its proximity to the air - i.e. the atmosphere.
How deep the surface water reaches and how much oxygen reaches the deeper regions of the oceans depends on the wind, the air and the water temperature. If the water on the surface is warm in the summer, it covers the cooler water layers like a blanket and there is hardly any exchange. Even strong winds do not change this. The oxygen-rich water remains on the surface and no longer reaches the deeper layers. In the lower levels of the oceans, more oxygen is then consumed than is replenished - creating a shortage.
After all, warmer oceans cause stronger storms and more frequent heavy rainfall. The reason for this effect is: how much water evaporates over the oceans is closely linked to the water and air temperature because warm air can store more water vapour. This means the warmer the seawater is, the easier it evaporates into the atmosphere that can absorb and transport more water. As a result, extreme weather conditions are increasingly brewing over the oceans. This is the reason why the oceans are also known as our "weather kitchen".
This means a double threat to coastal areas: on the one hand, there will be increasing weather extremes and on the other hand, there will be a rise in sea levels. This is also a consequence of warmer oceans because warm water expands.
This physical law of nature has contributed to the fact that global sea levels in 2018 were around 20 centimetres higher than in 1900. Until the beginning of the 21st century, the expansion of water due to increasing sea temperatures was the main reason for the rise in sea levels (*). Since then, the melting of ice sheets and glaciers in the Arctic and Antarctic have also been a major contributor - and at an increasing speed rate.
The warming of the seas and oceans therefore has dramatic consequences. The fatal thing is that the physical processes can not only intensify themselves but that they also affect marine ecosystems that are already damaged by heavy utilisation such as fishing, the removal of resources or pollution. The oceans climate service of absorbing a lot of heat has a high price. It is therefore important to take ocean warming into account if making decisions on the use of the oceans.
