Why does the sea temperature change

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Prof. Dr. Ulrich Bathmann

Prof. Dr. Ulrich Bathmann

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Born in 1954, Professor of Biological Oceanography at the Alfred Wegener Institute for Polar and Marine Research (AWI) in Bremerhaven. He teaches at the University of Bremen. Main areas of work: The interactions of biological oceanography in the polar regions with the global climate; the contribution of special plankton groups to the carbon flux in polar seas; the changes in the physico-chemical environment of the polar oceans and their effects on biology.

The oceans of our planet not only react to changes in the climate, they also actively influence it. Physical factors such as the salinity of the water, but also biological factors such as the occurrence and behavior of krill or fish species, are equally involved in this process.

The glaciers on Greenland are currently decreasing sharply. (& copy AP)

The world's oceans take on important functions, one of which is the regulation of the global climate. In the past 200 years, the oceans have mainly emitted greenhouse gases (such as carbon dioxide, CO2) and heat are absorbed and in this way the man-made greenhouse effect is mitigated. Many of these processes are currently changing rapidly; some counteract the greenhouse effect, others intensify it. The following text is based on scientific findings that are proven in the bibliography.

Water temperature rises

Fig. 1: Heat content of the upper 700 meters of the world's oceans relative to 1961. The dashed line shows the mean increase between 1961 and 2003. Modified according to WWF Report (see literature).
71 percent of the earth's surface is covered with water, 97 percent of which collects in the world's oceans. The oceans are the most important heat stores on earth. The temperature of the water layer in the upper 700 meters is currently rising rapidly with an increasing tendency. The consequences of this can be seen in annual and decadal climate fluctuations, in heat transport in the ocean circulation, the stratification of water masses as well as in biology and biogeochemistry.

The global mean of the surface temperatures of the oceans has been increasing at around 0.64 degrees Celsius per year for 50 years (Fig. 1); this range accounts for around 90 percent of global warming between 1961 and 2003. According to various models, this trend will continue into the 21st century.


Effects of salinity and currents

Salinity, the salinity of the oceans, does not currently remain constant. In the Arctic, salinity is decreasing, mainly due to increasing ice melt. In the North Atlantic, salinity is increasing due to the increased influx of salty water from the Mediterranean. In the subtropics of the Atlantic, more water evaporates on the surface - the salinity rises -, over the subtropics of the Indian Ocean it rains more intensely - the water becomes sweeter.

The density (gravity) of the water is determined by its temperature and salinity. Lower temperatures and higher salinity make the water heavier, it becomes denser. Heavy water sinks (thermo-haline circulation) and thus drives the ocean circulation: surface water is drawn into the depths, on the surface water then has to flow into such areas (meridional circulation). These circulations ensure, for example, that water and thus heat from the subtropics (Caribbean) flows far into Northern Europe (Gulf Stream). In Iceland and Greenland, the salty water has cooled down so much that it sinks deep into the ocean. This heat pump for Northern Europe has decreased by 30 percent over the past 60 years.

Sea levels and tropical storms as warning signs

Tropical storms (hurricanes, cyclones, typhoons) occur when the upper 50 to 100 meters of the ocean reaches water temperatures above 26 degrees Celsius. The storms stir up the ocean, and accumulated thermal energy is released into the atmosphere. Between 1970 and 2004, the intensity of such storms increased by 75 percent. The frequency of Atlantic hurricanes increases by up to 40 percent with water temperatures only 0.5 degrees Celsius higher in the tropics.

The height of the sea level is influenced by two processes, the water temperature and the dynamics of the glaciers. The effect of the increase due to the expansion of the water through heat is only a few centimeters. On the other hand, the contribution of melting ice sheets is much greater on land. During the last ice age, the sea level was a good 120 meters below today's level. When the ice caps melted, the sea level rose by 1 meter every 100 years (at times up to 4 meters). 125,000 years ago, when today's temperatures were exceeded by 3 to 5 degrees Celsius during the last warm period, the sea level was a good 6 meters higher than today.

The glaciers on Greenland are currently decreasing sharply. If the temperatures there rise by more than 3 degrees Celsius, the melting process is irreversible. This is likely to be achieved in the 21st century if effective precautionary measures are not taken. A complete melting of the Greenland ice would raise the sea level by around 7 meters, probably in a period of 100 to 1000 years. The ice sheet over West Antarctica is also warming up, but whether in a similarly threatening way is still being investigated.

Plankton as a carbon dioxide store

Microscopic, unicellular algae in the ocean, the phytoplankton, form the basis of almost all life in the sea. With the help of solar energy, they bind carbon dioxide and use it to build up organic matter (carbohydrates, proteins, fats). The share of phytoplankton in global primary production is 45 percent. This depends on light and nutrient salts (including phosphate, nitrate, silicate, iron) and thus on ocean currents, the mixing of the water and the geographical latitude.

More than 60 percent of the algae biomass is consumed again where it was formed, namely in the upper 100 meters of the oceans, which are flooded with light. This means that part of the bound carbon dioxide is inhaled again, the rest sinks into the depths with dead algae, as excrement products or in the form of migrating animals. Only what is not recycled there ends up on the sea floor in a few hundreds or thousands of meters, where food specialists again wait for the rain of feed from above. At the end of this food cascade, less than 1 percent of the once fixed carbon ends up at such great depths on the sea floor that the bound carbon dioxide is withdrawn from the cycle of ocean water and the atmosphere.