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The Conversation Spain

Humanity is altering the oceans, the main regulators of climate change

Shutterstock / Andrey Yurlov The turn of the decade brings us the gift of the Decade of Ocean Science for Sustainable Development. It is very timely, therefore, to emphasize that the oceans are not only the essential and central element of life on our planet, they are also the great regulators of climate change. One of the keys to planetary climate control lies in the deep global circulation, also known as the global conveyor belt, a great current that reaches the abyssal regions of all the oceans of the planet. This planetary current originates from surface waters at high latitudes in the North Atlantic and around the Antarctic continent. Every winter, these cold, salty waters sink, thus starting the global conveyor belt. In a few weeks, 1,500 trillion cubic meters of water are injected into the depths of the ocean. This represents an annual average of about 48 million cubic meters per second, more than 200 times the average flow of the Amazon River. Scheme of the global conveyor belt, originating and ending in the Atlantic Ocean, reaching the entire planet through the Southern Ocean. John Marshall & Kevin Speer / Nature Geoscience The beginning of this global circulation is accompanied, also in winter, by another sinking of surface waters. This pumping is caused by the wind and occurs in medium and high latitudes. There, the waters submerge to about 1,500 m, causing the temperature and other properties to vary in depth in a similar way to how they do with latitude. These waters make a transoceanic submarine trip, delimiting the great subtropical gyres. They are large systems of ocean currents influenced by the winds and the rotational movement of the Earth. The result is what is known as the thermocline circulation. Subtropical gyres, whose waters move clockwise in the northern hemisphere and counterclockwise in the southern hemisphere, dominate the central regions of the oceans. Arnold Gordon / Britannica Earth’s circulatory system The global conveyor belt and thermocline circulation can be thought of as the Earth’s circulatory system. The thermocline circuit runs through the transoceanic turns, continuously distributing energy and regenerating nutrients in the system. Every several years, the waters return to the surface and gases are exchanged with the atmosphere, as if it were the lung circuit of our living planet. In contrast, the global ribbon takes hundreds and even thousands of years to travel the entire planet, maintaining the memory of past climates. Cold waters sinking to high latitudes in the North Atlantic are eventually replaced by the return branch of the global conveyor belt, warm, nutrient-rich waters from tropical and subtropical regions. The result is a flow of heat and nutrients that is directed towards the high latitudes of the North Atlantic. The heat released there maintains the moderate climate of northern Europe and the supply of inorganic nutrients sustains the spectacular spring blooms of the North Atlantic Ocean. The spring flowering of North Atlantic phytoplankton is one of nature’s most spectacular manifestations. NASA Currents and Climate Earth’s climate is largely conditioned by the local radiative balance, which depends on the reflection of solar radiation (albedo) and the fraction of radiation emitted by the Earth that cannot pass through the atmosphere (effect greenhouse). But equally important is the transfer of heat from the tropics to high latitudes, which occurs thanks to atmospheric winds and ocean currents. In the ocean this is determined by the intensity of the global conveyor belt and the thermocline circuit. The strength and vertical extent of the global Atlantic conveyor belt has not always been the same. Paleoceanographic indicators in sediments indicate that about 20,000 years ago the North Atlantic conveyor belt was much weaker and shallower. As a consequence, heat transport to high latitudes was less and the Earth experienced a glacial maximum. Predictions suggest that throughout this century the subpolar region will warm and salinize, the latter due to the intrusion of subtropical salty waters. The forecast is that the global conveyor belt will slow down, although competition from heat and freshwater flows creates great uncertainties. Ocean circulation patterns today (top) and about 20,000 years ago (bottom). In the past, the waters of the North Atlantic sank only to intermediate depths, in a weaker way. Woods Hole Oceanographic Institution Waters with less oxygen and more acid The physical factors that control the climate are joined by the self-regulation of the living planet, in continuous evolution towards an optimized state. Two examples of the interaction between climate and life are the control of carbon dioxide through changes in primary production and the influence of marine plankton on cloud formation. Another example is the expansion of hypoxic ocean regions, or low oxygen zones. These occur on the eastern fringes of all the great oceans, in relatively isolated regions between the subtropical and tropical gyres. Its expansion may be due to changes in circulation patterns, warming of the waters and increased primary production. The oceans have also incorporated around 40% of the anthropogenic carbon dioxide emitted into the atmosphere, causing significant acidification. As a consequence, the saturation depths of calcite and aragonite have decreased, reducing the regions where calcareous organisms can grow. The combination of these stressors (warming, salinization, deoxygenation, acidification, pollution, overfishing) represents a significant threat to many marine species, with a high impact on marine biodiversity and on the evolution of the planet itself. Consequences of the Anthropocene The Holocene, the warm interglacial period that has characterized our planet for the last 12 thousand years, is being altered by humanity. The emission of large amounts of carbon dioxide has changed the radiative balance and has led the Earth to a new metabolic state, the Anthropocene. An important effect has been that the average temperature of the earth’s surface has increased by 1.2 ° C from the pre-industrial period, in the middle of the 19th century, to the present day. This has happened despite the high regulatory capacity of the oceans, which have absorbed around 90% of excess anthropogenic heat in the Earth system with an increase in their average temperature of just 0.15 ° C. If it were not for the anthropogenic effect, the Earth would have already slowly entered a glacial period, which would intensify until it found its maximum cooling in about 60 thousand years. However, the models tell us that the current interglacial climate will strengthen for another 20 to 25 thousand years. The next glacial maximum won’t take place for about 110,000 years. An unpredictable result? Observations and models tell us that Earth’s climate has changed and will continue to change. Large areas of our planet experience rising sea levels, severe droughts or torrential rains, more frequent intense hurricanes, severe heat waves, loss of biodiversity and an increase in infectious diseases. We cannot predict the future with certainty. The extreme complexity of the living ocean, the interplay of physical and biogeochemical processes at all spatial and temporal scales means that the system can take unexpected paths. Yet science almost unanimously warns us that imminent action is required if we are to maintain a favorable climate for humanity. It is imperative to define planetary limits and it is everyone’s job to respect them. Our daily actions will determine the evolution of our planet: health and planetary consciousness. Jimena Uribe Cortés, Author provided Our living planet, with the ocean as its central and essential component, is robust and has a high regulatory capacity. Its resilience has been tested throughout the history of our planet. The conjunction of living and non-living mechanisms has caused its low entropy and high complexity. This high complexity, the result of the infinity of processes that sustain and shape life itself, turns a potentially fragile terrestrial system into a living, dynamic and robust being. Gaia will evolve and unfold for the benefit of the entire system and not one of its parts. It is in our power to keep humanity in harmony with this evolution.This article was originally published in The Conversation. Read the original. Josep Lluís Pelegrí Llopart receives research funds from the European Union, the National Research and Development Plan of the Government of Spain, the Higher Council for Scientific Research and the Barcelona City Council.

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