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Global warming caused by the burning of fossil fuels, deforestation, and other human activities has a significant impact on ocean circulation. As the planet warms, the ocean absorbs much of the excess heat, causing the surface water to expand and become less dense. This can alter the balance of ocean currents, which play a crucial role in regulating the Earth’s climate. Through this article, we will understand the effect of global warming on ocean circulation.
Ocean currents play a vital role in stabilizing temperatures and distributing heat across the planet. It regulates and maintains climate patterns. But warmer ocean waters fuelled by accelerated climate change and global warming threaten the ocean’s ability to keep the planet and its creatures safe.
One of the most significant impacts of global warming on ocean circulation is the slowing of the Atlantic Meridional Overturning Circulation (AMOC), which is a major ocean current system that plays a critical role in distributing heat around the planet. The AMOC carries warm water from the equator to the poles and cold water back to the equator, helping to regulate the Earth’s climate. However, as the ocean warms and fresh water from melting ice enter the North Atlantic, the density of the water there decreases, slowing the AMOC. This can have a number of cascading effects on the global climate, including changes in weather patterns and sea level rise.
Another impact of global warming on ocean circulation is the weakening of the thermohaline circulation, a global ocean circulation system driven by differences in temperature and salinity. As the ocean warms, the thermohaline circulation weakens, which can alter the distribution of heat and nutrients around the globe. This can lead to changes in ocean ecosystems, such as the displacement of certain fish species and the decline of phytoplankton populations.
Oceanographers made quite a surprising discovery two years ago. Not only has human-induced climate change warmed our oceans, but it has also caused ocean currents to accelerate. From 1990 to 2013, ocean currents have accelerated by some 15%. At that time, scientists suspected that intense ocean winds were causing the currents to speed up. But a new modelling study has indicated another culprit: the ocean itself. The sea warms from top to bottom. The result is the development of constricted, warm surface waters where currents move faster.
The study indicates that climate change will continue to speed up ocean currents. The oceans absorb 90% of the heat caused by global warming. A faster ocean current limits the amount of heat the sea can capture. It will result in complications for marine life already living under stressed conditions due to ocean acidification.
Ocean currents form due to differences in salinity and temperatures. We called this the thermohaline circulation (Thermo = temperature, haline = salt). Cold, salty, dense waters sink to the bottom, and warm, faster-moving waters rise to the top. The greater the density difference between different layers of the water column, the greater the circulation and mixing of those layers. This forms a global ocean circulation system, the ocean conveyor belt. The ocean conveyor belt keeps the regions near the equator from scorching temperatures by bringing cold water from the poles to its coasts. Conversely, the conveyor belt keeps the Polar Regions from extremely low temperatures by bringing warm waters from the equator.
The ocean conveyor belt circulates the entire globe in a span of 1,000 years. That means that a single water molecule would take 1,000 years to complete one full cycle of the conveyor belt. The belt transports a volume of water equivalent to a hundred Amazon Rivers or 16 times the flow of every river in the world combined. The conveyor belt is the result of two crucial and simultaneous processes. Surface currents carry warm, less dense water away from the equator and towards the poles. Deep ocean currents carry dense, cold water away from the poles and towards the equator. It is important to note that cold water is denser than warm water, and salty water is denser than fresh water.
The conveyor belt is vital for the distribution of heat energy across the planet’s surface. It helps regulate the climate and weather and cycles gases and nutrients.
The conveyor belt starts near the north pole in the North Atlantic Ocean. Here, sea ice formation makes the surrounding water saltier and denser. Because of its increased density, this water sinks to the bottom. Surface water moves in to replace the dense, sinking water and thus starts a current.
This dense, sunken water mass starts moving south. It travels between continents, passes the equator, and down the coasts of Africa and South America. As the current travels around the edge of Antarctica, more cold, dense water gets added to it. Thus, it gets ‘recharged’. This recharged mass then splits into two and turns northward—one segment of the current travels into the Indian Ocean and the other into the Pacific Ocean.
As these two segments of the conveyor belt reach the equator, they warm up. These warm waters rise to the surface, a process known as ‘upwelling’. They again turn toward the south and then west toward the South Atlantic. They eventually reach the North Atlantic, completing the loop and beginning the global ocean cycle again.
But climate change is threatening this circulation. We’re already seeing the impacts of climate change, such as increased rainfall and melting glaciers. These are adding more warm freshwater into the sea. A massive influx of warmer freshwater disrupts the formation of sea ice. If sea ice doesn’t form, cold, salty water from the poles can’t sink and travel to the tropics and equator. This chain of events could result in drastic temperature changes across the whole world.
A warming planet is weakening ocean circulation. With water at the poles not as cold and dense as before, the currents simply cannot circulate water well enough. Additionally, the melting Greenland ice sheet is pouring more freshwater into the salty ocean. This will alter the density of the water layers to a great extent.
An increased difference in density among water masses leads to the development of defined layers in the ocean. This creates boundaries between the water masses, which reduces the transfer of dissolved compounds like oxygen. If oxygen cannot reach the deep sea, life down there cannot survive. It can lead to the formation of ‘depleted zones.’
In addition to oxygen, the thermohaline circulation also helps mix dissolved carbon dioxide from shallow waters into deep waters. This is why oceans can draw down and store more carbon dioxide than the atmosphere. The deep ocean is the largest carbon reservoir on Earth.
But a warming world is slowing ocean circulation. In turn, the absorption of carbon dioxide will also slow. Therefore, more carbon dioxide will be present in shallow surface waters than in waters deep within the ocean. More carbon dioxide in shallow waters means increased ocean acidification. Ocean acidification threatens the survival of marine life and marine ecosystems. Even if we remove ocean acidification, the fact that global ocean circulation is slowing down is cause for enough concern about marine life. The conveyor belt is a bringer of vital nutrients to aquatic species. If it slows down or completely stops, many species won’t survive.
It also means that our oceans will not be able to absorb as much carbon dioxide, leaving more of it in the atmosphere. This can further accelerate climate change and increase global warming.
Overall, global warming is causing significant changes in ocean circulation, which can have a wide range of impacts on the Earth’s climate and ecosystems. It is important that we take action to reduce our greenhouse gas emissions and slow the warming of the planet to mitigate the effect of global warming on ocean circulation and the planet as a whole.