Earth’s Underwater Powerhouse: How Global Climate Change Will Affect The Atlantic Thermohaline Circulation And A World That Relies On It
2018, High School, Prose
Heat is unequally distributed around the planet due to Earth’s rotation around the sun. Throughout the year, the equator receives the most direct sunlight and a concentration gradient of heat is created between the equator and the poles. Due to prevailing winds known as the Westerlies, this warm surface water at the tropics is then shifted to the North Atlantic Ocean via the Gulf Stream (Ananthaswamy 2012). During the December solstice when the Northern Hemisphere is tilted away from the sun, heat is transferred from this warm water to the air surrounding northern Europe, thus keeping a large population of the world from experiencing bitter cold weather (Trujillo and Thurman 2011). As the water loses heat and continues to migrate toward the Arctic Ocean, some of it freezes. Salt that accumulated from surface runoff is left behind, thus increasing the density of the water. The combined effects of cold temperature and high salinity cause the water to sink to the bottom of the ocean, thereby initiating a crucial stage in global thermohaline circulation.
As fundamental chemistry would state, gases more easily dissolve in cold water. The sinking of water in the North Atlantic Ocean therefore brings with it an important source of oxygen to deep water organisms living in the depths of the earth’s five major ocean basins (i.e., the Atlantic, Pacific, Indian, Southern, and Arctic) (Trujillo and Thurman 2011). In addition to oxygen, vital inorganic nutrients such as sulfate, nitrate, and phosphate are also distributed throughout the ocean floor, fueling the world’s biodiversity by allowing deep-water plants and species to grow and reproduce (Townsend 2012).
The process of thermohaline circulation is thus a fundamental aspect in the health and prosperity of many marine organisms and coastal communities. It seems almost impossible that this planet could even function without it, yet natural forces have shut it off in the past. Twenty thousand years ago, Earth’s orbit shifted so that the Northern Hemisphere was exposed to more direct sunlight than it typically received (Ananthaswamy 2012). Ice sheets in that region began to melt and the Atlantic thermohaline circulation nearly stopped, thrusting northern Europe back into an ice age. This change in the normal temperature difference caused the prevailing winds to alter, resulting in warm water near the equator moving further south toward Antarctica. As the seas surrounding the Antarctic were warmed, CO2 stored in the typically cold, dense water was brought to the surface and released into the air (Ananthaswamy 2012). Increased amounts of atmospheric CO2 further exacerbated the Southern Hemisphere’s warming by trapping even more heat.
What is worrisome is that this could all happen again, not as a result of some unpredictable random tilt toward the sun, but as a result of human carelessness driven by greed and ignorance. As people release more and more greenhouse gases into the atmosphere to fuel their need for convenience and wealth, heat energy trapped by the earth’s atmosphere increases. In recent years, the annual increase in global CO2 levels has been more than 3.5 ppm. Today, global levels have reached nearly 410 ppm with future estimates projected by an upwards linear trend (Lindsey 2017). This heat trapped by such greenhouse gases then gets radiated back toward the planet’s surface, melting land ice and generating a positive feedback loop whereby land beneath the ice absorbs even more heat energy due to its lower albedo. A graph produced by the World Glacier Monitoring Service detailing world glacial mass balance affirms this result by showing an exponential decline in yearly ice loss (Hill 2016). Of particular concern is Greenland, because if any or all 1.7 million square kilometers of ice covering the country melt (NSIDC 2018), the Atlantic thermohaline circulation would be at risk (Delworth et al. 2008). This is because the sinking of the surface water in that region is dependent on temperature and salt concentration (thermo means “heat” and haline means “salt”), so as temperatures rise, the water becomes warmer, and as fresh water from land ice mixes with the ocean, its salinity decreases. As a result, the sinking of surface water in the North Atlantic Ocean may greatly slow down or even stop completely (Delworth et al. 2008).
This hypothesis, however, is still debated. One study in 1997 suggested that if the world increased its CO2 emissions to 750 parts per million within a span of 100 years, the Atlantic thermohaline circulation could entirely shut off (Stocker et al. 1997). A report by the National Oceanic and Atmospheric Administration states that by the end of this century, continued release of greenhouse gases could reduce it by up to 30%. Another study showed that if CO2 levels are increased to twice or even quadruple their pre-industrial levels, the overturning could remain weak for up to 70 or 1,000 years, respectively (Wood et al. 2003). Regardless of specific statistics, it is clear that the world’s great ocean conveyor belt will be negatively affected by any further anthropogenic emissions of greenhouse gases.
What does this mean for the human population? Do people living far from the North Atlantic Ocean have anything to worry about?
The answer is a resounding yes.
Remember, all of the world’s oceans are connected by this process of thermohaline circulation. As mentioned before, thermohaline circulation supplies essential nutrients to aquatic species all around the world. If this circulation were to slow down or stop, many organisms would see reduced rates of growth and reproduction. With every organism linked to another via a complex series of food webs, a decrease in the productivity in one species, such as plankton, would have enormous effects on other animals that either directly or indirectly benefit from that species. Examples include fish, whales, and seals – animals that contribute greatly to many local cultures with regards to food, recreation, and business.
Various communities would also be introduced to uncommon weather patterns. Areas close to the North Atlantic Ocean in the Northern Hemisphere, due to colder temperatures, would receive less annual precipitation. A lack of rain could reduce agricultural productivity by as much as 12 percent, having a drastic impact on countries with subsistence economies and those whose main capita is gained through agricultural exports (Wood et al. 2003). The colder weather would add an additional burden on local agriculture by reducing soil fertility since decomposition of organic matter happens quicker in warmer temperatures. Conversely, communities in the Southern Hemisphere would have to adapt to warmer climates. This might lead to droughts and severe heat waves, retarding crop growth and causing more deaths related to either heat strokes or an increased spread of diseases.
Agriculture in the South Asian region would also be greatly impacted as warming surface waters in the Indian Ocean might lead to weakened seasonal monsoons (Delworth et al. 2008). Staple crops such as rice and tea depend on the torrential downpours of rain that occur during a summer monsoon. South Asian countries such as India, Sri Lanka, and Myanmar would not only face difficulties growing their local crops, which would increase the demand for food, but would also have to invest more money importing food from other countries, which would in turn raise the price of such food. Aside from agriculture, rain that falls during a monsoon helps to refill aquifers and can be caught to generate hydroelectric energy (NGS 2018). A weakened monsoon would not only leave their groundwater depleted, but would also increase the price to produce and consume energy, thus hindering their economy and any technological advancements.
If these are the possible outcomes of a reduced Atlantic thermohaline circulation, why do so many people seem to remain unconcerned? I believe this is because thermohaline circulation is a process that cannot easily be seen, and just like anything else that is not directly laid before the eye, people tend to ignore it. Why worry oneself over something that is slow-moving and happening in the ocean far from daily life? Even though it is not something that is talked about on radio shows or newspaper articles, it is a fundamental process that allows the planet to function as it does.
The importance of thermohaline circulation can be compared to that of a leg on a table. It does not seem all that important when one is concerned with eating or setting the table for guests. Often, a person will even throw a tablecloth over it to cover it up, but try imagining what it would be like to eat Thanksgiving dinner with one of the legs missing.
Indeed, it would not be too easy, if at all possible.
There needs to be an increased awareness of what thermohaline circulation is and its role in the daily lives of people all across the globe. It affects world climate, biodiversity, economics, and culture. It is a powerhouse that helps people. Unfortunately, it is people who are taking it for granted and it is people who are slowly destroying it. As global climate change becomes a more pressing issue, thermohaline circulation will have to be addressed.
Indeed, it would not be too easy, if at all possible, to live in a world without it.
Ananthaswamy A (2012) The Great Thaw. New Scientist 2889: 32-35
Delworth, T.L., P.U. Clark, M. Holland, W.E. Johns, T. Kuhlbrodt, J. Lynch-Stieglitz, C., Morrill, R. Seager, A.J. Weaver, and R. Zhang, 2008: The potential for abrupt change in the Atlantic Meridional Overturning Circulation. In: Abrupt Climate Change. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. U.S. Geological Survey, Reston, VA, pp. 117–162.
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Prior to my Environmental Science class, I had no idea what thermohaline circulation was or that it even existed. It seemed as essential a part of knowledge as the earth rotating around the sun, and yet, it wasn’t included in my second-grade syllabus. It really amazes me how such an influential part of the world’s climates and communities is hidden backstage, away from public view and unspoken about in mainstream media. Unfortunately, this seems to be the case with anything related to climate change. My decision to take Environmental Science my senior year stemmed not only from an interest in the fascinatingly complex system that is planet Earth, but also from a desire to understand how my actions would impact that careful system. I chose to write about thermohaline circulation because I want people to become aware of what it is and how it is being changed by human behavior. So long as people remain uninformed, nothing will be done to address its weakening. I want thermohaline circulation to be woven into common knowledge because that is the only way it is going to be valued and perhaps even saved.