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Thermohaline Circulation of the Atlantic Ocean

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Concerns regarding the effect of global warming brought up the issue of the breakdown of the ocean’s thermohaline circulation that may possibly herald the onset of another ice age. This possibility is not far-fetched because of the sensitivity of the phenomenon to wind force, sunlight intensity, precipitation, surface temperature and other environmental factors, all of which are affected by the present global warming being experienced by the earth. The phenomenon is based on the fact that water from the world’s oceans is in a state of continuous flow both on the surface and within its depths. Surface water movement, both forward and backward, in the form of currents and tides is caused by the force of the wind and gravity and is clearly visible. Unknown to many, water in the depths of the ocean is also in a perpetual upwards and downwards movement known as thermohaline circulation.

What is Thermohaline Circulation?

Thermohaline circulation is the slow cyclic movement of water throughout all the earth’s oceans caused by the difference in the sea water density in various parts of the world. Water of diverse densities tend to move in opposite direction with those of lower density floating whereas those of higher density or mass sinking deeper thereby creating an “overturning” of water from top to bottom and vice versa. Its name is derived from the prefix “thermo” meaning temperature and “haline” which is derived from salinity or saltiness, the two factors mainly responsible for the variation in sea water density (Ramstorf 2006). Temperature is inversely proportional to water density such that the lower the temperature, the higher is the density of water because of its property to expand with heat thereby increasing its volume relative to its mass whereas it contracts with cold thereby decreasing its volume compared to its mass (Van Aken 2007, p.23). In contrast, salinity is directly proportional to water density since the more solutes like salt present in water, the greater will be its mass or weight (Van Aken 2007, p.24).

Cause of Thermohaline Circulation

Thermohaline circulation is primarily attributed to the constant changes in temperature and fluctuations in freshwater content of the sea surface and tumultuous mixing in the ocean interior (Ramstorf 2006, p.1). Changes in temperature are due to the exposure of seawater to sunlight that warms it up and cause it to evaporate faster thereby increasing its salinity and to the wind flow and increase in latitude that cools it down. In very high latitudes, the cold temperature will cause freezing of seawater. Freezing will separate the water from its salt content in a process known as “brine exclusion.” The separated salt will then mix with the remaining water to increase it salinity (Van Aken 2007), hence highest surface densities are attained by surface water in the cold regions in the Atlantic between Norway and Greenland and near Antarctica during winter (Ramstorf 2006, Stewart 2008.).

The amount of freshwater that mixes with seawater depends on the amount of rainfall and ice water running from the north and south poles. The fresh water will dilute and decrease the salinity of surface seawater.

Finally, energy is required to sink and raise the water. This is propelled by the turbulent mixing of the colder surface water with the warmer deeper waters in the ocean interior. The downward movement of water molecules bounces off the more inert deeper water molecules thereby creating energy that dissipates in the form of heat. The mixing process can therefore slowly warm and expand the old deep water in the ocean interior making them less dense so that they can be lifted upwards into the surface (Ramstorf 2006, Toggweiler & Key 2001).

Thermohaline Circulation Formation

Ramstorf (2006) summarized the main components of thermohaline circulation as follows:

(1) Deep water formation is caused by the sinking of large volumes of surface water deep towards the bottom of the ocean. Exposure of the surface water to direct sunlight increases its rate of evaporation into the atmosphere. High levels of evaporation take place in between Northern Europe and Greenland and just north of Labrador, Canada (Pidwirny & Jones 2008). This will leave large volumes of surface water with less water and more salt. Simultaneously, the frigid wind in high latitude areas like the North Atlantic will cool the waters and further increase their density to great levels until they sink. The sinking water will exert greater pressure than the deeper water hence by law of convection it will move from area of greater pressure to that of lower pressure, hence the exodus of large masses of surface water deep into the ocean floor.

The largest volume of deep water is formed in the North Atlantic. The very large bulk of approximately 15x106m3 s-1 of surface water that cyclically becomes deep water (Toggweiler & Key 2001) in the said area then travels south then north again to reach the same area has been compared to a giant conveyor belt thereby gaining it another monicker.

Though smaller in volume, the coast of Antarctica is another major deep water formation site. Its surface water is covered in ice whereas its deep water is warmer at 1.5°C and fairly salty at 34.70-34.75 psu. Because of the previously explained process of “brine exclusion,” constant freezing of seawater in this region allows it to maintain its high salinity level despite the diluting effects of the addition of relatively hypotonic precipitation and melted water from the ice caps. This makes Antarctic surface water denser and allows it to sink deeper than the deep water in the North Atlantic (Toggweiler & Key 2001).

Certain unfavorable conditions prevent deep water formation in the other oceans of the world making the phenomenon characteristically localized in certain areas specifically in the Greenland-Norwegian Sea, the Labrador Sea, the Mediterranean Sea, the Weddell Sea, and the Ross Sea. For instance, the surface waters of the oceans near the equator like the Indian Ocean are too warm to sink whereas the surface waters of the northern Pacific Ocean receive large amounts of fresh water surface runoff from the mountains of western Canada and the United States bringing its salinity to a low level that would prevent its sinking.

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(2) Spreading of deep waters. The turbulent mixing of surface water with deeper water produces the relatively dense water masses known as the North Atlantic Deep Water (NADW) and Circumpolar Deep Water (CDW) in the Antarctic. The cold NADW flows southward at depths between roughly 2 and 4 km in a deep western boundary current (Te Raa, LA n.d.) along the coast of North and South America, crosses the equator until it reaches the circumpolar region south of the tip of Africa where it flows over the even denser Antarctic Bottom Water (AABW).

Meanwhile, the Circumpolar Deep Water (CDW) “penetrates onto the relatively deep continental shelves around Antarctica where it is cooled to the freezing point” and mixes with Antarctic shelfwater (deep current that is created by evaporation occuring between Antarctica and the southern tip of South America) causing it to further sink as the Antarctic BottomWater (AABW) (Toggweiler & Key 2001). The AABW fills the deepest parts of all ocean basins (Te Raa, LA n.d.) and flows northward into the Atlantic, Indian, and Pacific Oceans through deep passages in the mid-ocean ridge system at 0°C, where it partially surfaces across 3500 m, and exiting back to the south at less than 2°C between 2000 and 3500 m (Toggweiler & Key 2001, p.6).

(3) Upwelling. As previously mentioned, the turbulent mixing of the denser colder surface water with the lighter warmer deeper water produces the energy that propels the same amount of water upwards into the ocean surface in the process known as upwelling. It is more widely difficult to observe than deep water formation. And although it is more widely distributed than convection, it is not as diffusely spread throughout most of the ocean’s area in the Indian and Pacific Oceans as previously hypothesized but is instead limited to the areas cooled by the Antarctic Bottom Water (AABW). The specific location pinpointed by most oceanographers (Ramstorf 2006, Toggweiler & Key 2001) is the Southern Ocean south of the Antarctic Circumpolar region, within the channel of open water that circles the globe in the latitude band of Drake Passage at 55-65°S (Toggweiler & Key 2001). Ramstorf (2006) pointed out that the process is possibly aided by the wind.

(4) Near surface currents. Pidwirny & Jones (2008) defined ocean current as “a horizontal movement of seawater at the ocean's surface. It is primarily driven by the frictional force exerted by the wind on the surface of water causing it to flow in the same direction or pattern as directed by the earth’s rotation.

Contrary to the southward movement of deep waters in the Atlantic, the upwelled water flows in the opposite direction, that is, northward through the tropical and subtropical North Atlantic where it turns saltier due to the excess of evaporation over precipitation in the said region. It gains warmth as it is exposed to the high solar energy of the equatorial region and more salt as it mixes with the salty outflow from the Mediterranean Sea. As it enters the high latitude of the northern hemisphere, its heat is dissipated back into the atmosphere so that by the time the flow has crossed the 50°N parallel into the subpolar North Atlantic it has cooled to an average temperature of 11.5°C (Toggweiler & Key 2001).

Meanwhile, subsurface currents move relatively slower than surface currents. The southward streaming of NADW in the Atlantic is compensated by the formation of surface currents from the Benguela Current off South Africa via the Gulf Stream and the North Atlantic Current into the Nordic Seas off Scandinavia. However, the thermohaline circulation will contribute only roughly 20% to the Gulf Stream flow.

One complete circuit of thermohaline circulation from the deep water formation in the North Atlantic, its flow towards the southern poles, the upwelling of deep waters in the Antarctic and its slow movement northward back to the North Atlantic is estimated to take about 1,000 years (Pidwirny & Jones 2008). Concurrently, Toggweiler & Key (2001) mentioned that thermohaline circulation turns over all the deep water in the ocean approximately every 600 years by radiocarbon measurements.

Stability of Thermohaline Circulation

The recent concerns about global warming have raised concerns about the stability of the thermohaline circulation. Global warming can cause both sea surface warming and freshening (Rahmstorf 2006). Man-made greenhouse effect of increased temperature on earth can significantly warm surface water, which will then disperse the heat to air pockets near the sea surface. Warmed air pockets produce low pressure areas, which can transform to tropical storms when air from high pressure areas flows into low pressure ones. This phenomenon will cause more rainfall and influx of freshwater that would then decrease the salinity and subsequent density of surface seawater. Furthermore, global warming can possibly melt the ice caps in the polar regions. Since studies of ice cores from Antarctica suggests that our recent polar climate is at its warmest compared to the past 250,000 years, then more polar ice caps are expected to dissolve. Melting of the ice will increase the influx of freshwater into the ocean thereby effectively decreasing its surface water density in an abrupt and unpredictable manner. This possibility is backed by numerous studies reviewed by Stouffer, et.al (2006) that links rapid reorganizations of the THC to abrupt climate changes and the possibility of large changes of the THC in the future. The likelihood of collapse really happening is still uncertain however an alarming 30% fall in the amount of warm water current flowing north between 1992 and 2004 was discovered by UK National Oceanography Centre Harry Bryden due to the partial shutdown of the deep water formation in the European side of Greenland while its Canadian side remains fully functional (Pearce 2005). Even though their findings closely paralleled US government's National Oceanic and Atmospheric Administration readings, the absence of the expected temperature fluctuations in the northern hemisphere makes Bryden’s findings seem puzzling. Hence Schiermeier (2007) tried to clarify the issue by pointing several possibilities. First, the amount of freshwater influx needed to shut down the thermohaline has not yet been reached therefore Bryden’s discovery may simply be a coincidence caused by natural variation or “noise”. Next, the expected cooling off may have been countered by the simultaneous heat produced by global warming or perhaps thermohaline circulation reductions will not have the drastic effects that have been predicted on European cooling. Finally, the shutdown may produce other major consequences aside from the cooling of Europe, such as an increase in major floods and storms like those recently reported from different Asian countries, intense El Niño and El Niña events and warming or rainfall changes in the tropics, Alaska and Antarctica.

Importance of Thermohaline Circulation

The thermohaline circulation is important in regulating the marine ecosystem and earth’s climate. The upwelling portion of the thermohaline circulation raises the nutrient-rich deep water to the surface so that they can acquire oxygen and other substances essential to marine life, disperse and fall back into the ocean and provide the necessary materials to various sea-living creatures at different levels of the ocean (Toggweiler & Key, 2001). Hence, the breakdown of the THC will prevent adequate oxygenation of deep sea water, adversely affect deep-sea communities, reduce food resources to shallow-water food webs and may eventually cause the collapse of the northern hemisphere fisheries and production.

Effects on the Climate. As the upwelled waters slowly flow northward, it carries approximately 1015 W of heat and about 50 W per square meter of ocean surface of the heat (Toggweiler & Key, 2001, p. 1) dissipates into the environment and increase local mean surface temperature by as much as ~10°C annually in Europe and the northern hemisphere (Rahmstorf 2000, Stewart 2008). The farther distance of the sea ice margin in the Atlantic compared to the North Pacific may be suggestive of the effects of warm surface current in the former sector. Such change will also produce an albedo feedback characterized by decreased reflection and more absorption of sunlight by the seawater leading to its further warming. For a more concrete evidence of THC effect on climate Rahmstorf (2006) suggested the use of a climate model that would effectively disable THC by forcefully adding large freshwater influx to the northern Atlantic. This will prevent NADW formation by making its surface water salinity too low for sinking. Cooling of the northern hemisphere (locally up to 8 %u25E6C, 1–2 %u25E6C on average) with a concomittant weak warming of the southern hemisphere (locally up to 1 %u25E6C, 0.2 %u25E6C on average) were observed with the strongest response around the Atlantic accompanied by shifts of the thermal equator, inter-tropical convergence zone (ITCZ) and the associated tropical rainfall belt to the south (Vellinga & Wood 2002, Zhang & Delworth 2005), an El Niño–like pattern in the southeastern tropical Pacific, a La Niña–like condition in the northern tropical Pacific, and weakened Indian and Asian summer monsoons through air–sea interactions (Zhang & Delworth 2005). These changes are consistent with certain paleoclimatic findings thereby suggesting the relationship of THC to Quaternary glaciations.

What areQuaternary glaciations?

Quarternary glaciation is more commonly known as the last ice age or the Pleistocene glaciations which started about 1.8 million years ago and ended about 12,000 years ago. Its name is derived from the Greek word pleistos which means “most” and kainos which means “new” or “recent”. (The Pleistocene) Furthermore, the Pleistocene epoch is the 6th epoch of the Cenozoic era of geological time and the first part of the Quaternary period. Last, unlike the preceding ice ages that each lasted for approximately ten thousand years, this phenomenon has shorter freezing period followed a longer slightly cold period glaciations hence promptly termed as a glaciation.

The Quarternary glaciation is characterized by a cyclic pattern of glacial periods alternating with warmer intervals known as interglacial periods. Throughout its existence, 20 such cycles of glacial periods complete with advancing and retreating continental glaciers occurred as verified by Pleistocene geological record. At the end of its duration, it left surviving glaciers at Greenland and Antarctica.

Each glacial period is characterized by the spread of large, thick, melting stationary or moving ice sheets or glaciers that covered all of Antarctica, large parts of Europe, North America, and South America, and small areas in Asia. Stretching southward from the edge of the ice sheet, a zone of permafrost was formed that extended several hundred kilometers in the North America and Eurasia. The mean annual temperature at the edge of the ice sheet was −6°C and 0°C at the edge of the permafrost.

They have a very wide distribution affecting about 30% of the Earth’s surface during each cycle (The Pleistocene). In North America they stretched over Greenland and Canada and over the United States as far south as a line drawn westward from Cape Cod through Long Island, New Jersey, and Pennsylvania, along the line of the Ohio and Missouri rivers to North Dakota, and through N Montana, Idaho, and Washington to the Pacific. The ice sheets of Europe radiated from Scandinavia and covered Finland, NW Russia, N Germany, and the British Isles. Glaciers distinct from the main sheets were formed in the Rockies and the Alps. In South America, Patagonia and the S Andes lay under an extension of the Antarctic sheet, while in Asia the Caucasus, the Himalayas, and other mountain regions were glaciated. However, the more extensive continental glaciers of the Pleistocene glaciations focused their distribution on the North Atlantic Ocean, seemingly depending on this area for their formation (Leighly 1949).

Glacier formation. Glacier formation occurs when the environmental temperature on high altitude areas like mountains is low enough such that masses of piled up snow do not completely melt during the summer. Compaction pressure during accumulation causes the snow to crystallize into light firn ice. Further compression by new snow coupled with repeated melting and freezing will transform granules into interlocking crystals. It will be called a glacier once it reaches a thickness of about 40 meters. Gravity will eventually pull the glacier down the mountain slope in what is known as glacial movement or advance.

Cause ofQuaternary glaciations. Despite the recent advances in science and technology, the exact cause of quaternary glaciations still remains a mystery. Though many theories have been proposed, not one can fully explain all its aspects. Its cause seems to be multifactorial, involving the interactions of Earth's orbital cycles, solar-energy fluctuations, continental positions, and oceanic circulation (The Pleistocene).

The cyclic appearance of ice ages every 100,00 years, glaciations lasting about 60,000-90,000 years and interglacials about every 40,000-10,000 years that scientists observed seem to coincide with Milankovitch cycle of orbital change variations. In the 1930s, Serbian mathematician Milutin Milankovitch calculated and descried the behavior of the earth as it orbits around the sun. He proposed that the earth’s orbit changes from being nearly circular to slightly elliptical (eccentricity) every 100, 000 years. The angle of tilt of the earth’s axis becomes more skewed from 22.1° to 24.5° (obliquity) every 41, 000 years and the direction of the tilt of the axis (precession) changes at an interval of every 26,000 years (Lee 2010, Maslin, et.al. 2001). Aside from modifying seasonal length, such positional changes can also influence the amount of earth surface exposed to sunlight thereby affecting the quantity of solar radiation or heat received by the earth surface. The stronger the solar intensity that enter the earth’s atmosphere, the warmer it will become whereas the weaker solar intensity directed on earth, the cooler will be the environment. Furthermore, since the earth’s rotation also influences its gravitational forces, it can also affect the oceans’ tides and currents, wind formation and overall climate. Sudden climatic changes that can provoke the polar ice sheets to move forwards (advance) and backwards (retreat) can then herald the onset of an ice age. However orbital changes, on its own, cannot produce the level of climactic change that can cause the polar ice sheets to move to the extent that will lead to an ice age.

Another factor that may have contributed to the production of Quarternary glaciations is tectonics or continental position. Tectonic is responsible for shaping the Earth’s surface geography via the horizontal displacement and vertical movement of earth’s crust (Hay 1999). Movement of land masses can effectively block the connections between oceans and displace land masses to different latitudes. As such, some scientist even theorized that the separation of Australia and Antarctica may have been responsible for the growth of the Antarctic ice sheets (Hyde, et.al. 1990). On the other hand, the sudden appearance of broad and tall land masses can block the sun’s radiation thereby causing extreme seasonal climate changes, specifically favoring colder weather, reversal of atmospheric pressure and enhanced monsoonal circulation.

All the aforementioned factors can partially claim responsibility for the development of Quarternary glaciations. However, they cannot explain why most of the glaciers seemed dependent upon the climate on the North Atlantic region from where they focus their distribution. Since the North Atlantic region serves as key area for the global thermohaline circulation, this finding is highly suggestive that thermohaline circulation greatly affected Quaternary glaciations.

Relationship of Thermohaline Circulation to Quaternary glaciations

The ocean current is both a key player in the regulation of climate while at the same time is deeply affected by changes in climate. Because of these characteristics, many view changes in the ocean currents as greatly associated with the development of the last glaciations (Clark 2002). However, the exact relationship between the two still remains unclear and needs further investigation

In order to understand the relationship between thermohaline circulation and quaternary glaciations, an understanding of the events that occurred during the last Ice Age is imperative. Some of these data can be derived from the study of the information about temperature, atmospheric CO2 and methane concentration, volcanic eruptions, and wind speed and direction that dates back up to 400,000 years within the surviving ice caps of the Quaternary glaciations found over the Greenland and the Antarctica. Among the information gathered is the occurrence of abrupt temperature fluctuations in the polar regions over the past 100,000 years. Recorded temperature near Greenland revealed recurrent periods of abrupt warming called Dansgaard/Oeschger event that lasted from one to one hundred years followed by gradual cooling over long periods (Stewart 2008). These unusual findings can be best understood by correlating these events with the known history of thermohaline circulation during the same time frame.

History of Thermohaline Circulation. Although thermohaline circulation has been ongoing since the beginning of time, its cyclic pattern is far from static. Ramstorf (2000, 2006) noted the documented changes undergone by the thermohaline circulation during the Quaternary and organized them into three major modes namely: (a) switched-off mode characterized by the virtual absence of deep water formation in the North Atlantic (NADW), (b) warm or interglacial mode similar to the present day Atlantic with deep water forming in the Nordic Seas, and (c). cold or glacial mode with NADW forming south of the shallow sill between Greenland, Iceland and Scotland The switch-off mode is attributed to the massive influx of freshwater, either from the Heinrich events characterized by surging glacial ice sheets or from the Little Ice Age or Younger Dryas event characterized by meltwater floods.

Heinrich events and Dansgaard/Oeschger events were noted to occur within the time frame of each glacial period of the glaciations with the Heinrich events preceding the Dansgaard/Oeschger by 1000 years (Stewart 2008). Heinrich events is named after Hartmut Heinrich who lead the team of scientists who discovered six distinct past events after studying marine sediments most likely deposited on the ocean floor by floating icebergs because of their inaccessible location at the center of north Atlantic. The cause of this phenomenon is blamed on Laurentide ice sheet instability that causes blocks of ice known as icebergs to detach from glaciers, and then discharge into the sea where they subsequently melt and dump large volumes of freshwater into the Atlantic for several centuries (Rahmstorf 2006). Computer models of this event suggested that the degree of dilution of surface water would suffice to completely prevent deep water formation. However, the Heinrich events appear to have had little effect in Greenland probably because the event occurred during the cold or glacial mode when the THC is already incapable of reaching the northern Greenland region (Rahmstorf 2006). The melting of ice in Heinrich events will cause Greenland to become very cold. This is because the resultant disabling of North Atlantic Deep Water production will significantly decreases the amount of warm water transported to the north. Furthermore, the melting of the ice will also displace the boundary between cold and warm water in the north Atlantic or polar front further south than its present position. The location of the front, and the time it was at different positions can be determined from analysis of bottom sediments. (Stewart 2008)

Meanwhile, the shift in major deep water formation site from north (Nordic Sea) to south (Iceland) is the most likely explanation for the Dansgaard-Oeschger event (Rahmstorf 2006). The root cause of the said transitions between these modes is still unknown; however it occurs regularly every 1500 years which raises the possibility of an underlying mystery climate cycle. To further clarify the event, Maslin, et.al. (2001) claimed that an influx of meltwater into the North Atlantic decreased the salinity of its surface water thereby diminishing the amount of North Atlantic Deep Water formation and significantly reducing its current strength so that the warm deeper water will be displaced southward, thereby carrying heat away from high latitudes and producing a see-saw of northern and southern hemisphere temperatures (Stewart 2008). However, Rahmstorf (2006) pointed out that a large coinciding increase in salinity in the Irminger Sea suggests a northward flow of warm, salty waters from the Atlantic into the Nordic Seas, which could explain its wide area of heat distribution. Furthermore, the event can cause significant rise of temperature from 8 to 16 ºC within a decade in Greenland and areas surrounding the northern Atlantic.

During deglaciation, the melting ice sheets on the northern hemisphere specifically surrounding the North Atlantic resulted in the episodic dumping of meltwater into the ocean. The effect of this meltwater on the THC could explain the oscillations and abrupt cold reversals seen e.g. in Greenland ice cores during this time. (Rahmstorf 2006)

Since the climate changes of Dansgaard-Oeschger events occurred 25 times throughout its duration and appear to coincide with each of the glacial periods of Quaternary glaciations, it can be hypothesized that these changes may have been the main factor responsible for the development of the last Ice Age. However, when Ramstorf (2006) tested this possibility in models, the results showed that although these factors could indeed amplify glacial cooling, ocean circulation changes are probably not the key factor for glacial formation.

Summary and Conclusion

Thermohaline circulation is an important continuous loop system that interconnects all the earth’s ocean into a single vast reservoir of water. It is characterized by the overturning of the ocean’s water whose extremely slow flow from north to south then back north again allows mixing of their content and adequate dissipation of its heat content into the environement to cool the frigid atmosphere of the north while distributing its oxygen and nutrient content to the marine animals living in the ocean’s depths. However, as part of the world’s largest body of water, it is deeply influenced by changes in global climate while at the same changes within its flow will also affect global climate. This relationship was well demonstrated in the studies linking thermohaline circulation to Quarternary or Pleistocene glaciations.

Although four periods of Ice Age have already descended on the earth, knowledge about this period is still sparse. The long time interval of billions of years from the last one makes the study of this rare occurrence quite difficult. Only the surviving ice caps and ocean floor sediments from the glaziers of the Quarternary glaciations serve as font of information. Applications of the gathered data with computer models than can determine all possibilities and remove the improbable ones can theories about the Quarternary glaciations be formulated.

After review of the different literatures about the time period, it became clear the thermohaline circulation was already in operation by the time the last Ice Age descended. Also the Quarternary glaciation was characterized by abrupt climate changes. Concomittant to this, the thermohaline circulation shifted from the present warm to the cold mode and eventually to the switched-off one as it finally weakened as a result of its positive feedback with the climactic changes during the Ice Age. The trigger for the said transition from one mode to another unfortunately remains obscure. Although the assumption that weakening of the thermohaline circulation may be the principal trigger for the Ice Age’s formation still remains a possibility, its likelihood to be true is small. What is definite is that enhance the generation of freezing temperatures to significantly cool the surrounding atmosphere of the earth’s region.

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