Marginal seas and climate change
Marginal seas are oceanic regions that serve as a transitional zone between coastal areas and the open ocean. These seas, which include examples like the Mediterranean Sea and the South China Sea, vary greatly in size, depth, and water circulation patterns. Their relatively shallow nature makes them particularly vulnerable to local climate variations, river runoff, and direct human impacts, such as pollution from agricultural runoff that can lead to harmful algal blooms and the creation of "dead zones" where marine life cannot survive.
In the context of climate change, marginal seas play a crucial role as buffers, influencing both local ecosystems and economies through activities like fishing and tourism. Rising sea levels and increased temperatures pose significant threats, potentially leading to habitat loss and altered hydrologic cycles. For instance, the Black Sea could experience harmful upwelling of oxygen-depleted waters, drastically affecting marine biodiversity. Additionally, the melting of ice in polar marginal seas raises concerns about contributions to sea-level rise and disruptions to ocean circulation patterns. Overall, the health and stability of marginal seas are essential for maintaining ecological balance and supporting the livelihoods of adjacent communities.
Subject Terms
Marginal seas and climate change
Definition
Marginal seas are ocean regions that connect coastal zones to the open ocean. Marginal seas are found as indentations in the continental landmasses and are often separated from the open ocean by an archipelago or peninsula. They vary considerably in size, depth, the nature of their connection to the ocean, and the circulation of water within them.
The Mediterranean Sea is connected to the Atlantic Ocean through the Strait of Gibraltar, which is only 14 kilometers wide. In contrast, the Gulf of Mexico is connected to the Atlantic Ocean via the Caribbean Sea through the Yucatán Channel (271 kilometers wide) and the Florida Straits (180 kilometers wide). The largest of the marginal seas is the South China Sea, with an area of 2.97 million square kilometers, while the Irish Sea is one of the smallest, with an area of approximately 90,000 square kilometers. The Caribbean Sea has the greatest average depth, at 2,400 meters, and the shallowest marginal sea is the Persian Gulf, at 24 meters deep.
Water circulation in marginal seas varies depending on bathymetry, the nature of the connection with the open ocean, riverine input, and local climate. In seas such as the Black Sea, where riverine input exceeds evaporation, surface waters are less saline than is average ocean water. In other seas, such as the Mediterranean Sea, where evaporation exceeds riverine inflow and precipitation, can exceed that of average ocean water. These salinity variations have important implications for the vertical circulation of water in the sea and therefore the chemistry of the bottom waters. Because marginal seas are relatively shallow and small in area compared to the open ocean, they are strongly affected by variations in river runoff, local climate, and direct human impacts, such as nutrient runoff. For example, the sediments of the Arabian Sea are characterized by a regular banding that reflects the annual development of monsoons.
Significance for Climate Change
Marginal seas represent the buffer between land and the ocean system. They have a major economic impact on those nations that adjoin them as a result of their effects upon tourism, fishing, and the transport of goods. Marginal seas in many regions of the world suffer negative impacts from direct human activities. For example, runoff of fertilizers from agriculture triggers algal blooms in the of many marginal seas. When the die and settle to the bottom of the sea, decay of the consumes almost all the sea’s dissolved oxygen, creating regions devoid of sea life known as “dead zones.” A dead zone up to 18,000 square kilometers in area develops every summer in the Gulf of Mexico at the mouth of the Mississippi River. The development of could be exacerbated, assuming societal activities remain unchanged, in a warmer climate. Increased river runoff and warmer temperatures would lead to a situation in which greater amounts of nutrients are delivered to marginal seas, which generally have lower amounts of dissolved oxygen than does the open ocean.
Marginal seas are also sensitive to rises in sea level, such as those that may accompany global warming. Such a sea-level rise could inundate land that is currently populated. Rises in sea level coupled with changes in runoff to the ocean will affect the hydrologic balance of many marginal seas, with potentially negative ecosystem consequences. For example, decreased freshwater flow into the Black Sea could cause upwelling of the oxygen-depleted, sulfidic bottom waters onto the shelf, causing widespread loss of marine life.
Marginal seas in the Arctic and Antarctic are sites of concern because melting could contribute to sea-level rise and perhaps also to the shutdown of thermohaline circulation in the Atlantic Ocean. The collapse of the Larsen B ice shelf, in the northwestern Weddell Sea, in 2002, remains a dramatic example of the effects of global warming. The loss of sea ice in marginal seas will have major impacts on many aspects of the Earth system, including ocean circulation, sea level, radiative forcing, biological productivity, and community structure.
Bibliography
Black, Kenneth D., and Graham B. Shimmield. Biogeochemistry of Marine Systems. Oxford, England: Blackwell/CRC Press, 2003.
Mee, Laurence. “Reviving Dead Zones.” Scientific American 295, no. 5 (November 2006): 78-85.
Pew Oceans Commission. America’s Living Oceans—Charting a Course for Sea Change: A Report to the Nation. Arlington, Va.: Pew Oceans Commission, 2003.
Satar, Muhammad Naim, et al. "Upwelling in Marginal Seas and Its Association with Climate Change Scenario—A Comparative Review." Climate, vol. 11, no. 7, 18 July 2023, p. 151, doi.org/10.3390/cli11070151. Accessed 26 Dec. 2024.
Yanko-Hombach, Valentina, et al. The Black Sea Flood Question: Changes in Coastline, Climate, and Human Settlement. Dordrecht: Springer, 2007.