By Ayoush Srivastava
As climate change continues to accelerate, there is increased international attention towards its effects and the development of potential measures to slow its pace. The exponential growth of greenhouse gas emissions over the past 30 years, especially carbon dioxide (CO2) and methane (CH4), has increased the average global temperature by 0.2°C annually; this extra heat is mostly absorbed by the ocean but has had numerous effects to oceans such as the greater stratification of the water column, increased meltwater from ice sheets and terrestrial glaciers, and more unpredictable behaviour of ocean currents (Ramírez et al., 2017). Ocean circulation is essential for marine life to prosper, and the changes incurred by climate change could upset this delicate balance.
Ocean currents are defined as the continuous and directed movement of ocean water and can occur at any depth, whether in shallow waters close to the sea surface or deep beneath (NOAA, 2011). These currents are respectively termed surface currents and deep ocean currents. Surface currents are largely driven by global wind systems whose energy is provided by the sun’s heat (NOAA, 2011). Surface currents typically carry heat from the tropics to polar regions; the most prominent example is the Gulf Stream in the Atlantic Ocean. The Gulf Stream also influences the climate of its surroundings, and as a result, Northern Europe is significantly warmer than other regions at a similar latitude (NOAA, 2011). Deep ocean currents, however, result due to differences in water density and the seawater’s temperature (NOAA, 2011). In polar regions, heat from seawater is lost to the atmosphere, causing the temperature of the seawater to drop and eventually form ice sheets. This in turn leaves additional salt in the seawater, increasing its overall density, causing the dense seawater to sink, while seawater that is less dense rises (NOAA, 2011). This process is termed thermohaline circulation and forms the basis of a current system known as the global conveyor belt (NOAA, 2011). This system is just as important surface currents as it largely influences the Earth’s climate, but also the ocean’s nutrient and carbon dioxide processes (NOAA, 2011).
Although ocean currents can be classified from their differences in formation, their unpredictable flow makes them difficult to examine and measure. However, there exists two approaches to assessing the features of their flow field. The first approach is termed Lagrangian measurement; this method involves using a free-floating drifter and tracking its course across the ocean using satellites (Hays, 2017). The second approach is termed Eulerian measurement; instead of having a free-floating drifter move with the current, a current meter is moored, allowing scientists to analyse the current flow at a fixed location (Hays, 2017). Whilst these approaches provide valuable data, ocean currents are volatile and constantly changing, meaning it is extremely difficult to concretely define the paths of ocean currents. Fortunately, the use of numerical models provides an alternative to the Lagrangian and Eulerian approaches by using wind data and buoyancy fluxes to predict the flow of currents (Hays, 2017).
Analysing the flow paths of ocean currents is also vital to understanding the behaviour of numerous marine species. For example, some species tend to drift in ocean currents at some point of their lives (Hays, 2017). Plankton, ranging from zooplankton to bacteria, is well known for drifting with ocean currents throughout its lifespan (Hays, 2017). In addition, some coastal animals and marine plants embedded on the seafloor like mussels, barnacles, and mangroves rely on drifting to spread their larvae and seeds across the ocean (Hays, 2017). Furthermore, terrestrial species like tortoises can be stranded on debris and carried by the current to colonise new lands (Hays, 2017). These observations support the conclusion that the diversity of species on an isolated island is dependent on the flow patterns of drifting along currents (Hays, 2017). This theory is further supported by recording and examining the drift time between islands, providing a measure of connectivity between islands isolated at sea; the drift time itself is an indicator of the strength and direction of the current (Hays, 2017).
Whilst some species are dependent on drifting to survive and reproduce, other species may be knocked off course from their migratory paths due to currents. This has introduced an exciting area of research: how animals are able to perceive and adjust their course if led astray by a drift (Hays, 2017). A popular example is sea turtles; sea turtles, when hatched, are believed to largely drift with ocean currents. During this passive drifting, the infant sea turtles will often imprint on favourable foraging areas, but if carried to a cold and more unfavourable region of the ocean, it has been proven that these hatchlings can orientate themselves with respect to the Earth’s magnetic field (Hays, 2017). Therefore, if carried to an unfavourable area, the hatchling can perceive their location via geomagnetic coordinates despite not being able to measure the current (Hays, 2017). In comparison to hatchlings, adult sea turtles still can be deflected off-course despite being more powerful swimmers (Hays, 2017). Since adult sea turtles often migrate between foraging and breeding sites, their migration paths can be tracked and can be combined with data about ocean current paths to understand the efficiency of their routes (Hays, 2017). Interestingly, adult sea turtles follow routes that do not minimise travelling time despite reaching their intended destination; this suggests that adult sea turtles are either incapable of perceiving possible deflections from ocean currents, or their sense of position is too imprecise to find the most optimal route (Hays, 2017).
Unfortunately, ocean currents are changing due to variety of effects brought upon by climate change. Due to increased meltwater from terrestrial glaciers and polar ice sheets, changing wind patterns, and excess absorbed heat, the temperature and strength of ocean currents are shifting; however, this shift will be nonuniform across oceans, and thus episodic (Hays, 2017). Regrettably, these changes, despite being episodic, will affect the connectivity of populations and the migration patterns of innumerable species (Hays, 2017). An example of a species that would be severely affected by shifts in ocean currents is the European eel. This species typically lives in rivers and lakes across the European continent; their breeding ground, however, is in the Sargasso Sea in the Western Atlantic (Hays, 2017). Their migration is long and arduous, having to travel thousands of kilometres from the rivers and lakes at which they reside to their breeding ground (Hays, 2017). Their eggs and larvae will then passively drift via the North Atlantic Current back to the European continent to hatch (Hays, 2017). Considering the effects of climate change on the ocean, numerical models predict that European eel populations will decline as the developing eggs and larvae may not reach European rivers with the North Atlantic Current shifting from its path (Hays, 2017). Many other species that rely on ocean currents for their survival and reproduction needs may share similar fates as shifts in currents become more pronounced.
Hidden from our sight, ocean currents dictate and influence essential processes that maintain the livelihood of numerous species; however, to protect these species, swift action is necessary from the international community if the worrying effects of climate change are to be combatted.
References:
Hays, G.C. (2017). Ocean currents and marine life. Current Biology. 27 (11), 470–473. Available at: https://www.sciencedirect.com/science/article/pii/S0960982217300775 [Accessed 4th June 2021].
NOAA (2011). Ocean currents | National Oceanic and Atmospheric Administration. http://www.noaa.gov. Available at: https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-currents. [Accessed 4th June 2021].
Ramírez, F., Afán, I., Davis, L.S. & Chiaradia, A. (2017). Climate impacts on global hot spots of marine biodiversity. Science Advances. 3(2). Available at: https://advances.sciencemag.org/content/3/2/e1601198 [Accessed 4th June 2021].
Imperial Bioscience Review 