A lot of the focus regarding climate change is on the atmosphere or the land, but we can’t forget that 71% of Earth’s surface is covered by water. The possible changes related to the oceans are complex but far-reaching and could have geopolitical changes in addition to affecting the environment.
Climate change won’t just cause a change in temperature, but will also lead to acidification and changes in oxygen concentration (Bijma et al. 2013). One particular issue is called surface ocean stratification which could have significant impacts on the marine environment. Naturally, the ocean is stratified based on density with warmer, lighter and less salty water layered above colder, heavier and saltier water.
Increased stratification leads to a cycle of global warming. Warmer water absorbs less carbon dioxide, therefore increasing the amount of carbon dioxide in the atmosphere. In turn, this increases global warming and heats up the surface water even more, leading to greater stratification.
The major problem of ocean stratification is that warmer water also absorbs less oxygen, leading to ocean hypoxia and eventually anoxic “dead zones” (Bijma et al. 2013). Reduced mixing in the oceans will affect nutrient availability and primary production, ultimately impacting food-web dynamics (Hoegh-Guldberg et al. 2010). This could lead to significant reorganisation of the marine ecosystem on a regional scale and triggering a range of cascading effects (Poloczanska et al. 2013). One such example is the potential decline of large ocean predators who have a higher oxygen demand. This alone, could have significant changes on marine trophic levels (Bijma et al. 2013).
The other significant issue linked to climate change is ocean acidification. While warm water absorbs less carbon dioxide, cold water absorbs more. When carbon dioxide dissolves in water, it forms carbonic acid (H2CO3) which decreases the ocean’s pH level (McNeil & Matear, 2006). Due to the temperature difference between the oceans, the colder waters in the north and south will acidify faster, driving species towards the equator and increasing competition (Bijma et al. 2013).
Coral reefs, particularly cold water reefs, could be significantly impacted by a changing climate too. Coral bleaching has occurred in the Caribbean, Indian and Pacific Oceans on a regular basis, and is thought to be a consequence of rising background temperatures (Brown, 1997).
Corals can survive bleaching events, but they are put under more stress and mortality increase. Ocean acidification will cause other problems for corals, as the decrease in pH leads to reduced calcification rates. This means that reef-building processes will be severely diminished, with rates of erosion overtaking. Inevitably, this would lead to the extinction of some species, as well as the overall decline of the corals. At current rates, cold-water corals will cease to exist in many parts of the ocean by 2100 (Bijma et al. 2013).
Lastly, over the coming decades, ocean warming could facilitate the invasion and establishment of non-native species. There are already case studies that show that milder winters and warmer summers enable establishment, however climate change will drive species to migrate into more suitable waters, potentially exacerbating the threat (Stachowicz et al. 2002). The sheer scale of potential changes is deeply worrying, for example, the potential homogenisation of the Mediterranean biota. Multiple species that have previously been found in the Eastern and Southern shores have been documented to be moving north-westwards (Lejeusne et al. 2010).
References
Bijma, J., Pörtner, H. O., Yesson, C., & Rogers, A. D. (2013). Climate change and the oceans–What does the future hold?. Marine pollution bulletin, 74(2), 495-505.
Brown, B. E. (1997). Coral bleaching: causes and consequences. Coral reefs, 16(1), S129-S138.
Hoegh-Guldberg, O., & Bruno, J. F. (2010). The impact of climate change on the world’s marine ecosystems. Science, 328(5985), 1523-1528.
Lejeusne, C., Chevaldonné, P., Pergent-Martini, C., Boudouresque, C. F., & Pérez, T. (2010). Climate change effects on a miniature ocean: the highly diverse, highly impacted Mediterranean Sea. Trends in ecology & evolution, 25(4), 250-260.
McNeil, B. I., & Matear, R. J. (2006). Projected climate change impact on oceanic acidification. Carbon balance and management, 1(1), 1-6.
Poloczanska, E. S., Brown, C. J., Sydeman, W. J., Kiessling, W., Schoeman, D. S., Moore, P. J., … & Richardson, A. J. (2013). Global imprint of climate change on marine life. Nature Climate Change, 3(10), 919-925.
Stachowicz, J. J., Terwin, J. R., Whitlatch, R. B., & Osman, R. W. (2002). Linking climate change and biological invasions: ocean warming facilitates nonindigenous species invasions. Proceedings of the National Academy of Sciences, 99(24), 15497-15500.
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