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It is! But it's compensated by different mechanisms.
The ocean is salted, with in average 34.7 psu (practical salinity unit), meaning that there are in average 34.7 g of salt per litter of sea water. This is the salinity, i.e. the quantity of salt (NaCl) in the water.
With runoff and infiltration in the rocks, the water from precipitations is getting a tiny bit salty, accumulating minute quantities of the elements contained in rocks. It then forms the rivers and streams before lunging into the ocean. Every tiny drop of water is adding a little bit of salt to the ocean, making it a bit saltier. The mean ocean salinity since the Earth’s Ocean formation in Archean (4.2 billion years ago) has double (Hay et al., 2006).
Salinity is important to know and study, because with temperature, it is one of the major factors determining the water density, and influencing in the oceanic currents system: the thermohaline circulation, with the densest bodies of water (cold with a high salinity) plunging beneath the least dense bodies of water (warmer with a lower salinity. It is varying both geographically and with the ocean depth, and is currently strongly impacted by and impacting the climate and current climate change ((Durack, 2015; Intergovernmental Panel On Climate Change, 2021). The importance of ocean salinity for climate and life has been demonstrated (Cullum et al., 2016; Olson et al., 2022). Due to ocean dynamics, a saltier ocean favours warmer climates (Olson et al., 2022). When water evaporates, only the H2O molecules are evaporating and the salt remains in the ocean. So why isn’t the ocean saltier and saltier?
Schematic representation of the different elements influing of ocean salinity
The salinity of the ocean and seas is determined by the Balance between output of fresh water (evaporation) and fresh water input (precipitation and runoff, melting ice sheet and crystallisation when the saturation point is reached). If there is a lot of evaporation but no fresh water input, the ocean gets saltier and saltier, and if there is more fresh water input that what is evaporated, then the salinity decreases. The quantity of water that evaporates is not constant geographically, and is principally changing depending on the latitude (Gimeno et al., 2015). Similarly, the volume of freshwater brought in by precipitation and melting ice is not constant either, and also varies regionally (Dore, 2005; Sun et al., 2018; Trenberth, 2011). Both evaporation and precipitation processes are strongly impacted by current climate warming. Additionally, most of the precipitations are coming from water that previously evaporated from the ocean, and this is also impacted by rising temperatures and climate change (Findell et al., 2019). In this way, the salinity in every ocean basin varies (Ponte and Vinogradova, 2016) and the ocean and seas don’t have the same salinity in every basin. For examples, the Mediterranean Sea and Red Sea are way saltier than the polar oceans. This is because they are semi-enclosed seas whereas the polar seas do have a strong fresh water input due to the melting icesheets.
The balance between fresh water input and outputs also varies through time. During glaciation and ice caps formation, there is a large extraction of fresh water from the ocean, making the sea level drop and causes a salinity increase. Inversely, with the melting ice caps, the sea level rises and the salinity decreases. In the past earth climate, temperature and currents circulation also changed, influencing the evaporation and precipitation patterns at the Earth surface.
In extreme conditions, when the salt concentration is above the saturation point, we do observe crystallisation, with the formation of halite structures and immense salt banks before becoming a salt desert, as it is the case for example in the Dead Sea and Salar de Uyuni. Furthermore, the fresh water input, climate and evaporation conditions can change through time via tectonic movements: rising mountains are blocking precipitations input and modifying the runoff and rivers trajectory and strait enclosure is stopping the water supply from the connection with the ocean/other basin.
Photos Salar de Uyuni, an ancient prehistorical lake completely evaporated in Bolivia.
A tourist wandering the salt at sunse, by Brendan Van Son.
https://brendansadventures.com/photography-on-the-salar-de-uyuni-and-bolivian-altiplano/
To conclude, the salinity of the ocean results from the balance between the fresh water output (evaporation), satl removal via crystallisation (when the salt concentration in the water is above the saturation point) and the fresh water input (precipitations, runoff, ice sheet melting). If the ocean is not getting saltier and saltier, it is thanks to a constant fresh water supply that compensate the lost via evaporation. Salinity changes can have an impact on the oceanic circulation, and so, the climate distribution.
Bibliography
Cullum, J., Stevens, D. P., and Joshi, M. M.: Importance of ocean salinity for climate and habitability, Proc. Natl. Acad. Sci. U.S.A., 113, 4278–4283, https://doi.org/10.1073/pnas.1522034113, 2016.
Dore, M. H. I.: Climate change and changes in global precipitation patterns: What do we know?, Environment International, 31, 1167–1181, https://doi.org/10.1016/j.envint.2005.03.004, 2005.
Durack, P.: Ocean Salinity and the Global Water Cycle, oceanog, 28, 20–31, https://doi.org/10.5670/oceanog.2015.03, 2015.
Findell, K. L., Keys, P. W., Berg, A., and Krasting, J. P.: Rising Temperatures Increase Importance of Oceanic Evaporation as a Source for Continental Precipitation, JOURNAL OF CLIMATE, 32, 2019.
Gimeno, L., Vázquez, M., Nieto, R., and Trigo, R. M.: Atmospheric moisture transport: the bridge between ocean evaporation and Arctic ice melting, Earth Syst. Dynam., 6, 583–589, https://doi.org/10.5194/esd-6-583-2015, 2015.
Hay, W. W., Migdisov, A., Balukhovsky, A. N., Wold, C. N., Flögel, S., and Söding, E.: Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life, Palaeogeography, Palaeoclimatology, Palaeoecology, 240, 3–46, https://doi.org/10.1016/j.palaeo.2006.03.044, 2006.
Intergovernmental Panel On Climate Change: Climate Change 2021 – The Physical Science Basis: Working Group I Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, 1st ed., Cambridge University Press, https://doi.org/10.1017/9781009157896, 2021.
Olson, S., Jansen, M. F., Abbot, D. S., Halevy, I., and Goldblatt, C.: The Effect of Ocean Salinity on Climate and Its Implications for Earth’s Habitability, Geophysical Research Letters, 49, e2021GL095748, https://doi.org/10.1029/2021GL095748, 2022.
Ponte, R. M. and Vinogradova, N. T.: An assessment of basic processes controlling mean surface salinity over the global ocean, Geophysical Research Letters, 43, 7052–7058, https://doi.org/10.1002/2016GL069857, 2016.
Sun, Q., Miao, C., Duan, Q., Ashouri, H., Sorooshian, S., and Hsu, K.: A Review of Global Precipitation Data Sets: Data Sources, Estimation, and Intercomparisons, Reviews of Geophysics, 56, 79–107, https://doi.org/10.1002/2017RG000574, 2018.
Trenberth, K.: Changes in precipitation with climate change, Clim. Res., 47, 123–138, https://doi.org/10.3354/cr00953, 2011.
October 2024
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