A saltier world is to be expected

Sealevel rise and increased evaporation due to climate change cause salinisation of freshwater. Salinisation of wetlands may also lead to higher emissions of greenhouse gases.

Freshwater salinisation – a process in which these water bodies become saltier – occurs because of a number of human activities. These include agriculture, resource extraction (the withdrawal of materials from nature), use of road salt etc. Climate change can also drive salinisation or interact with salinisation caused by other factors.

Salinity is one of the main factors that regulate the distribution of species and hence also the species composition in water bodies. The functioning of animals, plants and microbes is affected by the equilibrium of ions in internal fluids and those in the surrounding waters. If the salinity in the environment changes, so will the salinity in the internal fluids, unless the organism is able to regulate its internal concentration of ions. Although some organisms have this capacity, it comes with an energetic cost that can affect the performance of the organism. Some organisms can conform to salinity changes and are to some extent tolerant to changes in their internal salinity.

Nevertheless, as the ability to cope with salinity changes varies between species, a saltier world leads to a world where the species composition and distribution of species change. Ultimately, this can change entire ecosystems, and affect the services that these ecosystems provide.

Salinisation as a result of climate change was a recurring topic in the contribution to the fifth assessment report of Working Group II of the IPCC (https://www.ipcc.ch/report/ar5/wg2/) , and this is also the case for the recently published final draft of the contribution to the sixth assessment report (https://www.ipcc.ch/report/sixth-assessment-report-working-group-ii/).

Climate change increases the risk of saltwater intrusions from the sea due to sea level rise and a rising occurrence of floods and storms. Another process that can lead to salinisation is increased evaporation. The problem of salinisation is also related to water scarcity – another threat from climate change (and a growing global population). As pointed out by Michelle van Vliet and co-workers in a commentary in Nature Geoscience, “Sustainable management of water resources for different uses will not only need to account for demand in water quantity, but also for water temperature and salinity, nutrient levels and other pollutants” ¹.

As mentioned, climate change can interact with other factors that drive salinisation. In a study on rivers and streams in the USA the projected impact of land use changes and climate change was investigated². The study showed that in this case land use was the main driver of salinisation. However, climate change interacted with land use and the greatest increase in salinity was predicted when these factors were combined.

Climate change not only relates to salinisation as one of the drivers – in fact salinisation can also be a driver for greenhouse gas (GHG) dynamics! This was, for instance, shown in an mesocosm experiment in lakes on the Qinghai-Tibetan plateau, where it was found that salinisation increased methane formation due to microbial processes³ (Mesocosms are enclosures that contain a part of nature and can be experimentally manipulated.) In another study⁴, salinisation related to drought in wetlands in California was investigated. The study led the authors to suggest that “…salinization may increase GHG emissions from estuarine freshwater wetlands”.

There is a scarcity of studies on salinisation in many parts of the world, including in South America, Africa, and Asia. Many of the regions where salinity studies are lacking also experience huge challenges due to climate change. To improve the geographic coverage of salinisation studies, and to address several other knowledge gaps, a group of researchers recently suggested a new “research agenda for a saltier world” ⁵.

Marko Reinikainen

References
1 van Vliet et al., 2017. Nature Geoscience 10: 800-802
2 Olson, 2019. Phil. Trans. R. Soc. B 374: 20180005
3 Huang et al., 2020. Front. Microbiol. 11: 1772
4 Chamberlaine et al., 2020. Ecosystems 23: 675-688
5 Cunillera-Montcusi et al. 2022. DOI: https://doi.org/10.1016/j.tree.2021.12.005

Acknowledgement: This work is part of the AQUACOSM-plus project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 871081. The funding has supported the author of this work.