Nazli Koseoglu points out that higher sea levels also mean more saltwater penetration into coastal freshwater aquifers.
Global sea level is rising at an accelerating rate in response to global warming. As temperatures increase, ice growth in winter falls behind ice melt in summer resulting shrinkage of nearly all surveyed glaciers worldwide. According to the U.S. Environmental Protection Agency, decline in ice cover increases amount of freshwater lost to the oceans and has already added about eight inches to the average sea level since Industrial Revolution. The IPCC forecasts [pdf] continuation of this trend in increasing sea levels over the course of this century with 0.4 to 0.8 metre additional increase only if the zero emissions are achieved as a result of historical emissions. On the other hand a more
pessimistic realistic scenario by the World Bank predicts up to 2 metres increase in the sea level assuming global carbon emissions remain unabated. A 2-metre rise in sea levels means an extreme reshaping of coastlines, possible flooding of many low-lying and coastal cities, and severe inundation of several islands.
Next to the well-documented concern for coastal and lowland flooding risk, another yet under-reported impact of sea level rise will be on the freshwater systems. When the freshwater level drops lower than the equilibrium in coastal aquifers, saltwater with higher density, thus pressure, is allowed further in land and salinize groundwater resources. This phenomenon is defined as salt intrusion (Johnson, 2007). Moreover as the sea level goes up beyond tolerable level, the interface between ground and seawater changes and intrusion risk increases, significantly impacting local drinking water availability of coastal communities. Basement and septic system failures and detrition of marshland ecosystems fed by coastal aquifers are other further hazards of the sea level rise associated with coastal aquifers are. How the sea level rise will affect in the coastal aquifers in schematised in the figure below taken from US Geological Survey sources.
Climate-related hazards threaten human-environment systems and their vulnerability increase with amplified exposure. There are wide variety of physical mitigation and social adaptation options of varying effectiveness that could be combined in dealing with reducing the pressure of sea level rise on the coastal aquifers. While physical measures are mainly barriers insulating and recharging aquifers or removing saltwater, socials measures are more about adapting behaviour such as changing or limiting withdrawal patterns from coastal aquifers. However each measure requires a definite level expertise for implementation and comes at a certain capital, operation or opportunity cost to communities at risk that are not always able to afford them [pdf]. This adds up to the immense external costs and injustices of global warming that we do not account for.
As elaborated in Chang et al. multiple factors affect the vulnerability to salt intrusion in coastal aquifers of different geological characteristics at different altitudes and sea level-groundwater dynamics has a high level of inherit uncertainty due to this complexity. The occasional mismatches in sea level rises at local and global scale also adds to the challenge of determining a rule of thumb indicator or transferable decision support tool to assess vulnerability to sea level rise and type of mitigation measure to be chosen.
Nazli Koseoglu is a PhD student from the School of Geosciences of University of Edinburgh, UK. Her PhD looks into the valuation and optimization of water use in Scotland to increase total social return. Prior to her current studies in environmental economics, she received MSc degree in Environmental Studies and BSc in Environmental Engineering. She thinks groundwater systems can not be considered in isolation from rest of the water systems and therefore wanted to contribute Life Plus 2 Meters project to highlight the linkages between sea level rise and groundwater dynamics.