Coastal communities around the world recognize that rising seas will lead to increased flooding and mostly negative impacts along their coastlines.  Planning for rising seas  mostly focuses on overland flooding; most have not started, nor even know, to consider how rising seas will also impact their groundwater. Apart from places such as Miami and Hawaii in the USA, where groundwater flooding happens today, this is a hidden vulnerability for most coastal communities.

The famous diked coastline of The Netherlands is a vision into a future with higher sea levels – walls and pumps, powered by wind or other energy sources, can keep low-lying land from groundwater flooding up from below. After hearing many questions and requests from coastal practitioners across California thinking about climate planning, we started off on our study, Increasing threat of coastal groundwater hazards from sea-level rise in California, to understand which portions of California’s coast could face hazardous rising water tables in the next century or so. We also wanted to understand why certain areas along any coastline would be more at risk than others.

Before we take you too far down the path of appreciating the very real importance of groundwater in this sea-level rise story, overland flooding in low-lying areas is still the dominant threat to coastal communities, resources, and infrastructure. But water tables can rise with sea-level rise and are hidden from view by our concrete jungles, scarce observations, and lack of prior studies. As water tables rise, buried and other infrastructure that was built for today’s less soggy ground may no longer work as planned. This list of infrastructure unfortunately includes some ways to defend against overland flooding – if we ignore groundwater, flooding may still happen behind our existing or future defenses.

Our biggest challenge with this study is that groundwater is hidden and observations are few and far between. It lurks underground, and the physical aspects that control how deep the water table is anywhere underground are not available for most places. One reason for this is that the shallow groundwater that rises with sea-level rise is not frequently used for drinking water, at least not in California, making it of less interest when trying to keep surfers hydrated.

It may be tempting to see this new information as yet another problem communities are having to understand and address, exacerbating the risk that communities will throw their hands up in the air and give up. We suggest that this new information on groundwater responses provides a missing piece of the climate puzzle that communities can use to make more robust decisions about their futures. Our team is continuing to partner with other groundwater scientists and policy-makers in several regions in the USA to increase visibility of this hidden threat and to continue to build on the science with the goal of helping communities around the world to plan and prepare.

Befus, K.M., Barnard, P.L., Hoover, D.J. et al. Increasing threat of coastal groundwater hazards from sea-level rise in California. Nat. Clim. Chang. (2020). https://doi.org/10.1038/s41558-020-0874-1

Cover photo by Greg Jurgajtis on Unsplash