This post belongs to a special series of posts written by students in Dr. Simon Engelhart’s Coastal Geologic Hazards course at the University of Rhode Island. In this course students learn about coastal processes, including storm surges and sea level rise, and how these impact people and the environment.
Do you belong to the 40% of the global population that lives with 100 km of the coast? You’ve probably thought a bit about how projections in sea level rise will impact your home, community, and lifestyle. This is understandable; sea level rise was expected to occur along all coasts at a similar rate globally (Global Sea-Level, or GSL). Well, it turns out, the most recent studies show that the water will rise at different rates, and different regions of the planet may experience significantly more (or less) sea level rise than others.
Research has shown that three factors contribute to what we now call Local Sea-Level Rise (LSL). First, as the water warms because of rising temperatures, it expands and takes up more space. Warming temperatures also melt ice and while ice in the ocean doesn’t contribute to rising water levels, it does melt ice on land, that is, glaciers. This eventually ends up in the ocean, raising the water level. Think about a glass of water with ice cubes — as the ice melts the glass will never overflow. However, if you add more water to an already full glass, then it will overflow. To complicate matters, an effect called subsidence/uplift can also change sea-level rise, depending on location. Subsidence is the effect that in actuality some areas are sinking, and uplift is the opposite: some areas are rising.
A group of researchers from Rutgers University, led by Robert E. Kopp, used a set of mathematical models to predict future sea-level rise taking into account these different factors. The researchers published a list of their predictions for major coastal cities in an article ‘Probabilistic 21st and 22nd-century sea-level projections at a global network of tide-gauge sites‘. The team predicted different rates of sea-level rise in their study depending on different scenarios for global rates of carbon dioxide gas emission. Even if sharp global carbon emissions cuts were to put in place immediately, the models predict that average global sea level will increase in the future. Kopp and his team show that the sea level around each city in the study has a different level of change, ranging from +0.5 to +1.2 meters by 2100. The factors depend on where the melting ice comes from, the thermal expansion (which won’t be the same everywhere), and whether each area experiences subsidence or uplift.
Interestingly, the rate of sea-level rise is generally higher in the Northern Hemisphere. Areas around the North Atlantic are particularly impacted by meltwater from the faraway Antarctic Ice Sheet. In areas where sea-level rise is predicted to be significant (0.7 m-1.3 m by 2100), the risk of major flooding is greatly enhanced. For example, in New York City, Kopp’s paper predicts there to be nine of what was once known as “1 in 100 years” flood events by the end of the 21st century. One in hundred year means that the flooding of a particular intensity has 1% chance to happen in a particular year (not that the flooding will occur once in 100 years). Because sea-level rise can vary locally, any planning must be based on local sea level rise projections rather than the global prediction. For example, Kopp and his team warned that the removal of underground liquids around the New Orleans area could increase subsidence and therefore result in enhanced local sea level rise rates.
To help us further refine climate change models, we must look to older geologic records beyond the last ~100 years of historical tide gauge data. To demonstrate the accuracy of reconstructions from pre-tide gauge data, Andrew Kemp and his team from University of Pennsylvania have shown in their paper ‘Climate related sea-level variations over the past two millennia’ that reconstructions based on data acquired from tiny organisms called foraminifera (“foram” for short) clearly line up with historical tide gauge data. By digging cores from a salt marsh and counting different species of forams throughout the core, the researchers can tell where that interval of the core was in relation to the tide. Since certain species of forams can only survive in very specific tidal elevations in a salt marsh, by identifying the exact species that live at each elevation, scientists like Kemp can reconstruct past sea level.
Kemp’s team dug cores in a North Carolina salt marsh and brought them back to the lab to search for forams and obtain radiocarbon dates to aid in the reconstruction. They found sea level went through a few distinct phases in the past 2100 years. From around BC 100 until AD 950 sea level was roughly stable. Sea level then increased for ~400 years at a rate of 0.6 mm/year. From medieval times until about the late 19th century, sea level was stable or slightly decreasing. Since then, sea level has increased at an average rate of about 2.1 mm/year. While it may seem small, this has been the steepest rate of increase in the past two millennia, and Kemp’s team, as well as many others like his, attribute this to human-induced climate change.
Although we have a very good idea of how most of the factors behind sea level rise work, disagreements still exist between sea level records and the climate/glacial record. This has been a very controversial topic in climate science, as there are many hypotheses outlined in a paper titled ‘Lack of evidence for a substantial sea-level fluctuation within the Last Interglacial’ by Natasha Barlow of the University of Leeds. Because similar global warm periods occurred so long ago (between 116-129 thousand years ago), sea level reconstructions from those time periods can be ambiguous due to large uncertainties in the data. For example, if the reconstructions of relative sea level data were correct, 1.15-3.45 million cubic kilometers of ice would have to have formed in a period of less than 1000 years following a warm period. Barlow and her team, however, found no evidence to support this, stating that it is physically close to impossible to form that much ice in such a short time span.
Because of these discrepancies in older reconstructions, we must be very cautious in drawing conclusions about the future sea-level rise. Sea-level rise might be most simply compared to a glass of water and melting ice, but in reality, the earth is dynamic and not as simple. Although we can be confident that the glass is full and beginning to overflow, we are still working on finding predictions of how much overflow we can expect in various parts of the globe.
Barlow, N.L.M., McClymont, E.L., Whitehouse, P.L., Stokes, C.R., Jamieson, S.S.R., Woodroffe, S.A., Bentley, M.J., Callard, S.L., Cofaigh, C., Evans, D.J.A., Horrocks, J.R., Lloyd, J.M., Long, A.J., Margold, M., Roberts, D.H., Sanchez-Montes, M.L., 2018. Lack of evidence for a substantial sea-level fluctuation within the Last Interglacial. Nat. Geosci. 11, 627–634. https://doi.org/10.1038/s41561-018-0195-4
Kemp, A.C., Horton, B.P., Donnelly, J.P., Mann, M.E., Vermeer, M., Rahmstorf, S., 2011. Climate related sea-level variations over the past two millennia. Proc. Natl. Acad. Sci. 108, 11017–11022. https://doi.org/10.1073/pnas.1015619108
Kopp, R.E., Horton, R.M., Little, C.M., Mitrovica, J.X., Oppenheimer, M., Rasmussen, D.J., Strauss, B.H., Tebaldi, C., 2014. Probabilistic 21st and 22nd century sea-level projections at a global network of tide-gauge sites. Earth’s Future. 2, 383–406. https://doi.org/10.1002/2014EF000239
Jeeban Panthi is a Ph.D. student at the University of Rhode Island and is pursuing his research on saltwater and groundwater interaction in Southern Rhode Island. Like water flows from mountain to ocean, Jeeban worked a few years in the mountain region (Nepal) and then came to Rhode Island for his study. He has had a few opportunities to go on glacier expeditions and white water rafting in rapidly flowing rivers. In his free time, he loves visiting new places, hiking, gardening, and reading and reviewing journal papers.
Omar Fahmy is a Masters of Environmental Science Management Student at the University of Rhode Island. Omar is also a professional stone mason. When not working Omar enjoys surfing and spending time with his family and friends on the southern coast of Rhode Island.
Zane Grissett is in the final year of his undergraduate degree (B.S. Geology & Geological Oceanography) at the University of Rhode Island. He is currently studying the environmental effects of a 9-million-year-old meteorite impact in Argentina, but when he has some free time, you can usually find him surfing or spearfishing somewhere along the coast of Rhode Island.
Feature Image: The flooding in Charlestown area in Rhode Island after the Nor’easter in March 2018 (Photo: Jeeban Panthi).