Sea level rise is one of the many consequences of global climate change. Many coastal cities and island regions at or near sea level face an imminent threat from sea level rise (SLR) within the next century. The number of people affected by sea level rise is only expected to increase as coastal erosion, high tides, and flooding affect infrastructure in coastal areas. Investigating sea level changes in the past will provide a better understanding of future sea level rise and associated threats initiated by climate change. When we contacted Christina Toms, Senior Environmental Scientist with the SF Bay Regional Water Quality Control Board, she noted that “...much of the damage from sea level rise will come before long-term sea level rises above a particular threshold; it will instead come from episodic, localized increases due to (more intense) storms/waves on top of elevated sea level baselines.” This damage will include flooding during “king tides” and storm surge, which will increase in impact and severity as overall sea levels increase.
Our class discussed a 2009 paper by Dr. Glenn Milne and colleagues that outlines the causes of sea level rise and the evidence used to reconstruct how sea level has changed in the past. Records of past sea level include: changes in oxygen isotopes in fossil corals and other animals, variations in patterns of fossil deposition, coastal archeological studies, and evidence provided by wave cut terraces. Glacial ice tends to have a different oxygen isotope values compared to seawater. As a result, when glaciers melt and contribute to higher sea level, the isotopic signature of seawater will change. This in turn is preserved in the shells of microscopic organisms and corals, and these recorded signatures have been changing over the past 20,000 years. For example, records from the Sunda continental shelf in Southeast Asia show changes are consistent with sea level rise between 21,000 and 14,000 years ago. In more recent times, the placement of ancient Roman fisheries relative to modern sea level shows sea level rise along the Mediterranean coast over the past 2,000 years. However, this change is not considered globally significant when isostatic factors (discussed below) are taken into consideration. The class also discussed the many long- and short-term geologic processes that affect sea level rise. In the short-term, melting ice on land and thermal expansion of existing water are the major drivers of this phenomenon. When land ice melts, it flows into the oceans, adding water (thus more volume) to the ocean basins. Additionally, temperature influences fluid density: warmer temperatures cause fluids to expand and colder temperatures cause fluids to contract. Warmer, less dense, ocean water experiences thermal expansion (warmer water occupies more space), thus contributing to sea level rise, for example during the most recent ‘warm blob’ oceanographic conditions in the past several years. “[tide] gauge data from the summer of 2015 provided a remarkable window into what future sea level rise might look like in the [San Francisco] Bay, and especially how it would impact ecosystems and localized flooding,” says Christina Toms. Some of the long-term geologic processes that affect sea level rise are the movement of tectonic plates and glacial isostatic rebound. In areas near the location of the melting ice sheets, sea level fall has often been observed as a result of glacial isostatic rebound. Glacial isostatic rebound is the rise of land masses that were previously weighed down by large ice sheets. For a point of comparison, think of how an inflatable raft sits lower in the water when people are using it, but pops back up when weight is removed. A similar situation occurs with continents: when land masses are covered with ice, they sit lower in the mantle (an interior layer of the Earth that acts as a liquid). When ice is removed (melts), continents will pop up, or rebound, to float at a higher point in the mantle. Even though the amount of seawater is increasing because of the melting of land ice, sea level local to the glacier may stay the same because the entire land mass is gaining elevation. Land masses not experiencing isostatic rebound may experience sea level rise and higher erosion rates. Additionally, variations in Earth’s gravity over geologic timescales can also affect global sea levels. During a period of weaker gravitational forces, gravity will exert less force on the oceans, which will decrease the pressure on the water column and encourage more water to pool locally. To measure present-day sea level rise, we use satellite technology (OSTM/JASON-2; GRACE), buoys (the Argo network), and tidal gauges monitored by the USGS and NOAA. In general, sea levels are rising; average global sea level has been rising over the past millennia due to long term geologic processes, but at a much faster rate over the past few decades. Some reef islands in the Solomon Islands have already been lost to a combination of sea level rise and erosion due to rising ocean levels. Islands in Alaska and the Gulf Coast of Louisiana, as well as communities in Virginia and Florida are just a few more examples of coastal communities that are already dealing with changes in sea level, and California has issued a new report on the rapidly growing evidence and knowledge about how this process will impact the CA coast. Taken together, knowledge of modern sea level rise and archives of past change allow us to understand sea level change on a range of timescales, future risks and potential management and mitigation tactics. For more information: Video on NASA sea level research California OPC report on sea level A UCD student blog post on sea level Written by the members of UC Davis GEL 232: K. Barclay, R. Banker, P. Edwards, C. Fish, K. Hewett, T. Hill, G. Hollyday, C. Livsey, H. Palmer, P. Shukla, D. Vasey. Comments are closed.
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ArchiveSea Levels: Past, Present and Future
How has El Niño changed in the past? Lessons from paleoclimate archives Paleoclimate into Policy: is there a bright future for learning from the past AuthorsWritten by the members of UC Davis GEL 232: K. Barclay, R. Banker, P. Edwards, C. Fish, K. Hewett, T. Hill, G. Hollyday, C. Livsey, H. Palmer, P. Shukla, D. Vasey. Categories
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