9.3 Sea Level
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Sea level is the average level of the surface of a given body of water on the surface of the earth. Water level is the height of water relative to some reference point.  Sea level changes on short and long term scales. Over the short term periods, tides are the most significant change in sea level, but storms also short term influences of sea level. Coasts can be storm-dominated or tide-dominated, which affects the relative sea level. The relative sea level is the position of the sea relative to the land.  Land can emerge from or submerge into the crust, which affects how we view sea level. Emerging land makes it appear that the sea level is decreasing relative to the land while submerging land makes it appear that sea level is rising relative to the land [2, pg 222].
These relative changes affect storm and tide sea-level changes. The definitions of a storm or tide-dominated coasts are relatively straightforward: storm dominated coasts are controlled by storms and tide-dominated coasts are controlled by tides. Storms and tides control sea level, as well as other geologic processes such as sediment deposition. 80% of coasts on Earth are storm dominated, 17% are tide-dominated, and the remainder is dominated by ocean current [2, pg 217].
Post-glacial sea-level rise records throughout thousands of years allow us to see changes in sea levels across the globe (https://en.Wikipedia.org).
Recording sea level is important because that is how elevation and depth of the oceans are calculated. Scientists can also use sea level to keep track of how other climate factors are changing. Though natural global cycles contribute to sea-level rise over long timelines, anthropogenic activity is speeding up the process significantly. The burning of fossil fuels and other human activities contributes to an increased percentage of greenhouse gases in the atmosphere, which accelerates the ocean warming.
There are two main factors of global warming that contribute to sea-level rise:
1. Melting Ice
While melting ice does add water to the ocean system, this only changes sea level when the ice was previously out of the water (i.e. landlocked ice, glaciers, ice caps) since ice takes up more volume than the water it contains.
Ice also has a higher albedo (reflectivity) than water, meaning that it can reflect more light than the open ocean does. Once the ice melts into the water, there is less ice to reflect and more to absorb heat, this creates a feedback loop where the oceans get warmer and the ice continues to melt, causing the oceans to get warmer, and the cycle continues.
2. Thermal expansion of water
When seawater warms, the particles become more spread out due to the higher kinetic energy, and this increased volume causes the sea level to rise.
Global mean sea-level history and projections are depicted in the chart above using geologic tide gauge data and satellite data. (https://en.Wikipedia.org)
The mean sea level of the ocean is measured over a long period and may help identify the effects of global climate change and anthropogenic effects.
Since 1900, the sea level increase has been recorded to be between .04 and .1 inches per year, while the increase since 1992 has been measured at .12 inches per year. This seemingly slight increase is quite large due to the compounding effects. Global warming has been affecting so many things from agriculture to marine life and the rising of modern sea level will affect many of these factors of our lives. Sea level rise will lead to a drop in available land space for human habitation, and may eventually cause flooding of islands and coasts.
The History and Future of Sea Level Changes
Sea levels have changed drastically throughout history due to the changing temperature of the Earth. During glacial periods ice sheets were much more extensive than they are today. Conversely, interglacial periods are periods of warmer temperatures and less extensive ice sheets. We are currently in an interglacial period. Sea levels have risen and fallen hundreds of feet between the periods. During the last glacial maximum, the Late Glacial Maximum around 14,000 BCE, sea levels were 400 feet lower than they are today due to water freezing in glaciers and continental ice sheets. Currently, sea levels are rising faster than they have in the past 6,000 years. Sea levels have risen 5-8 inches since 1900, and the rate they are rising per year is increasing. As long as atmospheric carbon dioxide levels continue to increase at the rate they are and the Earth keeps warming, sea levels will continue to rise. The Intergovernmental Panel on Climate Change (IPCC) predicts that sea levels could rise to 1 meter by the end of the century. If they do rise 1 meter, New Orleans, Miami and many other cities on the coast will be completely under water.
Risk Zone Maps: Interactive maps that simulate predicted sea rise levels:
https://ss2.climatecentral.org: Surging Seas, Risk Map Zones
https://vesl.jpl.nasa.gov/: Virtual Earth System Laboratory
https://coast.noaa.gov: NOAA: Sea Level Rise Viewer
Videos: further explore the basics of sea-level rise as well as causes, effects, and impacts:
https://www.youtube.com: AsapSCIENCE and the role of thermal expansion in rising sea levels.
https://www.youtube.com: “What If All The Ice Melted on Earth” ft. Bill Nye
https://www.youtube.com: “NASA’s Earth Minute: Sea Level Rise”