Q# 2: Summarize and explain factors that control relative sea level.Pirazzoli (1996) groups the causes of sea-level rise into three main topics: changes in the quantity of ocean water, deformation of the shape of the ocean basin, and variations in water density and dynamic changes affecting the water masses. In analyzing the components of the hydrological cycle, Pirazzoli (1996) points out that the volume of ocean water can increase from ground water due to overpumping. This can lead to relative sea-level rise through an increase in the volume of ocean water from ground water that was deposited within the earth years prior being released into the oceans as runoff. Changes in ocean water volume that can lead to relative sea-level rise also include deforestation and draining of wetlands (Sahagian and Zerbini, 2002).
Regional subsidence, uplift, and changes in isostasy can deform the shape of the ocean basin, influencing relative sea-level rise. Being that the Earth is composed of layers varying in density, these layers move and react at different time scales. However, sea water reacts instantaneously and the lag time reaction of the land aids in relative changes in sea level (Pirazzoli, 1996). Relative sea-level changes over the past hundred years has been attributed to the subsidence and uplift caused by loading and unloading during the Little Ice Age in the coastal area of Juneau, Alaska (Motyka, 2003). Thomson et al. (2002) discuss how drought conditions in the Mississippi River Delta Plain lead to primary production stress by increasing the salinity and decreasing the amount of sediments delivered to the delta. This study further discusses how the lack of primary production can decrease coastal stability turning vegetated marshes into unvegetated mudflats, which would exacerbate the rate of relative sea-level rise (Thomson et al., 2002). Overpumping can lead to land subsidence, causing relative sea-level rise (Pirazzoli, 1996). The compaction or subsidence of sediments with a constant water level would aid in a relative sea-level rise decreasing the elevation. Subsiding Holocene sediment deposits in the Mississippi River Delta area were found to have doubled the rate of sea-level rise (Penland and Ramsey, 1990).
Thermo-isostasy is a term used to describe the increase in density that occurs at the mid ocean ridges. When new material is produced and extrudes from the mid ocean ridges, it cools on the ocean crust, increasing in density and subsiding the material underneath (Pirazzoli, 1996). This occurs in tropical waters where corals grow and increase in size, as the ocean floor beneath subsides. The depression of material due to lava repeated extruded has been termed volcano-isostasy (Pirazzoli, 1996). As the lava flows and cools, the material becomes dense and depresses the underlying material.
Conversely, areas undergoing tectonic uplift or isostatic rebound, due to glacial retreat, would exhibit a decrease in relative sea-level is the rate of uplift and rebound is greater than the rate of sea-level rise. In Juneau, Alaska there has been debate on whether the rising earth is caused by tectonic uplift or isostatic rebound from deglaciation (Motyka, 2003). Motyka (2003) dated the locations of coastal forests and marine terraces finding a correlation between uplift and regional glacier retreat during the mid 18th century, identifying glacial retreat and expansion as the primary mechanisms responsible for subsidence and uplift in the Juneau area during the last several hundred years.
Changes in sea level are affected by changes in sea water density. Sea water density controlled by the temperature, salinity, and pressure of sea water. Density of sea water increases with depth and with increasing salinity and pressure. However, as temperature increases, the density of water decreases. Lower density sea water requires a larger volume due to the thermal expansion of water molecules. Over 400 m an increase of 1°C or a decrease of 4 ppt in salinity would cause a 60 cm rise (Pirazzoli, 1996). With temperature increases caused by climate change, thermal expansion is thought to be more influential in sea-level changes. In a study performed by Lombard et al. (2005), the authors looked at thermosteric sea-level changes, changes in sea-level cause by temperature alone, over a 50 year period (1948-1998). Lombard et al. (2005) found regional sea-level trends from 1993-1998 were mainly explained by thermal expansion. The authors also found that thermosteric sea-level trends have a decadal lifespan and are influenced by El Nino Southern Oscillation, Pacific Decadal Oscillation, and North Atlantic Oscillation.
Works CitedLombard, A., Cazenave, A., Le Traon, P., and Ishii, M. 2005. Contribution of thermal expansion to present-day sea-level change revisited. Global and Planetary Change. 47, 1-16
Motyka, R. J. 2003. Little Ice Age subsidence and post Little Ice Age uplift at Juneau, Alaska, inferred from dendrochronology and geomorphology. Quaternary Research. 59, 300-309
Penland, S., and Ramsey, K. E. 1990. Relative sea-level rise in Louisiana and the Gulf of Mexico: 1908-1988. Journal of Coastal Research. 6 (2), 323-342
Pirazzoli, P. A. 1996 Sea-Level Changes: The Last 20,000 Years. John Wiley an211 pps. 211pp.
Sahagian, D. and Zerbini, S., eds. 2002. Global and regional sea-level changes and the hydrological cycle. Global and Planetary Changes. 32, vi-viii
Thomson, D. M., Shaffer, G. P., and McCorquodale, J. A. 2002. A potential interaction between sea-level rise and global warming: implications for coastal stability on the Mississippi River Deltaic Plain. Global and Planetary Changes. 32, 49-59
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After lingering on that thought for awhile, a gentleman approached the microphone and the room became quiet. There were a few introductory speeches and then 
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