Sea Level Rise and the Uncertainty of Future Climate Conditions

Sea level rise (SLR) is one major consequence of global warming and the resulting climate change. Sea levels have already risen 1.8 cm/decade during the 20thcentury and this trend is expected to increase rapidly in the coming years. Sea level rise is mostly controlled by temperature and salinity changes in the oceans and continental ice sheet buildup and melting. As such, a rise in sea level is seen when there is warmer and less saline water as well as when continental ice melts.  Hu and Deser (2013) were able to analyze SLR rates for the future by using the Community Climate System Model Version 3 (CCSM3). This system was able to present 40 different climate change projections in order to understand the uncertainty that is related to the future climate variability.  The results of this experiment show the extreme uncertainty of the climate moving forward and that the SLR varies by a factor of 2 depending on global location with large increases in sea level in some areas and decreases in others. —Chloe Mayne

Hu, A. and Deser, C., 2013. Uncertainty in future regional sea level rise due to internal climate variability. Geophysical Research Letters 40, 2768—2772

Hu and Deser used the Community Climate System Version 3 (CCSM3) in order to model the rates of future sea level rise.  This system used 40 projections, called ensemble members, and began at the end of the 20th century. Each projection included similar levels of greenhouse gases, ozone, solar and aerosol over a period of 60 years from 2000—2006. In addition, the states of the ocean, sea ice, and land were also static. The differences in projections came from using atmospheric conditions found on days between December 1999 and February 2000. to encompass the variety of atmospheric conditions that are plausible over the next 60 years. It is, of course, impossible to predict future precisely, but this model is able to represent an array of future possibilities for sea level rise as greenhouse gas concentrations continue to rise and add to global warming.
The result of Hu and Deser’s study looked at the mean SLR over the last 20 years of the 60-year period. The global mean SLR was very similar for most cities at around 11+/- 0.2cm for the 20 year period.  On a more local scale, the regional SLR varied by a factor of 2. An example of the local scale SLR can be seen when looking at the large range for San Francisco, which was 4.3-9.6 cm.  Some of the ranges seen showed a peak in the distribution frequency (a large range for the SLR) while other cities showed no peak at all, with a constant rate of SLR.
The geographic distribution of the sea level projections was found to be positive across the globe, except for in the Southern Ocean. In the North Atlantic and Arctic the rate of SLR was projected to be around 5 cm/decade while only about 1-2 cm in the Pacific and 2-3 cm in the Atlantic. These numbers are due to the increase in the amount of heat that the ocean can store as less heat is being lost to the atmosphere. Most uncertainty is found in the high and middle latitudes around the Arctic Ocean, Southern Ocean, North Atlantic and North Pacific. Less uncertainty is found in areas that are tropical as well as in the North Indian Ocean.
Sea level rise is also associated with changes in ocean circulation, which are considered a dynamical component. These changes occur as a result of wind and buoyancy forcing.  Looking at two specific projections for SLR, it is possible to see the trends of sea level pressure, SLP, which is a measure of wind forcing. The results showed that SLP was weaker or stronger depending on which ocean basins were considered, probably resulting in different ocean circulation patterns and differences in sea level rise.  In addition to wind, buoyancy forcing, as a result of the Atlantic Meridional Overturning Circulation (AMOC), plays a large factor in ocean circulation. The AMOC moves warm waters north where they become cold and dense and sink deep down into the ocean. These waters then eventually move south and are cycled through the ocean conveyor belt. As GHG emissions continue to grow, it is expected that the AMOC will slow down and sea levels will increase in the Atlantic without the cold water in the north being forced deep into the ocean.

Disregarding ice melt, the primary increase in sea level rise was found to be expansion of seawater as the temperatures rise but it is clear from their study that there is a great amount of uncertainty as to future SLR. 

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