An important implication of climate change is the potential for rapid sea-level change. With recent warming trends, the change will almost certainly be a rise. There are two main mechanisms for sea level change: mass change or density change. Mass change is a result of altered levels of water availability. The primary source of water that adds to current sea-level rise is land-ice melt2
. Accelerated levels of glacial melt will increase the rate of sea-level change. The second source of sea-level change is more directly linked to temperature. As air temperatures increase, so too will sea surface temperatures. Thermal expansion accounts for approximately 90% of the density change and freshening (lowering salt concentration) accounts for the other 10%2
There are two main sources of land ice: the Greenland ice sheet and the Antarctic ice sheet. Combined they contain enough ice to raise sea levels by about 64m if they melt completely2
. Currently melt from these ice sheets and smaller glaciers contribute about 1.2mm2
rise per year, but this is expected to accelerate as positive feedback effects alter the way in which the local climate behaves. The Greenland ice sheet has recently been contributing 0.75mm per year to sea-level rise, but it's rate of melting has been accelerating at an alarming rate5
. The West Antarctic ice sheet is also experiencing accelerated melt rates. The East Antarctic however is growing by 45 billion tonnes per year, which is equivalent to slowing sea-level rise by 0.12mm per year6
. Increased precipitation on the ice sheet is the suspected cause of the growth, and it is expected that precipitation will increase over the East Antarctic ice sheet as atmospheric temperature continues to rise6
The temperature of the ocean varies based on both depth and latitude, as does salinity. Additionally, the amount that water expands when heated (or contracts when cooled) increases at higher temperatures. This means that thermal expansion will have a greater effect on sea level closer to the equator where water is warmer. Expansion due to lowered salt levels will be more pronounced closer to the major fresh water inputs, the ice caps at the poles. Initial changes will be on the surface, as deep water mixing occurs only in two main locations, one in the North Atlantic, and the other in the South Pacific.
The instrumental record of sea level reaches back to the 1870s, but is less accurate than the satellite records that we use today. Measurements from before the 1990's are based on tidal gauges, which lack the precision of the satellite measurements, both because the measuring stations are only on land, and thus lack mid-ocean data, and because the tidal measurements are based on sea-level relative to land which is not constant. The positions of the measuring satellites however are precisely tracked, and their orbits monitored2
. The figure below shows both the available data on sea level and the projected sea level change both before and after the instrumental record. The width of the band indicates level of certainty, thin being high certainty and thick being low.
Of concern: The direct effect of sea-level rise is clear: higher water levels will inundate low lying land. This can be mitigated at great cost by creating structures to hold back the water. The Netherlands provide the best example of a country already at great risk of flooding. The projected extra
annual cost for the Netherlands to sustain their current infrastructure, and protect against further rise is $2.2 billion7
. Most countries have not got the infrastructure in place to protect themselves. Bangladesh for example could face losing up to 15% of it's land area by the year 21008
. As sea-levels increase coastal land will become less useful even before they are inundated. Again in Bangladesh it is expected that increased soil salinity could reduce crop output significantly8
. About 10% of the world's population live in low elevation zones (10m or less above sea-level) slightly over half of these people are concentrated in urban areas4
While the direct effects of sea level change will not affect Canada (The basis of the report was to write a letter to a hypothetical politician providing an explanation that a layman could understand)
as much as many countries, the indirect effects: massive amounts of environmental refugees and reduced food availability, will tax our own resources, and damage our economy. More importantly, global resource strain, and mass movements are very likely to cause political instability, and it is possible that Canada, being a rich country, both in resources and material wealth, could be resented for it's comparative lack of problems.
Uncertainties: There are few uncertainties remaining regarding sea level change. As technology has improved, so too has our ability to accurately measure the relative level of the ocean. In both the 42 and 10 year calculations there has been an under prediction of sea-level rise, though the difference between calculated and observed has narrowed considerably since the advent of the information age.
Ocean currents are controlled by both variations in temperature and salinity. The alteration of the temperature and salinity gradients the Atlantic Meridional Overturning Circulation (MOC) could decrease as a result of both warming and increased desalinization2
. As the MOC is strongly related to local climate, this could result in drastic changes to the local climate2
My opinion: It seems pretty clear that ocean levels are continuing to rise. Highly accurate satellite observation over closely matches ground based measurements. The continuing rise is clear,as is the acceleration of that rise. Satellite monitoring of the oceans should continue, as future policy decisions should be made based on what is happening, rather than past projections of future events. More research should be placed in holding flood waters at bay at a relatively low cost, as there are many vulnerable cities which need to be protected, and many of these are in fairly poor countries. The MOC should also continue to be monitored as our current data resolution is too low to make any decent predictions about it's potential changes in behaviour with respect to temperature and salinity2