Background Information: Sea Level and Climate Change
Atmospheric and oceanic processes have a powerful effect on changes in sea level. These changes are associated with variations in space and time of temperature, salinity, ocean currents and the combined results of the atmosphere and the ocean interaction which causes irregular patterns, such as the El Niño effect. The following are changes of sea level occur in a wide range of time scale:
- Wind Waves are periods of a few seconds.
- Tides are regular periodic cycles with periods of a few hours to many years.
- Storm Surges sometimes locally named as King Tides are temporary disturbances due to the effects of atmospheric pressure and to wind stress setting water in motion.
- Seasonal Oscillations in response to seasonal cycles of atmospheric conditions reaching prominence in the case of monsoon conditions.
- Longer Period Changes in response to climate change through thermal expansion of Mean Sea Level of the upper layers of the ocean and/or the addition of mass to the oceans by changes in precipitation [or evaporation] and/or melting of land ice.
Although the fears of global warming tend to focus upon the latter long term changes of mean sea level, all the above are relevant to some degree. For example:
The incidence of storm wave conditions could change to create more serious problems of coastal erosion or perhaps, as some scientists suggest, climate change might well cause zones of exposure to tropical cyclones to shift in latitude or increase in frequency along with storm surges as the change tends to increase atmospheric pressure gradients. Similarly, monsoon and less dramatic seasonal patterns could well be affected in ways which could see more or less frequent incursions of the ocean onto land or exposure of the beaches.
The volume of water in the oceans may increase and sea levels may rise if there is an increase in global temperature. This volume increase would be due to two factors. Firstly, if the atmosphere warms, the ocean will also be warmed slowly, and water expands as it warms. Secondly, some of the water now locked into land-based ice will melt and change into a liquid form [water]. If this occurs, some scientist predicts that ocean levels may well rise through several centimeters by the middle of the next century.
The fear is that rising sea levels may flood low-lying atolls like Kiribati, Tuvalu and the Marshall Islands. Urban centers of high Pacific countries may suffer damage from high water levels or storm surges.
If warming oceans also intensify rainstorms by speeding evaporation and condensation in the overlying atmosphere, water levels will rise even further. This is particularly true if severe storms such as hurricanes and typhoons become more frequent.
Statistics on Sea Level Rise
According to many studies, the global mean sea level may have risen by one or two centimeters since 1990 at the rate of one to two millimeters per year. Climate change (global warming) is expected to cause a further sea level rise as the years progress.
There remains much uncertainty as to the size of the sea level response despite the attention of world experts. For example, the Intergovernmental Panel on Climate Change (IPCC) met in Rome in December 1995 to prepare the Second Assessment Report which was reviewed by 2000 scientists from 130 countries over a two-and-a-half-year period. The complete assessment contained 1500 pages and 10 000 references. Based upon a 'best estimate' of the elements of climate change the report suggests that sea level may rise by 50 centimeters from the present to the year 2100. The uncertainty is such however that the report acknowledges that this estimate lies within a range of probabilities from a low sensitivity estimate of 15 centimeters to a high sensitivity estimate of 95 centimeters.
It is theorized that this rate even based upon past events and practices would continue beyond the year 2100 even if concentrations of greenhouse gases were to be stabilized in the interval. The following interesting findings have been extracted from published works:
The IPCC points out that regional sea level changes may differ from global mean values owing to land movement and changes in ocean currents. Also it is well to note that changes in the mass of the ocean would cause some warping of the earth's crust due to the geographic shape of the ocean basins. Sea level rise is unlikely to be the same everywhere.
- The tropical Pacific experienced temporary sea level anomalies of the order -30 to +30 centimeters during the El Niño South Oscillation (ENSO) period according to a study by French scientists undertaken between 1979-1990.
- Tide gauge measurements at Pago Pago in American Samoa and at Honolulu in Hawaii, showed a steady sea level rise of around 1.5 rnm per year in the last 100 years.
The combination of ocean expansion with a reduction in sea ice, possibly some melting of ice shelves and a small positive mass balance of the Antarctica ice sheet, could cause a rise in sea level of up to 40 centimeters by 2030.
Though it is true that a rise in temperature would lead to melting, there is evidence that ice from Antarctica would not melt to cause a catastrophic sea level rise! Instead, what is more likely to happen is that melted ice would return as precipitate ice. The interior of Antarctica is a desert where very little snow falls because the air is too cold to hold much water vapor. An increase in air temperature would enable more water vapor to be carried inland and to fall as snow, thus increasing the ice in Antarctica.
Monitoring Sea Levels
Forecasting sea level rise is still an extremely inexact science. In the Pacific region, sea level monitoring stations have been installed throughout the islands under the program of the South Pacific Sea Level and Climate Monitoring Project to measure relative sea level changes due mainly to climate change. In view of the uncertainties of the climate change models this is regarded as the most responsible approach at this time.
Relative sea level trends (in nun per year) at individual stations from their starting date to the present. The values are calculated based upon statistical methods at individual stations and details are available from NTF. An important point to note here is that when the sea level records become longer, the trend will be more stable and reliable. It is also to be noted that the observed trends in sea level include natural variability, events such as El Niño and effects due to several other atmospheric, oceanographic and geological processes. For these reasons, the length of the sea level data set is not yet long enough to calculate meaningful,trends. Longer term data sets for all stations are necessary to identify the effects of different signals.
However, a growing number of people in the Pacific region are concerned about the climate change and sea level rise. Consequently, they are interested to see the present relative sea level trends of the Pacific region in general and the trends of their own islands in particular, regardless of the length of sea level data. It is useful to note the impact on their lives caused by the frequent occurrence of short term events.
