Tide gauge records in the region are not long enough to give any conclusive evidence on sea level rise. Fairly long records of at least 50 years are needed because of the influence of natural variability in the climate system (Douglas, 1992).
Some of the stations, including the oldest ones such as Mombasa (1932) and Port Louis (1942) have significant record gaps, making it difficult to examine the trends with certainty.
From the available data, sea level has been rising in some stations whereas in others it has been observed to fall (e.g. Zanzibar at - 3.9 mm/yr), probably due to decadal variability.
The decadal variability can be explained by the Aberdeen record (1862-1965). There is a consistent fall in sea level during the first 23 yrs, then a consistent rise during the remaining 77yrs.
The East African region is of special interest in the aspect of sea level rise. Whilst there is worldwide trend of rise in sea level, both tide gauge and satellite altimetry (e.g. Church et al, 2006) indicate a falling trend in sea level in some parts of the region.
Some Observations on Sea Level Trends in East Africa
The falling trend in sea level, though a rare occasion, is not unique to the East African region. The sea level at Helsinki has been declining at an average rate of 2mm/yr over the past century (1879 and 2001).
There are records from a number of sea level stations which are not in the PSMSL database but can be used to observe the trend in sea level (e.g. Port Victoria, Saint Paul, Dar es Salaam, Lamu, Dzaoudzi, Reunion, etc.).
Comparison cannot be made on sea level trends at different periods. E.g., the sea level at Port Louis (1942-1947) showed a rising trend (+2.8mm/yr), that of nearby Port Louis II (1986-2003) indicates a falling trend of 1mm/yr.
Some stations have longer records than those in the PSMSL database. However, use of such data requires reducing the data to a common datum. Recommended data sets of the PSMSL are the monthly and annual “Revised Local Reference” (RLR) means.
