Tides and Sea Level 2025

The Guide contains a full summary of the scientific findings. The following is an extract of the new Section:

Projected sea water flooding at Diego Garcia out to 2130

1.  On longer timescales of decades to the next century, sea-level rise due to global warming and the consequential expansion of the oceans and melting of land based ice sheets is likely to become an increasing problem for low lying islands such as those in the Chagos. Because storm surges and regional climatic factors are stochastic events their influence is not included in the longer-term projections made in this section, but their significant impacts should not be discounted.

2.  By 2130, which is approximately the duration of a 99-year lease for the US base on Diego Garcia, mean sea level under two realistic IPCC warming scenarios (SSP2-4.5 and SSP3-7.0) is projected to rise by 820 mm and 1050 mm respectively. The effect is that flooding levels seen in March 2012 in the south of the island would occur 31% and 48% of the time (i.e. twice a day on every high tide) as opposed to the current infrequent incidence (0.7%) and the depth of water over the submerged areas would reach over 1 m.

3.  By 2130 the US base small boat basin fixed jetties would be submerged under both scenarios for the same time period (31% and 48% of the time by a water depth of 0.06 and 0.56 m respectively). Submerged areas of the island could be considerable particularly under the SSP3-7.0 scenario. The low-lying southern areas of the atoll and the main road to the space tracking facility and areas around East Point Plantation are particularly likely to be submerged for considerable periods of time.

4.  The airfield runways (3.96 m above the 2024 mean sea level) would remain above water at all times and the deep-water wharf would also remain above the flooding level although the effective wharf height would be reduced from ~2 m to 1.34 m (SSP2-4.5) and 0.9 m (SSP3-7.0) respectively.

5.  By 2130 flooding in the “Downtown” area of the US base where accommodation and administrative buildings are situated will occur even in calm conditions under both SSP2-4.5 and SSP3-7.0 scenarios. Under the latter, flooding would be present on any tide level above the present Mean High Water Neaps.

6.  Although the present ‘hard shoreline engineering’ on the seaward edge of the Tank Fuel Farm is likely to protect it from overtopping, there is a risk of flooding in this area from the lagoon shore or from around the edges of the concrete sea wall. The same applies to this type of shoreline structure if implemented elsewhere to protect against overtopping and flooding. Defences such as this will be prohibitively expensive and ultimately ineffective.

7.  Without significant infrastructure changes in the medium term (50-100 years), island life would become progressively more difficult and the military operation of the base compromised. Given the steady time based and accelerating increase in sea level, these difficulties are unlikely to be realized until the latter half of this century (e.g., by 2050 the projected rise in sea level is only 23 cm under SSP2-4.5 and 24 cm under SSP3-7.0).

8.  Much of the island vegetation will be killed by repetitive saltwater immersion and the island freshwater lens, which supplies the US base, will be damaged and possibly destroyed.

9.  Given the likelihood of the future degradation of the reefs surrounding Diego Garcia due to sea temperature bleaching of corals and man’s interference with the natural ecology and morphology of the island, any prospect that natural processes will act to combat climate change and rising sea levels seems very slim.

Richard Dunne

13 January 2025

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