Coastlines facing the prevailing trade winds retreat through time, while those in leeward positions expand. Coastline expansion and retreat are in balance such that total land area of all the considered islands is virtually static over the last 50 years. Small islands (<20ha) are substantially more dynamic than large ones. Sea level rise data from BIOT are limited and largely focused on the largest island, Diego Garcia, which shows that while island coastal margins are dynamic, the change in dry land area does not appear to be significant. Smaller islands of less than 0.5ha in BIOT however are potentially more exposed to erosion and loss of land area.
Whilst there is little observed net change on the islands, coastal erosion is observed across the atolls, particularly on ocean-facing shores. Here, man-made coastal structures and loss of coastal vegetation may be exacerbating coastal erosion. There are economic implications of breaches from the sea into the lagoon in Diego Garcia, due to the potential damages to infrastructure and goods and disruption of operations associated with the military base.
During October and November 2019, extreme water levels caused significant coastal erosion, flooding and deposition of sand and rubble, mostly affecting Diego Garcia, and to a lesser extent across the northern atolls. These extreme high sea levels were attributed to a combination of seasonal high tides, rising regional sea levels due to climate change, and a positive phase of the IOD. Positive IOD events are also linked to a temporary and regional sea level elevation further contributing to the local impact.
Degradation and loss of coastal vegetation, due in part to erosion, increased sea water incursion and droughts, as well as human impacts, compound the situation, and current policy is to reduce coastal vegetation removal as much as possible.
High evidence, medium agreement
More regular monitoring of change in reef islands, particularly accurate shoreline monitoring is required to improve confidence.
BIOT islands are likely to display multi-decadal stability. However, sea level rise will exacerbate patterns of island mobility on islands <20ha, eroding where exposed to wave energy, and expanding to their leeward sides. Human modifications on Diego Garcia will amplify these dynamics and margins are more likely to overtop and erode than on uninhabited islands. Crests of fringing reefs may not be able to grow and keep pace with future rising sea levels, further increasing shoreline wave exposure. Coastal vegetation also plays a critical role in island stability but loss of plants and trees due to coastal salination of freshwater and drought will exacerbate erosion of island margins. Consequently, there is a potential risk of damage to infrastructure. On the northern atolls, there is also a progressive risk of damage and loss of cultural heritage sites if island migration rates increase under SLR.
By 2100, extreme positive IOD events are projected to become almost three times as frequent. If BIOT island fringing reefs suffer further degradation and mortality due to bleaching, they will lose their ability to provide wave attenuation and expose the small low-lying islands to raising sea levels as well as extreme high sea level events related to storms, particularly as storminess increases with more extreme IOD events.
There will also be localised impacts on important nesting sites for seabirds and sea turtles due to erosion and beach mobility. Furthermore, temperature change may skew turtle sex ratios in hatchlings at internationally important nesting sites, and marine heatwaves can impact on hatchling success.
High evidence, medium agreement
A greater understanding of BIOT island dynamics and changes in coral reef sediment supplies, including understanding how and when the islands were formed, would increase predictive capability of island mobility. New research on projected erosion and deposition patters on 10, 25 and 50 year timeframes will help inform long term planning.