To make progress in the chain of impacts of climate change and ocean acidification on human communities, the focus is put on the impact of physical disturbances on land (soil, water, flora and fauna) and marine resources (reef and fisheries) of low-lying islands and coastal plains of high mountains islands.

Land resources are going to decline as a result of various processes. Firstly, the increase in atmospheric temperature leads to increased evapotranspiration, causing the soil to dry and an increase in the consumption of brackish shallow groundwater by plants. This groundwater absorption should not be overlooked. The coconut tree restored at least 150 litres of water per day to the atmosphere through transpiration. Under these conditions, the expected increase in groundwater pumping by coconut trees and other types of vegetation should significantly strengthen the pressure that is exerted on these reserves that are already used by humans to meet there needs. The degradation of the quality of the soils and the decreasing water resources will further reduce the possibilities of cultivation. Consequently a drop in production should arise, especially for island agriculture, representing a serious challenge regarding food security. An increase in external dependency will follow, especially for rural atolls in many coral archipelagos. Soils will also tend to degrade under the effect of salinization due to rising sea levels and more frequent coastal flooding on the islands and coastal plains that cannot elevate. Moreover, few edible plant species tolerate salt, even though coconut tree can support salt up to a certain threshold beyond which they die. The reduction in exploited areas, especially coconut groves, should reduce the availability of building materials. Also, the gradual evolution of island farming practices towards species that are less resistant to climatic and marine pressures than indigenous species – for example the banana tree being less resistant than the pandanus and the coconut trees – may increase the magnitude and frequency of food shortages (this is what happened for example in the Maldives following the damage caused by the tsunami in 2004) and trade deficits (the case of the West Indies following the passage of Hurricane Dean in 2007) in the future.

Climate change will cause quantitative and qualitative changes in water resources, which depend on several factors. The most important is the sea level, whose elevation will inevitably reduce the volume of underground freshwater reserves. According to the principle of Ghyben Herzberg that governs the functioning of aquifers, any rise in sea level causes a reduction in volume. More frequent or even systematic coastal flooding during high spring tides, are the source of repeated intrusions of salt water into the groundwater, thus contributing to the deterioration of its quality. The islands and coasts under strong coastal erosion should be more affected by the decrease in the volume and quality of underground lenses. Another important factor is rainfall, which determines the rate and frequency of recharging the underground freshwater lens and rivers that cross the coastal plains. To date, there is no reliable mean of forecasting the evolution of rainfalls. Moreover, there are still uncertainties regarding the freshwater resources of certain high islands. It is thus impossible to identify the islands and archipelagos that will be most affected by the degradation of water resources. A reduction in the volume of available water is to be expected in areas where droughts will be more frequent and/or drawn-out. Consequently, the water will become more salty, causing the increase in the frequency and severity of crop mortality peaks (for coconut and taro, in particular) which are already being observed. The removal of water from the groundwater during a drought has the further effect of reducing its thickness, which means that in periods of water shortage, groundwater, which is crucial for the survival of many islanders, may become unfit for consumption. As rainwater tanks on the islands become empty when the drought lasts, this issue could undermine the habitability of certain low-lying islands. Individual access to water should also decrease as a result of the high population growth characterizing these areas.

As stressed in the International Panel On Climate Change (IPCC) reports, there is currently very little information concerning the impacts of climate change on the distribution of fishery resources. The strong pressures that are already at work on coral reefs in some of the most populated areas should increase everywhere where population growth remains strong. As different factors in these areas contribute to the degradation of reefs, available reef resources per inhabitant will decrease. Moreover these resources play an important role in the daily diet of islanders, including the islands where the need for imported products is high. This is even more an issue when considering that the possible changes in ocean currents might reduce the presence of pelagic species in certain ocean regions, there- by preventing the consumption transfer on these species. The fishing industry as a whole is therefore being questioned, from the natural resources to the fishing means (ships, ports, etc.), the latter also being destabilized by rising sea levels, extreme events and other sources of stress (economic crisis for example). On top of this, overfishing leads to severe reduction in fish stocks in coastal waters and lagoons as well as offshore.

Even if island systems will have a differentiated response to the signs of climate change and ocean acidification, and despite the uncertainties that remain, it is clear that environmental constraints, which are already strong, are still going to increase. As a consequence, the already limited island resources are to decrease or to become more random than today. Therefore the viability of certain reef islands and island states themselves might eventually be challenged. However, at present the main threat for the sustainability of these islands is unsustainable development that has, over the past few decades, degraded the resources and reduced their resilience to natural pressures. In other words, the main challenges nowadays in coral islands and coastal plains reside in pollution, land disputes, depletion of natural resources, etc., and not only the effects of climate change and the ocean acidification. This conclusion is not a denial that climate change and acidification have and will have a major impact, but it is rather a justification that existing insular communities are going to have to meet a challenge that is yet unmatched with the disturbances that they are already facing today. With relatively poor flexibility, they will have to deal with the impacts of climate change that will in turn be multiplied by the environmental disturbances of recent decades, the latter having greatly increased the vulnerability of ecosystems. Under these conditions, climate change and acidification will act as accelerators of the impacts of current developments. By reducing the area of the islands in a context of high population growth, climate change will in certain cases, generate land conflicts. Furthermore, by generating a decline in reef resources while the need for food is increasing, climate change and acidification will most likely accelerate the deterioration and death of reefs in some areas. The pressure on water resources will also increase. In total, it can be expected that the concentration of the population will increase in the capital cities that are currently the only areas to benefit from alternative solutions (desalinated water, imported food). This will not be without consequences, notably on food security and human health.

It is now feared that due to the combination of the effects of unsustainable development, climate change and acidification, certain archipelagos will no longer be inhabitable within a few decades. If immediate proactive policies could be triggered for the readjustment of territories, for environmental protection and for the modification of the relationship between human communities and their economies and the marine and coastal resources, a major step forward would be made towards adaptation to climate change and ocean acidification. The identification of anthropogenic pressure factors presently at work finally provides many clues for imagining and starting to implement adjustments to environmental changes. Human responsibilities are powerful levers that must be used to reduce future threats.

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