Global warming of 2-4°C such as has been predicted by some scientists would affect sea levels in two main ways: through melting of polar land ice and through expansion of seawater as it warms up. Predictions on the extent of sea level rise vary from about 0.4 m to 1 m and are very difficult to estimate, given the large number of variables involved.
Coastal communities such as saltmarsh and mangrove systems can keep pace with a slowly rising sea level by trapping sediment and growing upwards, and have successfully done so over the ages. They may have to retreat landwards but this is not a problem unless prevented from doing so by sea walls and barriers. Building of extensive walls to keep back the sea as levels rise, could result in the loss of extensive areas of coastal wetlands. Apart from the loss of wildlife, this could also result in the loss of commercially valuable marine fish and other species as nursery areas are lost. Experiments are underway on the east coast of England and in other countries, to remove or lower sea walls and allow and encourage the growth of saltmarsh or mangrove which can act as a natural and adaptable (and cheap) barrier to the sea.
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