Removing mangrove forests that protect tropical coastlines is costing countries $42bn in damages each year and creating millions of tonnes of CO2 emissions, according to a UN report.

Carbon-rich mangroves provide an important natural service of mitigating flooding and coastal erosion, as well as housing fish stocks and clean water supplies vital for many communities in developing countries, where 90 per cent of the world's mangroves are found.

Yet destruction of mangroves is happening three to five times faster than terrestrial forests, driven by climate change, land conversion for commercial aquaculture, and pollution, which exposes developing nations to threats from sea level rise and flooding, the UN Environment Programme (UNEP) said.

In addition, emissions resulting from mangrove losses are thought to make up nearly one-fifth of global emissions from deforestation.

The UN calculates the costs of removing mangroves in African nations such as Cameroon, Gabon, and Angola as $11,286 per hectare in seawall replacement and $7,142 per hectare in benefits for protection of rural infrastructure against shoreline erosion - rising to $151,948 per hectare for urban mangroves.