Mangroves are some of the world's most important trees. They provide food and resources for people and animals, protect coasts, and store huge amounts of carbon. The world's largest mangrove forest - the Sundarbans in the Bay of Bengal - supports millions of livelihoods. In terms of the services they provide, they are worth nearly US$200,000 per hectare per year. But these coastal forests are threatened by rising seas and human development. In a study published today in Nature, we show that some of these forests will drown unless we help them.