Mangroves, the tangled forests that populate many tropical coastlines around the world, are at risk of being submerged in the Indo-Pacific region as sea levels rise, scientists have found.
The international research team, who published their findings in Nature, warn that many mangrove sites could be essentially drowned by 2070.
Mangroves are of immense value, Catherine Lovelock, a coauthor of the report and professor at the University of Queensland, told Mashable Australia. The environment supports livelihoods by attracting fish, and in many communities in Asia and Africa, the forests also provide fuel in the form of charcoal and timber. She estimates mangroves have a value of US$194,000 per hectare per year.
If mangroves die, coastal communities could lose the flood and storm protection the trees offer. Mangroves act as a buffer, slowing down waves and tides as they approach land. They also help keep the coastline above sea level, trapping sediment with their roots and raising land height.
"Usually, as the sea has risen, mangroves have been keeping pace by adding to their soil level," Lovelock said. "Now, the coasts aren't what they were 100 years ago." Many of the 27 sites the team monitored were lagging behind the local rate of rising sea levels.
One reason for this could the damming of rivers for irrigation, which has lead to a serious reduction in sediments flowing to the coast -- and thus to the mangroves. There's been around a 30% reduction in sediment flow globally, she estimated. Along the Chao Phraya River delta in Thailand, the scientists found there had been an 80% reduction in sediment delivery.
The big deltas of the world, including the Mississippi, are also thought to be sinking at different rates, reducing the movement of sediment. "Scientists think they're sinking because of oil, gas and water extraction," Lovelock said. "It means you need more sediment to accommodate the sinking.
"Mangroves have to be at about mean sea level, they can't grow well below that."
The impact of rising sea levels and reduction in sediment will be felt differently around the world. "Places with big sediment supply and large tidal range ... have a better chance of keeping pace with sea levels," she suggested. The team's modelling showed that mangroves in east Africa, the Bay of Bengal, eastern Borneo and northwestern Australia could persist.
Places with low tidal range and small sediment supplies, on the other hand, are especially vulnerable. "The places that stood out to us were the island nations of the Pacific. They don't have big rivers that deliver sediments and they don't have big tidal ranges," she said. The researchers are particularly concerned about areas in Thailand, Sumatra, Java, Papua New Guinea and the Solomon Islands.
It's not all doom and gloom: Lovelock suggested there were steps the world could take to adapt.
In her opinion, we need to look carefully at the damming of rivers. There are big dams planned along the Mekong river in southeast Asia, Lovelock pointed out, but if you reduce sediment, then it heavily impacts the coastal zone.
Countries should also consider whether mangroves could shift as sea levels rise -- habitats could move inland as the sea floods land that was once terrestrial, but is now turning marine. "[Man-made] barriers may need to move to accommodate this trend," she said. "Land planning for the future should take into account where those forests want to go.
"In lots of places, there's a marine wall or shrimp farms, but we need to be thinking about where are the appropriate places these environments would move to," she said.
"Australia is lucky to have this undeveloped coast in many settings, but in other countries, people are living right there."