Mapping the world’s ‘blue carbon’ hot spots in coastal mangrove forests

Human actions have boosted carbon dioxide concentrations in the atmosphere to levels higher than any measured over the last 160,000 years. Rising concern over the risk of severe impacts from climate change is spurring research into ways in which ecosystems may mitigate global warming by storing excess carbon in plants and soil.

Our research group has studied the ecology of mangroves for over 40 years. Mangroves are tropical forests that thrive in salt water, forming a canopy with the atmosphere and extensive roots in the sediment of the intertidal zone – the area that is above water at low tide and underwater at high tide. Scientists refer to them as “blue” carbon ecosystems, in contrast to “green” carbon ecosystems on land, such as forests and grasslands.

In a recent study, we estimated that the wood and soil of mangrove forests along the world’s coastlines hold 3 billion metric tons of carbon – more than tropical forests.

Mangroves are common along tropical and subtropical coastlines around the world, and among the most biologically important systems on the planet. NASA Earth Observatory
We have also shown that previous studies have overestimated blue carbon storage in some river delta areas, such as the Amazon in Brazil; the Sundarbans region in India, where the Ganges, Brahmaputra and Meghna rivers converge; the Zambezi delta in Mozambique; and the Indus river delta in Pakistan. Others have grossly underestimated blue carbon storage in carbonate (peat-dominated) coastlines, such as Belize, the Florida Keys, Puerto Rico, Mexico’s Yucatan, Cuba, the Dominican Republic and several Caribbean islands.

The key to improving these estimates, we found, is to factor in how rivers, tides, waves and climate shape coastal landforms to create different environmental settings. Using this approach, we have produced a more accurate estimate of global blue carbon “hot spots” – an important first step toward protecting them.

Adapted to many settings

Mangroves can extend along tropical shorelines to the very edge of warm temperate climate zones, controlled by changing frequency of frosts. They grow where tides and salt from oceans meet rivers carrying sediments from continents, mixing to form different types of intertidal zones.

Deltas exist where large rivers with very turbid waters and very little salt deliver sediment to the coast, forming extensive mud banks. In contrast, lagoons and coasts formed mainly of carbonate rock, such as chalk or limestone, have beautiful blue salty waters and firm sandy or coraline sediment along shorelines. In a middle category, estuaries form where rivers meet the sea and tides mix fresh and salt waters, forming brackish water that changes seasonally as river levels rise and fall.

Mangroves grow very differently in these various settings. In deltas, some of the largest mangrove trees in the world reach the heights of rain forests, with extensive roots penetrating into soft silty mud. In contrast, mangrove trees growing in the sediments of a carbonate shoreline are so much smaller that they look stunted, like ornamental trees in public parks.

Grunts and other fish shelter among mangrove roots, U.S. Virgin islands. USGS/Caroline Rogers

Carbon stocks in coastal environmental settings

Why do mangrove trees grow so differently in different environments? In our view, the explanation lies in how climate, rivers, tides and waves form conditions for mangrove tree growth.

The mud in deltas and estuaries with large tides contains high concentrations of essential nutrients. This creates benign environments where trees grow to their full potential. In contrast, coastlines where sediments are made of carbonate and tides are small tend to be nutrient-poor. This stunts tree growth and produces scrub forests along the shore.