June 5, 2020

Mangroves at risk if carbon emissions not reduced by 2050, scientists predict

Rapid sea-level rise a threat to highly valuable ecosystems

An international research team, including scientists from the University of Wollongong (UOW), has predicted that by 2050, mangroves will not be able to survive rising sea-levels if global carbon emissions are not reduced.

Using sedimentary archives from when the Earth underwent deglaciation up to 10,000 years ago, the researchers estimated the probability of mangrove survival under rates of sea-level rise corresponding to two climate scenarios – low and high carbon emissions.

When rates of sea-level rise exceeded 6mm per year, corresponding to what is estimated to result under high emissions scenarios for 2050, the researchers found that mangroves very likely (more than 90 per cent probability) stopped growing at the pace required.

In contrast, mangroves can survive by building themselves up vertically when the sea-level rise remains under 5mm per year, which corresponds to that projected under low emissions scenarios during the 21st century. 

The threshold of a 6mm sea level rise is one that will be “easily surpassed” on tropical coastlines if society does not make concerted efforts to cut carbon emissions, said lead investigator of the study, Professor Neil Saintilan from Macquarie University.

“We know that sea-level rise is inevitable due to climate change, but not much is known about how different rates of sea-level rise affect the growth of mangroves, which is an important ecosystem for the health of the earth,” Professor Saintilan said,

The team comprising scientists from Nanyang Technological University, Singapore (NTU Singapore), Macquarie University, University of Hong Kong, Rutgers University, and UOW published their findings on 5 June 2020 in leading academic journal Science.

Co-author Associate Professor Kerrylee Rogers, from UOW’s School of Earth, Atmospheric and Life Sciences, said, “Understanding how mangroves have responded to sea-level rise in the past provides us with crucial information about how our shorelines will change in the future with climate change.

“The ecosystem services provided by mangrove forests, such as carbon sequestration and wildlife habitat, are highly valuable and this study provides us with the information to start planning for their resilience now.”

Professor Benjamin Horton, from NTU Singapore said, “In 30 years, if we continue on a high-emissions trajectory, essentially all mangroves, will face a high risk of loss.

“This research therefore highlights yet another compelling reason why countries must take urgent action to reduce carbon emissions. Mangroves are amongst the most valuable of natural ecosystems, supporting coastal fisheries and biodiversity, while protecting shorelines from wave and storm attack across the tropics.”    

WHY MANGROVES MATTER

With roots that rise from under the mud, mangrove stands grow in a process called vertical accretion. This feature is important to their ecosystem as it helps to soak up greenhouse gas emissions (carbon sequestration) at densities far greater than other forests, and provides a buffer between the land and sea - helping protect people from flooding on land.

The study, which covered 78 locations, explored how mangroves responded as the rate of sea-level rise slowed from over 10 mm per year 10,000 years ago to nearly stable conditions 4,000 years later. The drawdown of carbon as mangrove forests expanded over this time contributed to lower greenhouse gas concentrations.

The study found that mangroves will naturally encroach inland if their ability to vertically accrete is hindered. In doing so, mangroves will have to compete with other land-uses and may become squeezed behind coastal protections.

 Co-author Assistant Professor Nicole Khan, from The University of Hong Kong said, "Most of what we know about the response of mangroves to rising sea level comes from observations over the past several years to decades when rates of rise are slower than projected for later this century. This research offers new insights because we looked deeper into the past when rates of sea-level rise were rapid, reaching those projected under high emissions scenarios.

“Our results underscore the importance of reducing emissions and adopting coastal management and adaptation measures that allow mangroves to naturally expand into low-lying coastal areas to protect these valuable ecosystems."

ABOUT THE STUDY

“Thresholds of mangrove survival under rapid sea-level rise” by N. Saintilan, N. S. Khan, E. Ashe, J. J. Kelleway, K. Rogers, C. D. Woodroffe, and B. P. Horton is published in Science, 5 June 2020

This research was funded by Macquarie University, the Singapore Ministry of Education Academic Research Fund, the National Research Foundation Singapore, the Singapore Ministry of Education, and he Australian Research Council.