Mangroves threatened by sea level rise

Mangrove ecosystems may cease to function if greenhouse gas emissions aren’t reduced, according to a new study published in the journal Science.

Using ancient sediment data, an international team led by Australia’s Macquarie University predicted the probability of mangrove trees surviving under two scenarios – low and high carbon emissions.

They estimate that if sea level rise exceeds six millimetres a year – in line with high-emissions scenarios for 2050 – there is a 90% probability that mangroves will not be able to grow quickly enough to keep pace with the rising water.

They are more likely to survive if sea-level rise is less than five millimetres, but the six millimetres threshold “will be easily surpassed on tropical coastlines” without concerted efforts to cut emissions, says lead author Neil Saintilan.

The loss of mangrove ecosystems, which support some of the highest rates of carbon sequestration, would result in increased carbon dioxide in the atmosphere and fewer buffers against storm surges, the researchers say.

There are about 80 species of mangrove trees, which grow in warm and shallow tidal waters. They will naturally move inland if they can’t build vertically, but coastal developments frequently impede such movement.

Saintilan and colleagues from Australia, Singapore, Hong Kong and the US investigated mangrove vertical accretion during the final stages of the Holocene deglaciation, 10,000-7000 years ago, by examining sediment cores from 78 tropical and subtropical sites around the world.

At that time, sea-level rise was higher than today and quite dynamic, allowing the authors to identify critical thresholds of mangrove survival.

“Most of what we know about how mangroves respond to rising sea levels comes from observations over the past several years to decades when rates of rise were slower than projected for later this century,” says co-author Nicole Khan, from the University of Hong Kong.

“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”.

The sediment core data also allowed the researchers to conservatively estimate that the amount of carbon sequestered as mangrove forests expanded during the early Holocene was around 85 gigatons.

That is more than the amount of carbon estimated to have been stored in boreal peats as they developed through the same Holocene period, Catherine E Lovelock, from Australia’s University of Queensland, notes in a related Perspective article in the journal.

Providing an evidence-based relative sea-level-rise (SLR) threshold for mangrove survival can help stimulate solutions for coastal management, Lovelock writes, but she suggests there are “caveats to consider in the new study”.

“Over the late Holocene, rates of SLR were mostly declining, whereas Earth currently faces accelerating SLR, which might alter the threshold.

“There could be lags in responses of mangrove cover to relative SLR that exceed the 6- to 7-mm/year threshold, and lags can vary with species traits and disturbance regimens, such as frequency and intensity of storms that exacerbate the impacts of SLR and which are predicted to intensify with climate change in many regions.”