A 2015 UNESCO survey found that people living in 84% of biosphere reserves considered natural hazards to be an important issue and 89% of those surveyed reported their biosphere reserve as being exposed to natural hazards. UNESCO’s global network counted more than 600 biosphere reserves at the time.
A follow-up survey two years later showed that 94% of biosphere reserves surveyed were exposed to natural hazards and had suffered significant damage. For example, the Savegre Biosphere Reserve is Costa Rica’s only coastal and marine biosphere reserve. It is exposed to tsunamis, storm surges and flooding from heavy rainfall. These natural hazards threaten not only the population but also a rich and very localised biodiversity that is sometimes endemic to the biosphere reserve.
Since 2014, the National Tsunami Warning Centre of Costa Rica (SINAMOT) has recorded over 350 offshore tremors with the potential to generate a tsunami, such as underwater landslides, volcanic eruptions and earthquakes. In addition, Savegre Biosphere Reserve was severely affected by flooding in 2022. Climate-related hazards like flooding are expected to become more frequent and intense in the years to come.
Although mangrove forests along the coast can bolster the population’s resilience to both geological and climate-related hazards by breaking the strength of incoming waves, these have become fragmented in Savegre Biosphere Reserve.
To address this situation, UNESCO experts worked with national and local emergency agencies, local government officials, Costa Rican experts and at-risk populations from 2021 to 2022 to strengthen the resilience of coastal communities by developing an integrated approach to preparedness for coastal hazards.
UNESCO mapped mangrove cover in the biosphere reserve and held a community workshop on how nature-based solutions like mangroves could mitigate coastal hazards. The workshop was funded by the Earth Network and the Government of Italy.
UNESCO has also identified opportunities for restoring the coastline. With UNESCO’s support, the biosphere reserve plans to implement local projects to restore its coastal mangrove forests to shield the reserve from future hazards.
A triple helix combining restoration, research and awareness-building
In addition to its work in the Savegre Biosphere Reserve, UNESCO is currently restoring mangrove forests in seven other biosphere reserves in Latin America and the Caribbean. As Nöeline Raondry Rakotoarisoa, Director of the Division of Ecological and Earth Sciences at UNESCO, reminds us, ‘we need look no further than nature for the solution to many of our problems’.
UNESCO has also been leading research since 2012 on the development of ‘floating mangroves’ which would grow on platoons placed near the shore. Floating mangroves would provide coastal inhabitants with an alternative source of timber and charcoal to rooted mangroves. Floating mangroves, thus, have the potential to reduce mangrove deforestation. However, more research is required to comprehend fully the economic and physical viability of floating mangroves.
In order to raise public awareness of the vital importance of mangroves for coastal populations, UNESCO is organizing an exhibition on the mangroves of the world at its Paris headquarters throughout the month of September, with the financial support of World Life Spaces.
Mangroves: the unsung heroes of coastal protection
Mangroves grow on the boundary between land and sea. Rooted in soils exposed to the tides, they support a rich biodiversity and provide a valuable nursery for fish and crustaceans, supporting the food security of coastal communities.
Over time, these exceptional trees have adapted to withstand or recover from the impact of natural hazards such as storm surges and tsunamis. When swell waves pass through mangrove forests, the tangled, above-ground roots and branches of the trees reduce wave height and energy. This decreases the waves’ ability to erode sediments and damage coastal buildings, while reducing the risk of flooding for low-lying areas behind the mangrove strip. Over a distance of 100 m, wave heights can decrease between 13% and 66%. Over a distance of 500 m, mangroves can reduce the height of waves by 50% to 99%.
Dense mangrove forests also reduce wind speeds locally, which has the added advantage of preventing swell waves in and immediately behind the mangroves. This can potentially reduce damage to nearby infrastructure.
There is growing evidence that mangroves can reduce the impact of tsunamis by lowering the destructive energy of water flowing inland. This was the case notably during the Indian Ocean tsunami in 2004. Mangrove belts several metres wide have been shown to decrease tsunami wave height by between 5% and 30%. Wide, dense mangrove strips can also limit the area flooded by tsunamis. However, tsunamis over 4 metres deep may be taller than the mangroves and able to damage or even destroy these. Similarly, engineered structures are rarely built to the height of mangroves.
Mangrove trees can grow up to 60 m in height, depending on the species, climatic conditions and available nutrients. Mangroves also exist as shrubs, ferns and palms.
They are found in the tropical and sub-tropical coastal areas of the Americas, Africa, the Middle East and Asia-Pacific. It is estimated that more than three-quarters of the world’s mangroves are now threatened by coastal development, overexploitation, waste disposal and other pressures.
