Known for the robust and diverse marine life they support, coral reefs are appreciated for complex ecology and economic benefits provided through tourism and fisheries revenues (Figure 1). These ecosystems occupy only 0.1 % of the ocean’s surface area, but support 25% of all marine life. Unfortunately, coral reefs are experiencing significant losses globally due to coastal development, coral and sand mining pressure, bioerosion, and destructive fishing practices. Rising ocean temperatures and changing seawater chemistry further threaten these important habitats. Prolonged exposure to frequent and intense warming events can bleach and kill corals, alter ecosystem diversity and abundance, and eventually lead to the dissolution of reef carbonate structures.
Flood protection, the hidden value
As the population grows and coastal development increases, so does the risk of coastal flooding against the backdrop of a changing climate. In the United States, for example, nearly 40% of the population lived near the coast in 2010 and that population is expected to increase 8% by 2020. The risk of coastal flooding is enhanced as sea-levels continue to rise and as intense storms become more frequent and longer in duration. Typically overlooked, coral reefs provide important flood protection benefits to people and property by acting as submerged breakwaters attenuating wave energy.
Ecosystem services are a broad group of benefits that humans gain from the natural environment. Fish production, an ecosystem service in the provisioning category, and tourism, a cultural form of an ecosystem service, are quantified globally. However, flood protection benefits, a regulating ecosystem service, are rarely considered in coastal management because the loss of an expected benefit (such as flood protection by intact ecosystems) can be unfamiliar to decision-makers. A study published this year by Michael Beck (The Nature Conservancy and UC-Santa Cruz) presents a global, process-based valuation of flood protection provided by coral reefs at sub-national levels. An improved understanding of these types of protection benefits could better inform environmental management decisions.
Shedding some light
Beck et al. (2018) estimate expected global benefits of coral reefs for protecting people and property using process-based flood models for storm return periods of 10, 25, 50, and 100 years. Flooding is compared between “with” and “without reefs” scenarios to determine protection benefits. In this case, the “without reefs” scenario is a one-meter reduction in the height and roughness of coral reefs. This scenario does not completely remove the reef structure, but represents what could happen (relative deepening of coral reefs by one meter) through a combination of the interacting adverse impacts highlighted above.
Flood protection on global- and national-scales
On a global-scale, coral reefs reduce expected flood damage from storms by more than $4 billion dollars annually across 71,000 km of reef coastline (“with reefs” scenario). Deepening coral reefs by one meter in the “without reefs” scenario more than doubles annual damages and substantially increases both area flooded and the number of people impacted.
Reefs provide more relative benefits for higher frequency and lower intensity storm events (e.g., 10 and 25-year return period storms). Specifically, in the “without reefs” scenario, flood damages from a 25-year storm increase by 141% ($36 billion in additional incurred damages) and for a 100-year storm damages increase by 90% ($130 billion in additional damages). Including anticipated sea-level rise under conditions that mimic the business-as-usual climate future (see Box 1) further enhances coastal flooding. For a 100-year storm in the year 2100, the area flooded increases by 64% even if reefs remain intact (“with reefs”). Considering sea-level rise, and removing the upper meter of the reef, the area flooded increases by 116% compared to the “with reefs” scenario.
On a national level, coral reefs are most beneficial to Southeast Asian countries (top three overall: Indonesia, Philippines and Malaysia) where annual averted damages are in the range of $450-640 million for each of the nations. The remainder of the top 15 countries receiving the most averted flood damage benefits include Latin American and Caribbean nations (Mexico, Cuba, Dom. Republic, Jamaica, Bahamas and Belize), United States, Saudi Arabia and additional Southeast Asian countries (Taiwan, Vietnam, Myanmar and Thailand). Further, for small island nations and developing countries where flood protection benefits by coral reefs are large compared to gross domestic product, the protection of reef systems should be an important component in coastal management plans.
The flood protection estimates presented in Beck et al. (2018) are conservative in that they do not assume the total disappearance of reefs, only the relative deepening of corals by one meter. These modest reef losses could occur quickly, especially with the increasing frequency of coral bleaching events. When coupled with anticipated sea-level rise, flood damages could be more severe if reef loss is greater. The results of this study value coastal protection from natural infrastructure in terms of annual expected benefits and; therefore, can be considered by decision- and policy-makers together with other metrics for built infrastructure. These estimates provide a compelling case for investments in reef management and restoration and can hopefully steer coastal developers toward decisions that reduce risks to both people and reefs.
Beck, M.W., Losada, I.J., Menéndez, P., Reguero, B.J., Díaz-Simal, P. and F. Fernández. 2018. The global flood protection savings provided by coral reefs. Nature Communications 9:2186. https://doi.org/10.1038/s41467-018-04568-z