Above: Windward Cemetery, Grenada. Image credit: Scott Schill.
“Reefs break waves every day”: I know you’re thinking that’s an obvious statement. But there’s more to it than meets the eye. Reefs break waves — waves that would otherwise crash into and erode coasts on which hundreds of millions of people and trillions of dollars in development sit.
In fact, this wave attenuation by coral reefs is the single most obvious way nature benefits people globally — and it’s a benefit that conservation and science and policymakers need to pay a lot more attention to as climate keeps changing, coral reefs keep dying, and more and more people inhabit and build on the world’s coastlines.
How Have We Missed This?
Most coral reef science and conservation today focuses on (i) the loss of the stunning diversity and productivity of corals and fishes and (ii) what that loss means for the future of coral reefs and the people that rely on their fisheries. But despite the obviousness of my title, there is extraordinarily little science that focuses on the role of reefs as barriers (even on the Great Barrier Reef).
Above: Figure 1: Grenville, the second-largest city in Grenada. This is just a typical day of wind waves. The cottony white lines offshore waves breaking on some of the reef breaks. Copyright: Google Earth.
The importance of reefs as barriers really hit home for me as I gazed on the Caribbean’s Windward Isles from seat 10A on a return flight home to the States from Grenada recently. But you can make these observations right at your desk. Use Google Earth to look first at the eastern edges of the Windward Isles — look for towns and small cities, and then look for the crescent of what may at first seem like clouds ringing those communities (Figure 1, above). These are the waves breaking on the fore-reefs that protect these communities. I know because, as a surfer, I have mind-surfed waves all around the globe this way.
But this is not just about wet dreams on Google Earth; it’s about people’s everyday lives.
The Daily, Global Disaster of Shoreline Erosion
While in Grenada, I met Dexter Miller on Petit Martinique (one of that country’s islands). We met on one of the few flat pieces of land on the island. Almost all his town’s livelihoods and lives happen on that one piece of land. Fishing is their main income, and the ports and docks are right there; boat-building is the second biggest income generator, and they make the boats right on this one strip of land, which also contains the town’s store and gas station; and this area also doubles as the soccer field.
But the shores of his town — this one flat piece of land — have begun to erode rapidly. And even more rapidly in the past few years. This land loss is not about the future of intensifying storms and sea-level rise; nor is it even about past storms like 1999’s Hurricane Lenny (“Wrong Way Lenny”), which everyone on the island still remembers vividly.
The erosion is happening now with no storm in sight — we watched it biting in and taking away part of their shore every day. This is the daily disaster.
What’s behind this increasing erosion? Past sand mining certainly contributed to it — but the residents stopped that mining. What the erosion is likely most about is that the fringing reefs of Petit Martinique are dying.
What few are recognizing — even in the conservation community — is how drastically that fringing-reef breakdown increases the wave energy hitting shores every day. Mere inches of lost fore-reef depth (either because reefs are not growing or sea levels are rising) means substantially greater wave energy transmitted over the reefs and hitting shorelines (Field et al. 2011). This increase in wave energy is an engineering fact, well worked out for artificial breakwaters around the world.
Indeed, just look at Petit Martinique in Figure 2 (below). What’s telling is what you don’t see on the southern side of the fringing reef. You can see the reef below water, but you don’t see the waves breaking there as you do in front of St. Vincent. While the reef of Petit Martinique is still getting protection from bigger storm waves (waves break ~ when Wave Height = Depth of Reef Tops), their fringing reefs are not protecting it from the daily disaster.
Figure 2: Petit Martinique (below) and Petit Saint Vincent (above). Look at what you don’t see on the south side of the fringing reef just east of Petit Martinque that you do to the east of Petit Saint Vincent — breaking waves. Copyright: Google Earth.
Now consider that this daily disaster is playing out just behind Dexter’s town in the village of Windward (see the above photo), where their ancestors’ graves are being washed into the sea (see image above). It’s also happening in coastal towns and villages throughout Grenada and the Grenadines; the windward isles of the eastern Caribbean ecoregion; throughout the Caribbean; throughout the tropical Atlantic; throughout the tropics globally.
These Coral Reefs Are the Wall Between Us and Rising Seas
Conservationists often say that we are going to have to be proactive and smart about how we respond to climate change, because it will be impossible and too expensive to build a wall around whole towns, cities or islands (except, of course, places like New York City).
But consider for a moment that in fact these fringing coral reefs are the wall — the natural infrastructure, sometimes 30, 60 or even hundreds of feet high — that rings villages, cities and whole islands. Think for one moment about the cost of erecting such a wall; and the benefits lost without it.
To save them, we are going to have to act fast and be much more proactive. On Petit Martinique, they are already heavily defending their shores — often house by house — with hastily erected breakwaters and gabions. Each small gray shore defense that goes in increases erosion just a little downshore and requires another gray solution in that spot. In Dexter’s town, they are pushing to get a whole breakwater wall — a project that will cost EC$3.5M at a minimum.
I can appreciate why they need to act now, but all of these fixes will be temporary. As sea levels rise and (far more importantly) if the barrier reefs continue to break down, then ever more wave energy will make their temporary solutions ineffective. These likelihoods are not currently accounted for in the engineering and design models for breakwaters (or it would become clear how unviable they might be).
So, What’s the Answer?
A significant part of the solution is going to have to be revitalizing and rebuilding those reefs. Those tasks will be challenging, but we do have the knowledge to implement them — specifically, how to better manage reefs to reduce sedimentation, pollution and overfishing so that the reefs can grow. We will also need to be more proactive in reef restoration to spur that growth. Increasing ocean temperatures and acidification will add to the challenge and (some would say) make the endeavor hopeless; I think these predictions are overly dire and do not account for resilience or evolution.
Yes, these approaches might seem expensive. But compare their costs against the cost of all of the individual “grey” infrastructure projects such as those being erected on Petit Martinique. And remember that new climate adaptation funds are already starting to flow in the billions of dollars; these small island developing states are primary recipients of those funds, and right now the gray engineers and businesses are selling just one approach.
The recognition that wave attenuation is a globally critical ecosystem service will challenge conservation’s priorities as well. These fringing reefs are rarely the most scenic, beautiful, remote or diverse coral ecosystems. Indeed, it’s that they are directly in front of villages, towns and cities that makes their revitalization such a high priority. We will also likely have to get comfortable with added hardened structures like reef blocks, as they provide substrate for oysters and corals; add quickly to some of the attenuation benefits; and include the industries and engineers who would otherwise promote only sea walls.
This is not exactly the work I signed up for some years back when I became a marine conservationist, but personally I am excited to be part of work that jointly meets conservation and hazard mitigation goals; that’s fulfilling to me.
M. Field, A. Ogston, and C. Storlazzi. 2011. Rising sea level may cause decline of fringing coral reefs. EOS 92:273-274.
M. Johnson et al. 2011. Caribbean Acropora restoration guide. Best practices for propagation and population enhancement. Arlington, VA: The Nature Conservancy.