As the world begins to pay more attention to climate change, the gravity of the situation is realized when we consider what we stand to lose as a result of it.  No one wants to see individual species die off, or watch coastal towns evacuate and move inland.  But it’s also shocking to realize that an entire type of ecosystem could be lost, and consequently affect all marine life as we know it today.  This is the unfortunate reality for the most beautiful and enchanting of underwater habitats; coral reefs.  Coral reefs are in danger, largely due to global climate change.  This article explores this complex and critical problem, starting with the basics of coral reef ecology and expanding upon the effects of climate change on corals and other sensitive marine species.  The call to action is simple; understand the problem, spread the word, and think about what you can do as an individual to save corals.  Because we stand to lose a lot more than colorful underwater photos if we let coral reefs meet their end.

Photo courtesy of NOAA.

Photo courtesy of NOAA.

What Are Coral Reefs?

Coral reefs are shallow underwater habitats found in tropical waters.  They are composed of a massive amount of biological diversity, the highest on Earth in fact, and are known for their beauty and tranquility.  While coral reefs sustain about 25% of the ocean’s marine life (during at least one life stage), these wondrous ecosystems cover only 0.2% of the earth’s surface.  An estimated one million species of fish, invertebrates, algae, corals, and more can be found near coral reefs.  There could be anywhere from 600,000 to 9 million species of animals and plants in coral reefs.  The range of this estimate is so large because so few reef species have actually been documented, possibly as little as 10%.  There are probably hundreds of thousands of species that are still to be documented.

A variety of colorful stony corals. Photo credit: Assaf Zvoloni.

A variety of colorful stony corals. Photo credit: Assaf Zvoloni.

The most critical residents of a coral reef are the corals themselves.  It is a common misconception that corals are plants.  Corals are actually a symbiotic relationship between photosynthetic algae (zooxanthellae), and hundreds or thousands of individual coral polyps, which are animals.  Coral polyps are immobile, so they depend on the algae to generate food for energy by photosynthesis since they cannot move about to acquire their own food.  The zooxanthellae are responsible for the brilliant colors seen in coral colonies.

Corals provide food and shelter for other organisms living on the reef, but most importantly, some corals are responsible for building the reef.  The reef-building coral species are called ‘stony corals’ because they create a limestone ‘skeleton’ that they attach to and grow on.  The limestone is constantly built upon by the many corals living on a reef, and this structure becomes the backbone of the entire reef.  This skeleton provides substrate not only for corals, but for algae, sponges, and other invertebrates to attach to, and for mobile organisms to find shelter.  There are over 800 species of this important reef-building coral, about one-third of which are at serious risk of extinction.

Island with fringing reef off Yap, Micronesia "Coral reefs around the world". 2 September 2009 . . (Photo credit: Wikipedia)

Island with fringing reef off Yap, Micronesia “Coral reefs around the world”. 2 September 2009 . . (Photo credit: Wikipedia)

The actual structure of a reef is important to the assemblage of species that live there, and to the function of the reef itself.  There are three main types of reefs: fringing, barrier, and atoll.  Each type eventually evolves into the next based on geological activity.  Imagine an island with coral reefs lining the shores all the way around.  These reefs are called fringing reefs.  Over time, the island may begin to subside into the ocean.  The fringing reefs will continue to grow upward to stay closer to the ocean’s surface, even as the island retreats.  When there is a considerable distance between the reef and the subsiding island, it becomes a barrier reef and protects the island shore as a wave breaker.  Finally, once an island has fully subsided, what remains are reefs that once encircled the island.  There is now a shallow lagoon in the middle where the island once sat above sea level.  This type of reef is called an atoll.

The three major zones of a coral reef: the fore reef, reef crest, and the back reef (Photo credit: Wikipedia)

The three major zones of a coral reef: the fore reef, reef crest, and the back reef (Photo credit: Wikipedia)

The classification of reefs continues within each individual reef, with various cross-sectional profiles and zones that remain pretty consistent even among the three reef types.  The reef flat is the area that extends from the beach out toward a sharp drop-off.  Corals will gradually begin to appear as the reef flat becomes deeper and deeper, but the slope is very gradual.  The reef flat could be anywhere from 10 meters to over half a mile wide.  Once the reef flat reaches the steep drop, a reef crest will occur.  The reef grows actively upward in this region, and takes on a lot of wave energy.  This is the part of the reef that can cause trouble for vessels.  Since the wave energy is so high upon the reef crest, the delicate corals cannot grow there.  Instead, red algae dominates.  This type of algae also creates a form of limestone skeleton, so the reef continues to be built.  The corals will grow closer to the base of the reef crest where the wave energy is lower.  Finally, on the seaward side of the reef crest begins the reef slope.  The land mass plunges at this point to great depths, often called a drop-off.  This is the part of the reef that is the most exquisitely diverse with high animal activity.  Some of the best scuba dives are ‘wall dives’ along these reef slopes with the ocean floor miles below!  Corals can still grow pretty far down along the reef slope in clear waters where the sun can penetrate deeper.

Where Are Coral Reefs Located?

Coral reef climate limitations and abiotic (non-biological) factors control the distribution of corals.  The abiotic factors that most specifically limit coral reef distribution are temperature, salinity, light, nutrients, exposure (to waves, air, etc.), sediment levels, and seawater chemistry.  Corals are very delicate and vulnerable organisms that can tolerate very little changes in these abiotic factors.

Coral reefs all occur in tropical waters, which are warm, shallow, and low in nutrients.  Nutrients like nitrogen and phosphorus are necessary for use by the photosynthetic algae that live with corals, but they are able to acquire the amounts they need to survive even in nutrient-poor waters.  They do so by efficiently recycling and conserving nutrients among the various organisms that they associate with.

This map highlights in green areas where coral reefs exist. Credit:

This map highlights in green areas where coral reefs exist. Credit:

Worldwide, the greatest concentration of coral reefs occur in Indonesia at about 17.95%, followed by Australia (17.22%) and the Philippines (8.81%).  The United States and its territories are number 16 on the list, with about 1.33% of the world’s reefs.  In total, about 80 countries have coral reefs within their waters, all roughly within the boundaries created by the Tropics of Cancer and Capricorn.  In addition to the greater coral reef cover, the Indo-west Pacific boasts the greatest diversity of reef life as well, with perhaps 10 times more species than other reefs worldwide.  Many of these exquisite reefs are situated among Indonesia and the Philippines in what is known as the “Coral Triangle”.

Western coasts, such as in North and South America and Africa, can support rich populations of pelagic fish important to fisheries due to upwelling of cold nutrient-rich water.  These same conditions, though, prevent tropical corals from growing along these coasts.  Interestingly, there is a group of corals that can be found closer to polar regions in colder water, or in deeper darker water in the tropics.  These corals are called ahermatypic, meaning they lack zooxanthellae and therefore do not need light for photosynthesis.  There are just as many species of this type of coral as there are other corals that do have zooxanthellae (hermatypic corals).  They rely entirely on their tentacles to capture zooplankton from the water for food, as they cannot receive any energy from photosynthetic algae.  This type of coral can produce a hard skeleton as well, and can actually form deep-sea reefs in otherwise soft substrate.  While an important habitat type in the depths, these reefs still do not support anywhere close to the diversity of life that shallow, warm-water reefs do.

Life on Coral Reefs

A colony of Thistle soft coral. Photo courtesy Wikimedia commons.

A colony of Thistle soft coral. Photo courtesy Wikimedia commons.

In addition to the important reef-building stony corals mentioned earlier, there are also just as many species of soft corals and sea fans.  These corals are soft and fleshy and grow in various forms, including lobed, branched, or encrusting.  While they are not major contributors to reef-building, they serve the same important functions as stony corals by creating shelter, habitat, and food for other reef species.

Other animals in coral reefs include invertebrates such as crustaceans, flatworms, and sea stars among so many others.  There is also a variety of sponges, a very primitive multicellular animal of diverse shapes and colors.  Sponges are of great importance on coral reefs for various reasons.  They are filter feeders, and so they contribute to filtration of water which keeps nutrient levels low, a necessity for coral growth.  They contribute to bio-erosion of the limestone coral skeletons, which allows for new growth.  Sponges also provide food and habitat for other reef-dwelling creatures.

A massive variety of invertebrates make their homes on coral reefs! Photo Credit:

A massive variety of invertebrates make their homes on coral reefs! Photo Credit:

Of course the most well-known coral reef animal species, besides the corals themselves, are the masses of fish that swarm constantly amongst the reef structures.  Coral reef fish species come in all shapes, sizes, colors, and patterns.  Some are brilliantly colored while others are drab or silvery.  Some are sleek, round, compressed, or just plain bizarre-looking!  Some may not even look like a fish at all.  About one-third of all identified marine fish species live on coral reefs, which explains the great amount of functional and morphological variance among these fish: when so many different species live in the same habitat, each must be able to carve out its own special niche in order to survive.  There are many factors that cause this differentiation, including methods of feeding, camouflage requirements, reproductive displays, mimicry of other fish, and more.

Photo credit: NOAA

Photo courtesy of NOAA.

Coral reefs, mangroves, and seagrass populations overlap in many parts of the world. Credit: UNEP Vital Water Graphics

Click to enlarge: Coral reefs, mangroves, and seagrass populations overlap in many parts of the world.  Credit: UNEP Vital Water Graphics

Though coral reefs are largely animal-occupied, there are a few plant species that exist in conjunction with reefs.  The two types most associated with reefs are seagrasses and mangroves.  Seagrasses usually grow in the soft sandy substrate in the calm water on the shoreward side of the reef, rather than on the reef itself.  Most reef animal species do not directly feed on the grasses, but the grasses still provide food once they die and decay.  This food is in the form of particulate organic matter that filter feeders can absorb.  However, one animal species does feed directly on the grass; green sea turtles.  The particular species of grass the turtles feed on is appropriately named ‘turtle grass’.

Credit: Heather Dine, Florida Keys National Marine Sanctuary Seagrass meadows, such as this one composed of turtle grass and manatee grass, are an important shallow water habitat.

A seagrass meadow. Credit: Heather Dine, Florida Keys National Marine Sanctuary

Mangroves are trees and shrubs which can grow out of soft sandy substrate in salt water, much like seagrasses.  In places where these plant communities occur close to a coral reef, they mutually benefit from the association.  The plants stabilize the soft substrate near the reef which reduces sedimentation, thereby improving water quality.  The reef acts as a breaker which reduces wave energy to create the calm waters on the other side, where the seagrasses and mangroves can grow.  These plant communities also serve as excellent nursery habitat for animal species that will eventually live on the coral reef.

The diversity of reef life can be devastated by even small disruptions.  Because everything is connected so tightly, to reduce or eliminate a species will cause ‘trophic cascades’, meaning the effects will flow up and down the food chain to affect every other species that falls within it.  This delicate balance along with the scarcity of coral reefs makes them vulnerable to even the smallest changes, whether natural or anthropogenic.  But humans don’t live on coral reefs, so why should we care?

The Value of Coral Reefs to Humans

Not only are reefs important wild ecosystems, but they are of great value to humans as well.  Many tropical countries are small islands with struggling economies.  In these nations, fisheries are often the primary food source for the islanders themselves, but also as a means for them to earn income by selling their catches elsewhere.  These same small island countries also rely heavily on tourism for foreign currency.  The existence of attractive and healthy reefs is a huge driver of tourism, particularly by recreational scuba divers.

The Great Barrier Reef dissipates wave energy before it reaches Australia's shores. Credit: ARC Centre of Excellence for Coral Reef Studies/Marine Photobank

The Great Barrier Reef dissipates wave energy before it reaches Australia’s shores. Credit: ARC Centre of Excellence for Coral Reef Studies/Marine Photobank

As discussed previously, coral reefs also act as barriers to shorelines from high wave energy that can erode away beaches.  This is very important for small island nations that depend upon their beaches and shoreline habitats for food sources and income generation.  In many cases, development happens all the way up to the shoreline.  Eroding beaches can cause expensive structural damage and present a safety hazard to people.  As climate change increases the intensity of storm surges in the tropics, reef protection of shorelines will become more and more important.

Another way reefs are useful to humans is simply because of the vast biodiversity.  This can be explained in a couple ways.  First, this diversity means there are more opportunities for discovery of organisms that can create or improve pharmaceutical products.  Secondly, biodiversity is valued in an aesthetic way and through a feeling of responsibility over maintaining it.  For many people, just the desire to keep a diverse ecosystem in its natural state is enough reason to hold it at high value.

Why Are Coral Reefs in Danger?

The various threats to coral reefs include natural disturbances such as storms, and anthropogenic disturbances such as overfishing and pollution.  Corals are very fragile organisms, and are easily disrupted by physical contact such as with boat anchors, fishing trawl nets, and dredging equipment.  Corals are also extremely sensitive to changes in their environment.  Even small changes in water chemistry and temperature can be enough to wipe out an entire reef.

This graphic shows not only the distribution and severity of coral reef decline, but also highlights the major contributing human-induced factors. Credit: UNEP Vital Water Graphics

Click to enlarge: This graphic shows not only the distribution and severity of coral reef decline, but also highlights the major contributing human-induced factors. Credit: UNEP Vital Water Graphics

As mentioned earlier, corals are able to thrive in nutrient-poor waters due to nutrient conservation and recycling.  Because of this efficiency, corals are able to thrive in areas where algae, which need much higher concentrations of nutrients, cannot.  When the nutrients in the water increase even slightly, algal growth can overcome a reef and out-compete corals and seagrasses.  Anthropogenic factors such as coastal development and sewage runoff are some ways that excess nutrients build up in reef waters, contributing to this macro-algal growth and coral decline.

Other changes in the abiotic environment can have negative impacts on reefs.  Strong storms create large waves that can easily cause physical damage to reefs.  Extended periods of low tide can cause fatal exposure of corals to sunlight and desiccation.  Changes in sea surface temperatures, salinity levels, and sea levels can be resultant factors of weather events like an ENSO (or El Niño), which can be just enough disruption to damage corals.

This damage to this coral is quite apparent, as three COTS prey upon it. Image copyright © Borut Furlan.

The crown-of-thorns starfish is among the top predators of corals, and can destroy entire reefs during a population explosion.  Image copyright © Borut Furlan.

Corals are also prey to many animals, such as crabs, worms, snails and sea stars.  In fact, another way nutrient enrichment further affects corals is by causing population explosions of the crown-of-thorns starfish, which is a top predator of corals and has been known to decimate entire reefs.

While these natural threats to corals do exist, it is widely accepted that the greatest threats to corals are human-induced.  Even some of the “natural” threats can be exacerbated or caused by an anthropogenic disturbance.  For example, increased water temperature and storm surge are expected effects of global climate change, which is now scientifically accepted to be caused by human activity.  Corals do have the ability to recover from mild or short-term disturbances from things like weather events or brief periods of exposure to warm water.  However, if these conditions persist, corals will certainly die.  Many of the human-induced threats such as global warming are not expected to be short-term at all.  Vast expanses of dead coral reefs may be a more common sight for scuba divers and snorkelers of the future.

Climate Change: What Is It and What Can We Expect?

To better understand how global climate change can affect coral reefs, let us first look at what climate change is.  It begins with our already extremely thin and vulnerable atmosphere, which controls the climate underneath.  It acts as a sort of membrane to our planet, allowing light and gases to pass through and be used up as necessary on the earth’s surface, and then reflect enough back out so that the planet does not warm too much.

Graphic representation of the greenhouse effect Source: CO2 Cooperative Research Centre

Click to enlarge: Graphic representation of the greenhouse effect. Source: CO2 Cooperative Research Centre

Since the beginning of industrial practices, we have produced an unnaturally high level of a very natural and important gas: carbon dioxide (CO2).  Various human activities release carbon dioxide into the atmosphere.  Burning fossil fuels to power our homes and vehicles is the greatest contributor of excess CO2, along with burning or cutting down forests and even some agricultural practices.  Carbon dioxide, among a few other gases, are what are referred to as ‘greenhouse gases’.  They have this name because they function like the outer plastic layer of a greenhouse; letting light in but trapping heat inside.  These gases have the same effect on the Earth.  They allow sunlight to penetrate through to the earth’s surface, but they do not allow all of the infrared radiation from the sun to reflect back out into space.  This trapped energy causes the planet to warm up.  This is not entirely a bad thing, in fact some level of greenhouse gases in the atmosphere is necessary in order to keep the Earth at a temperature warm enough to sustain life.  But too much of a good thing easily turns into a bad thing, and such is the case with greenhouse gases.

This simple graphic explains some of the indicators of climate change we are currently seeing. Credit: John Cook,

Click to enlarge: This simple graphic explains some of the indicators of climate change we are currently seeing. Credit: John Cook,

The effects of climate change are many.  At the top of the list is global warming, which is simply the rise of average temperatures worldwide.  It is estimated that Earth’s average temperature has risen 1.5°F over the last century alone, with an additional 0.5°F to 8.6°F rise estimated for the next one hundred years.  This may not sound like a lot, but for ecosystems that have evolved over millennia in a stable climate, the smallest changes can have drastic consequences.

In addition to warming atmospheric temperatures, the seas are warming as well.  This is what is causing the greatest problems for coral reefs, which have a very narrow temperature tolerance.  Climate change is also causing changes in weather patterns, increases in storm intensity, ocean acidification, and sea level rise due to warming seas and melting glaciers.  Some parts of the world are experiencing changes in rainfall which is causing more flooding in some areas and more droughts in others.

This graphic describes some of the many ways climate change will impact humans as well. Photo courtesy of the CDC.

Click to enlarge: This graphic describes some of the many ways climate change will impact humans as well. Photo courtesy of the CDC.

Climate change is certainly not only impacting sensitive wild populations; it is affecting human civilization as well.  We can expect to see negative impacts to our agricultural resources, wild food sources, fresh water resources, power generation, transportation methods, natural disaster response, and individual health.  Though a certain degree of climate change will happen no matter what because of damage that has already been done, we can each make small changes that could, combined, reduce the severity of climate change.  Sustainable practices can range from simple actions such as unplugging electronics, to recycling, to choosing clean fuel transportation or carpooling.  Check out the US Environmental Protection Agency’s website for information on what you can do.

How Does Climate Change Affect Animals?

The life that exists on Earth now has evolved with fairly consistent climate conditions in each ecoregion.  Evolution is slow, and it takes a very long time for a species to adapt to a new condition.  For this reason, many species disappear from areas that have changed too much as a result of human activity, introduction of exotic species, or complete removal of habitat.  This may not seem like a big deal if a certain animal population can no longer inhabit a certain spot on the map, but imagine if that species’ habitat changed across the entire globe, such that they have nowhere suitable to move to.  Even worse, imagine immobile organisms like plants and sedentary marine invertebrates that cannot move even if they wanted to.

As droughts become more frequent with global warming, many aquatic habitats will dry up, killing aquatic animals and removing important sources of fresh water for terrestrial animals. Photo Credit:

As droughts become more frequent with global warming, many aquatic habitats will dry up, killing aquatic animals and removing important sources of fresh water for terrestrial animals. In addition, dry wetlands presents an opportunity for invasive plant species to colonize.  Photo Credit:

Animals depend on healthy habitats that must meet specific requirements, such as temperature conditions, food and water sources, shelter, and proper nursery areas.  Climate change threatens to change just enough of each of these habitat features that many species will no longer be healthy enough to reproduce, or may not be able to survive at all.  The increase of flooding events will cause more erosion, which destroys habitats and reduces water quality through sedimentation.  Droughts can kill plant communities and remove critical water sources for animals.  Drought conditions also prevent seasonal wetlands from emerging, which are critical habitats for many species and are in some places the only available nursing grounds for amphibians and aquatic insects.  The changing climate affects timing and growth patterns, which is very important for migratory species that depend on certain conditions to be in place when they arrive at their migration destination.  The best example of this is for migratory birds, which could arrive to find that many of their food sources bloomed or hatched much earlier as a result of above average temperatures, and are depleted by the time the birds arrive.

How Does Global Warming Affect Animals?

Though the faces of climate change are many, perhaps the most significant and well-known condition is global warming.  In addition to the effects to terrestrial species just mentioned, the warming planet is causing massive problems for marine species in particular.  The list of a few of the possible global warming effects on animals includes the disappearance of habitat for Arctic mammals as sea ice melts away, the bleaching and mortality of coral reefs, and the death of cold water fishes such as salmon and trout (note that cold water fishes often make up the majority of the fisheries that we depend upon ourselves).

Global Warming and Polar Bears: The Unfortunate Poster Child of Climate Change

These heartbreaking photos were captured of an emaciated polar bear earlier this year in Iceland. This may become a more common site as global warming destroys polar bear habitat. Photo Credit: Kerstin Langenberger

These heartbreaking photos were captured of an emaciated polar bear earlier this year in Iceland. This may become a more common site as global warming destroys polar bear habitat. Photo Credit: Kerstin Langenberger

Perhaps the best and most familiar example of animals affected by climate change is the tragic story of polar bears and global warming.  Just this past summer, an image you have probably seen went viral after a group of German tourists spotted an extremely emaciated polar bear.  The photographer and tour guide, Kersten Langenberger, captured the photo just before the boat made the compassionate decision to turn and leave so as not to stress the bear any further.  The bear was floating on a sheet of ice (called a ‘floe’) in Svalbard, a Norwegian archipelago within the Arctic Sea.  These areas of the Arctic are seeing less and less ice each year, which can be attributed overwhelmingly to global warming.  For polar bears, this is a serious problem that could ultimately lead to their extinction, possibly within our own lifetime.

Polar bears rely on sea ice for camouflage, shelter, and as a vehicle to access more seals, which are their primary food source.  While polar bears are great short-term swimmers, they cannot swim great lengths to access food without expending great amounts of energy.  Blocks of floating ice allow them to wander further out into open water where chances of finding prey are much greater.  In addition, seals themselves will bask on ice, and polar bears are able to sneak up on their prey thanks to the camouflage of their white fur against the ice.  Less ice means blocks are further and further apart and smaller, and seals can easily spot a polar bear against the contrast of the dark water.

A polar bear floats on a small piece of sea ice amidst a mostly ice-free ocean. Photo courtesy of

A polar bear floats on a small piece of sea ice amidst a mostly ice-free ocean. Photo courtesy of

Global warming is causing changes in the Arctic climate faster than anywhere else in the world, and the result is significant reductions in sea ice.  Scientists forecast that this is not going to change any time soon.  Sea ice in the Arctic has been declining at least since 1979, and at an alarming average rate of 11.3% each decade.  Eventually enough research showed significant effects of sea ice reduction on populations of polar bears and even individual bears that polar bears were finally listed as a threatened species in May 2008.  But it’s not enough.  Scientists predict that within the century, two-thirds of the polar bear population could disappear.

The challenge is being able to link models of future population dynamics of polar bears with future environmental forecast affects.  It is difficult enough to model future scenarios, and even more so to make the claim that one will affect the other.  So why do we even need to make these future projections if we can already tell how melting ice is negatively impacting polar bears?  One reason is because the criteria for listing a species as threatened or endangered with the United States Endangered Species Act (ESA) requires evidence that a species is at risk of extinction within the foreseeable future.  However, even if polar bears’ status is elevated under the ESA, having increased protection as an individual species is going to do very little.  What we need is change globally, starting with dramatic reduction in greenhouse gas emissions.

Other Marine Species Affected by Climate Change

A North Atlantic Right Whale breaches, lifting its huge body out of the water. There are very few of these giants left due to past whaling practices. Credit: Courtesy of the New England Aquarium

A North Atlantic Right Whale breaches, lifting its huge body out of the water.  There are very few of these giants left due to past whaling practices. Photo Courtesy of the New England Aquarium

Though famous, polar bears are far from being the only animals affected by global warming.  Many of the marine species also affected by climate change are other cold-water species, such as right whales, penguins, seals, lobsters, and cod.  These animals are affected in much the same way as polar bears; by increasing ocean temperatures and melting sea ice.  Seals use sea ice to rest between hunting, and therefore they are losing habitat right along with the polar bears.  Right whales are a tragic example of human-induced species decline.  Fewer than 500 individuals remain as a result of whaling and overfishing, and now global warming (also human-induced) may finish off this species completely.  For these whales, as well as many other marine animals toward the top of the food chain, the problem is a reduction in food sources starting from the bottom up.  Zooplankton are the primary food source for whales and for important commercially-fished species such as cod, herring, mackerel, and lobster.  The warming seas and shifts in currents are causing a reduction or a displacement of zooplankton patches, which these species all rely on for food.  The effects climb up the chain, reducing survival of the top predators.

Global warming is not the only phenomenon having a negative affect upon marine species.  Though many consider “global warming” and “global climate change” to be synonymous, the warming global temperatures is actually only one component of many changes in climate the world is experiencing.  Other results of climate change are rising seas due to melting glaciers and increased ocean temperatures, as well as an increase in storm intensity.  The combination of these two creates a recipe for disaster for beaches.  As seas rise and intense storms crush the coasts, beaches are eroding away completely.  In fact, just weeks ago one of the most powerful hurricanes ever, Hurricane Joaquin, swept through the Bahamas, Bermuda, the Antilles, and many other areas in September-October 2015.  The storm left behind $60 million in damages and 34 fatalities.

Eggs from a loggerhead turtle nest are exposed to the elements after being unearthed by erosion. Photo Credit: Palm Beach Post

Eggs from a loggerhead turtle nest are exposed to the elements after being unearthed by erosion. Photo Credit: Palm Beach Post

Because of climate change, storms of this intensity can be expected to occur more often.  This is certainly detrimental to our own coastal communities, but it also causes many problems for animals that use the beaches as part of their habitat.  Sea turtles are a prime example.  Every sea turtle nest is made in a sandy beach, and usually it is the same beach from which the females themselves hatched decades before.  A female that returns to her native beach to lay eggs may find the beach no longer exists, and she may not even successfully lay a clutch.  If a sea turtle does successfully deposit her eggs, the chance of an intense storm eroding the nest away is increasing with climate change.  Sea turtle nests are already quite vulnerable to predation from carnivorous mammals and, sadly, humans as well.  The addition of further damage to the nesting beaches does not bode well for sea turtles.

There’s even more at stake for sea turtles as a result of climate change.  Like any developing egg, there are temperature thresholds within which the eggs must remain in order to survive.  Furthermore, the temperature in the sea turtle nest actually affects the sex of the turtle!  Warmer temperatures create females, and cooler temperatures create males.  The eggs toward the top of the nest and closer to the sun-warmed sand are often females, and the eggs further down in the cool moist sand often become males.  With rising global temperatures, a couple interesting but dangerous things can happen.  First of all, if the sand warms a little higher than normal, there could be an imbalance of the sex ratio in favor of more females.  Balanced sex ratios are very important to the health of animal populations, so this could actually cause more problems than it may seem.  Secondly, if the sand warms even more, an imbalanced sex ratio is the least of our worries; the baby sea turtles may not survive to hatch at all.

How Could Climate Change Affect Coral Reefs?

Expansive colonies of bleached corals. Photo by Ray Berkelmans.

Expansive colonies of bleached corals. Photo by Ray Berkelmans.

Global warming is perhaps the greatest of coral reef threats.  As sea temperatures rise, the delicate coral undergo much stress that results in what is called ‘coral bleaching’, which if prolonged, can result in mortality.  When ocean waters are abnormally warm, the all-important and colorful algae are expelled, leaving a clear polyp on top of a white calcium carbonate skeleton.  A bleached coral is still alive, and corals have the ability to recover from isolated or short bleaching events.  However, longer periods of bleaching or repeated instances of bleaching will cause coral mortality.  Right now coral reefs are currently experiencing one of the worst global bleaching events in history, and many will likely not survive.

In addition to warming temperatures causing coral bleaching, another concerning climate change variable is ocean acidification.  As we expel more CO2 into the atmosphere, the ocean does a great job of absorbing the extra CO2 that doesn’t get used up or reflected back into space.  At one time this was once celebrated as the Earth’s “self-regulation” of the problem of excess carbon dioxide.  In fact, the ocean has absorbed about half of our CO2 output over the last few decades.  However, it is now understood that this comes at a price.  When CO2 dissolves into the ocean’s surface waters, it becomes carbonic acid.  At the rate this is happening, it is causing a noticeable change in seawater pH to become more acidic.  For many marine species that are sensitive to their chemical and environmental environments, this is a big problem.  For corals, the acidification means a decrease in the amount of carbonate in the water.  Because of the extra CO2, much of the available carbonate is becoming bicarbonate instead.  Carbonate is what corals use to make their calcium carbonate (limestone) skeletons, so a reduction in this necessary ion means much slower coral reef growth.

The damaging effects of climate change are enough, but coupled with earlier-mentioned anthropogenic disturbances like pollution, dredging, and overfishing could be a death sentence for corals.  In remote tropical regions, many reefs still thrive and can adapt to some of the more natural disturbances without the added burden of human interference.  But in more developed parts of the tropics, the combination of climate change and human disturbance could mean these will be among the first reefs to go.

The Outlook for Coral Reefs: It’s Not Pretty

This graph shows the warming trend that has been in effect since the 70s. Photo courtesy Wikimedia commons.

Click to enlarge: This graph shows the warming trend that has been in effect since the 70s. Photo courtesy Wikimedia commons.

While corals do have the capability of adapting in many unfavorable situations, this is usually only successful if said conditions are isolated, mild, or short in duration.  The environmental stressors that most affect coral reefs are only going to continue to get worse as we experience climate change, and this is what makes it seem unlikely that corals will be able to adapt successfully.  The current warming trend has been happening since the 1970s, and scientists have seen very little, if any, ecological adaptation of coral reefs during that time span.  It seems that a few decades is not long enough for evolution to kick in to save corals, but the rate of climate change is happening on a noticeable scale decade to decade.  Will corals be able to keep up with this?

Since the warming trend began, coral reefs have been declining.  There is perhaps no reef out there that is pristine and healthy enough to be considered a reference point, which is very important in the study of ecology to determine what the goal is when managing certain ecosystems.  However, ecological management has begun to evolve as well in the face of climate change.  There is more and more focus on adaptation rather than mitigation or attempts to restore areas to a “natural” state.  For many habitats, there is simply no going back to that original or ideal state no matter what we do.  Instead, scientists and managers are working on understanding and predicting the effects of climate change so that we can plan for how to keep wildlife healthy in those future conditions, rather than dwelling on an unattainable condition of decades past.  The concept of adaptive management focuses on making decisions despite uncertainty (we don’t have a crystal climate change ball), and then constantly re-assessing and learning from the results of those management decisions and adapting the strategy as we go.  This will be important for the sustainability for coral reefs and all other sensitive ecosystems we hope to protect.

A volunteer diver with REEF does a population survey of fish on a reef. REEF volunteers are citizen scientists, collecting underwater data to expand knowledge about marine life populations. Photo by Ron Lucas.

A volunteer diver with REEF does a population survey of fish on a reef. REEF volunteers are citizen scientists, collecting underwater data to expand knowledge about marine life populations. Photo by Ron Lucas.

Unfortunately, monitoring of reefs around the world only began in the late 1980s.  The last few decades of research have simply not provided enough information about the destruction of these complex ecosystems.  It’s difficult to say which reefs are in the most danger of elimination because many reefs are still not monitored due to a lack of resources.  However, monitoring has increased in recent years thanks to volunteer-based ‘citizen science’ projects such as ReefCheck, REEF Volunteer Fish Surveys, and the Global Coral Reef Monitoring Network.  These projects allow recreational divers and snorkelers to submit observation data, which culminates into a useful database of the conditions of reefs all over the world.

While a lot of scientists are focusing on figuring out how to keep coral reefs from disappearing in the future, other efforts are in place to restore existing reefs.  Some of these methods involve creating artificial reefs by installing rigid structures that could attract the fish and invertebrates that colonize reef substrate.  There are many coral reef nurseries around the world that hand-raise corals in a controlled setting, and then carefully transplant them once mature to a natural reef in hopes that they will colonize successfully.  As with any type of transplant activity, coral transplants have varying degrees of success.  Another method of reef restoration involves sediment stabilization with mesh, blocks, or other rigid structures.  Stabilizing existing natural reef structures and sediment may be enough to encourage corals to recolonize in some areas.

Though it is extremely important to recognize the perhaps bleed outlook for coral reefs, it is even more important to remain positively focused on what can be done to prevent it.  No fate is set in stone, and the predictions that we have now are based on well-tested climate models that show us what we can expect along a gradient of possible scenarios.  Any one of those scenarios can change, if our course of action changes it.  Education is key; understanding what climate change is, why it is happening, and what we can expect will allow us to decide how to react.  If we want to share with future generations the astoundingly beautiful and rich marine wonders that are coral reefs, taking action to address sustainable practices is the first step!



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