Outdoor Furniture. Indoor Furniture. Sunroom Furniture. Area Rugs. Playa Del Carmen. Special Offers. Our Story. Hat Sizing. Contact Us. Log in. Close cart. It is the sea country home for the first Australians — more than 70 Traditional Owner groups — whose connections to the marine environment date back more than 60, years. Today the Reef is a Marine Park and World Heritage Area, supporting a range of commercial activities and attracting millions of visitors each year who come to enjoy its beauty above and below the water.
The Great Barrier Reef is unique as it extends over 14 degrees of latitude, from shallow estuarine areas to deep oceanic waters. Within this vast expanse are a unique range of ecological communities, habitats and species — all of which make the Reef one of the most complex natural ecosystems in the world.
The reef itself is large enough to be seen from outer space. This reef features species of coral, species of fish, and species of mollusk. The Great Barrier Reef is valuable to the entire world because it harbors several near-extinct species of aquatic animals. Length: 1, miles 1, km. The corals in the Red Sea, especially in the northernmost part found in the Gulf of Eilat or Aqaba, are more resilient than most.
They are often studied for their ability to withstand high water temperatures. Length: miles 1, km. This reef is even more diverse in species count, including threatened species, than the Great Barrier Reef. Length: miles km. This reef is home to species of fish, including whale sharks, and species of mollusk.
Location: The Bahamas between the islands of Andros and Nassau. The Andros Barrier Reef, home to marine species, is famous for its deep-water sponges and large populations of red snapper.
It sits along a deep trench called the Tongue of the Ocean. Area: 15, square miles 40, square km. Location: Indian Ocean northeast of Madagascar. The Saya de Malha Bank is part of the Mascarene Plateau and features the largest continuous beds of seagrass in the world. This reef is more round in shape than most long, elliptical reefs, which is why it is most often measured by area rather than length.
Corals use calcium carbonate in waters to build a hard exoskeleton. But the key to their success is their symbiotic relationship with algae. They contain microscopic algae called zooxanthellae that photosynthesize just like any other plant would , providing corals with the majority of their energy. This important relationship can be disrupted when corals are put under stress. One of the biggest stressors is warmer temperatures.
When exposed to warmer waters, corals expel their algal symbionts, meaning they lose their source of energy. Corals do not always die immediately after bleaching. Some do, but most survive. Throughout history, corals have experienced infrequent coral bleaching events. But when corals are hit with bleaching event after bleaching event, in short succession, they struggle to recover. These events often kill them off. With climate change, the global average temperature is rising.
We often think about atmospheric or terrestrial temperatures, but our oceans are warming too. We might expect that corals have been under increasing pressure from warming waters. Do we see evidence for this in the real world? Satellite data allows us to track specific changes in water temperatures around specific reefs, and the level of thermal stress that they are exposed to.
The first study to do this at a global scale found that the percentage of global reefs that were impacted by bleached stress tripled over the year period from to In a more recent study, published in Science , Terry Hughes and colleagues tracked the frequency of coral bleaching events across pantropical locations from to They not only quantified the total number of bleaching events but also their intensity. Severe episodes represent events where mass, rather than localized, bleaching has happened.
In the chart, we see the number of events that occurred across these reef locations by year. Moderate events are shown in blue; severe events in red. What we see is a general increase in the number of bleaching episodes over time. From the millennium onwards, it was rare that there would be less than ten. We also see large spikes in particular years: , , , , and But it has an impact on the climate across the tropics and subtropics more broadly.
In the chart we see the number of bleaching events across the locations we looked at previously, colored by the phase of the ENSO cycle at the time.
We see examples of this in the and spikes. This was particularly pronounced when we look at severe bleaching events, which is shown in the next chart. This is no longer the case. This applies to the total number of bleaching events, but also the severe ones.
When we look at sea surface temperature data over these decades, this makes sense. In the chart we see the tropical sea surface temperature anomaly — the change in the sea surface temperature relative to the average from As I already mentioned, occasional bleaching episodes have been a constant phenomenon for most corals.
Most of them survive. In the chart we see the frequency of the coral bleaching events across our locations from onwards. The height of each bar represents the number of locations that experienced the given number of events over these decades.
This is shown separately for the total number of events the pale bars and severe events the dark red bars. By looking at the two extremes of the chart we see just how widespread global bleaching is. On the far left we see that only 3 of the locations did not experience a bleaching event over this period.
And, only 9 did not have a severe event. On the far right we see that many experienced bleaching events every few years. Nearly one-third of locations had eight or more episodes since Just over half of the reefs experienced two or three severe bleaching events. Some experienced many more. This means the vast majority had a severe bleaching event at least once per decade. The frequency distribution tells us something about how common bleaching events are. Shorter recovery times can mean a higher likelihood of corals dying off.
In the chart here we see the return time of severe coral bleaching events across the locations. This measures the number of years after a severe bleaching event that we would expect another one to hit. In dark blue we have the return times in the s and 90s; in green we have the return times since the year Overall, the estimated return time of severe bleaching events has declined from once every 27 years in the s to once every 5. This reduction is visible in the chart. If we look at the blue bars, which show us the return times in the s and 90s, we see a roughly equal distribution: around one-fifth of reefs experienced severe events every 1 to 3 years; one-fifth every 4 to 6 years; all the way to 13 years or more.
Contrast that to the green bars, showing this distribution since It has shifted to the left, towards shorter times. Much fewer locations have return times of ten years or more. And the number of locations with 1 to 3 years; or 4 to 6 years has increased a lot. This is much shorter than the typical 10 to 15 years that most corals need to recover from severe bleaching. Some can recover faster, but most do not. Shorter and shorter times between events will ultimately lead to the mortality of many corals.
It will also change the composition of the reefs that remain. Some species are more resilient than others to warmer waters; these might survive while the less resilient species die. The ability of coral reefs to adapt to warmer waters has been a hot topic for conservation. In a follow-up article I take a look at the latest research on how adaptable reefs are to warming and what their future prospects might look like.
Coral bleaching is a phenomenon that many people have heard of, but have never seen in the real world. Does it immediately die? Is a bleached coral a dead coral? Can it recover? The frequency and intensity of coral bleaching events have been increasing as a result of climate change [I looked at the global trends in bleaching events here ]. As our oceans continue to warm in the coming decades, we would expect corals to experience even more. Corals build a hard exoskeleton using calcium carbonate in ocean waters.
When corals are put under stress through extreme temperatures this important relationship breaks down. They expel their algal symbionts and lose their source of energy. Hughes et al. They combined this with underwater assessments on 63 reefs spread right along the coast.
These reefs covered the full spectrum of heat stress and exposure to bleaching. They measured a number of key metrics: initial mortality; the extent of coral bleaching; longer-term changes in coral cover; and changes in the composition of these reefs. All of these metrics were measured relative to the levels of heat stress that corals experienced. This gives us an understanding of what levels of warming need to be reached for corals to die; become bleached; and what stresses they might still be able to recover from.
DHW measures the amount of stress that corals have been under in the previous weeks. The higher the value of DHW, the more heat stress a reef is under. Many millions of corals died almost immediately in the northern Great Barrier Reef in March This happened at the peak of the extreme temperatures over the course of only two to three weeks.
This initial mortality was not caused by the normal process of bleaching where corals lose their algal symbionts and ultimately starve. Instead, temperature-sensitive corals died immediately from being exposed to stress that they had no hopes of recovering from. In the chart we see the percentage of corals on each of the 63 sampled reefs that died off immediately. This is plotted against levels of heat stress, where further to the right means more heat stress.
These deaths are not driven by bleaching. But, more corals can die from bleaching episodes over the months and years that follow. On the GBR, over the winter that followed the extreme warming episode in , corals took one of two paths.
Some survived and recovered their algae. This means they regained their color. Other corals did remain bleached and continued to die off over the winter months. They could not recover their algae and the vital energy source that they provided.
In the chart we see the relationship between the reduction in coral cover — the amount of corals that die off — and the extent of coral bleaching. As we might expect, there is a strong relationship between the amount of corals that are bleached and the amount that die off. But above this level of bleaching, reefs lost more and more coral with further bleaching. In the right-hand panel, we also see how the change in coral cover relates to exposure to heat stress.
As we might expect, reefs that experienced more heat stress tended to see a larger reduction in coral cover. More heat stress led to more bleaching which led to more long-term coral loss. There was a threshold beyond which corals died off very quickly. Just like any other animal, corals span a wide range of species. These species have different characteristics, shapes, and grow at different rates.
Some grow like platelets; others like branching trees. Some grow rapidly; others much more slowly. This means that reefs can have very different compositions depending on the types of corals that are present.
Coral taxa respond differently to heat exposure. Some are highly intolerant to heat stress and die off immediately, while others can withstand high levels of stress and recover quickly. For example, the tabular staghorn Acropora, Seriatopora hystrix, and Stylophora pistillata are fast-growing branching species that dominate many reefs in the Indo-Pacific region. It becomes more dominant in the resilient species and loses all of the vulnerable ones.
Researchers can measure changes in reef composition using a metric called the nMDS score. A higher score means a larger change in the make-up of a reef ecosystem. In the chart we see the shift in composition across the sampled Great Barrier reefs in This is measured relative to the amount of heat stress that each reef experienced. Again, we see that the relationship is not linear. At lower levels of heat stress, bleaching levels are lower and there is not much differentiation between winners and losers.
This leaves only the species that can withstand high levels of heat stress. They become the dominant species, which can completely transform reef ecosystems. With satellites, surveys and climate modelling we can now track changes in the atmosphere and the oceans at high-resolution. This means scientists can understand when and where we would expect coral bleaching to be a problem in any given season. To make these forecasts we need to make assumptions about what levels of heat stress lead to seriously bleaching, and mortality.
But, these recent studies on the Great Barrier Reef suggest the current guidelines on tolerable levels of heat stress might be too low. The experience of the heatwave on the Great Barrier Reef suggests that these guidelines are too high. Corals can bleach and die at lower levels of stress. The loss of many of its corals is almost an inevitability.
The biggest thing we can do to protect them is to slow global climate change by reducing our greenhouse gas emissions as quickly as possible.
Globally we have seen an increase in the frequency and intensity of coral bleaching events. This has been driven by increasing temperatures, with warmer waters putting corals under stress.
The average time between mass bleaching events has fallen five-fold, from once every 27 years in the s to once every 5.
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