Global warming isn’t just warming surface temperatures around the world, it’s also harming our reefs as ocean water temperatures
rise higher and higher every year. Find out what it means for the economy, the ecosystem, and local marine life.
by Alexis Koss & Lisa Koss
Numerous things can cause coral bleaching to occur, including small changes in ocean temperatures (colder temperatures
affect coral too!), runoff and pollution, overexposure to sunlight, and extreme low tides (
source). Mostly driven by climate change, sea surface temperatures are rising each year and even just a small increase
of 1 °C can cause global bleaching events to occur (
source). The longer a bleaching event lasts, the chance for a reef to recovery rapidly diminishes.
Both coral and algae, called zooxanthellae, depend on each other to live. Without the algae, the coral cannot prosper
and will lose it’s distinctive color. When the coral begins to be stressed, the algae will be expelled from the coral,
usually due to rising water temperatures the coral cannot withstand.
When this happens, it is believed that coral programmatically kill some of their weaker cells, so they have a chance
for recovery later when the bleaching event is over (
source). As soon as the algae are absent from the coral for long periods of time, the entire coral will start to
turn white and very pale because it’s major food source is missing.
Healthy
Stressed
Bleached
Dead
If the bleaching event lasts too long, a coral will have no chance to recover. A bleached coral is not only very susceptible
to disease, but it is also starving while the algae are expelled. Once a coral is under stress for too long of a time
period, it will die and turn a brown, black color since the coral’s tissues have disappeared and algae have reappeared
to use the dead coral as a host (
source).
It only takes a few weeks for coral to become bleached, but can take months for an entire reef to recover back to its
original, healthy state. No doubt, some rate of mortality will be seen as coral try to recover during their bleaching
stage.
Coral Bleaching in 1983
Bleaching Event History
Over the last few decades, many coral bleaching events have occurred worldwide, causing severe damage. Most notably,
there were bleaching events during 1998, 2002, and 2005 due to higher than average summer sea surface temperatures
caused by global climate change.
The map shows reports of coral bleaching from
1983 throughout
2010. You can hover over each data point for more information and you can also pan and zoom on the map.
1998
During the
1998 coral bleaching event in the summer, it was recorded as one of the hottest on record for coral reefs during
the 20th century. A survey of over 650 reefs showed that nearly 1/2 of them experienced moderate to high levels of
bleaching. Fortunately, most reefs recovered, with only a small percentage of coral deaths (mostly in the Palm Island
area).
2002
Fortunately, during the
2002 coral bleaching event, it was only slightly more severe than the one witnessed in 1998. The biggest signs
of bleaching were noticed in early 2002, but by the end of April, most reefs had recovered. NOAA observed around 54%
of the nearly 650 reefs surveyed showed moderate to high levels of bleaching, similar to the 1998 event. Less than
5% of reefs suffered a high amount of deaths.
2005
In the
2005 coral bleaching event, the bleaching was mostly localized to one area. However, despite being confined to
the Keppel Islands, up to 98% of corals were bleached on the surrounding reefs, resulting in 30%+ coral deaths.
For a more detailed look, the following line graph allows you to view the number of bleaching cases reported each year.
The peaks, as seen in 1998-99, 2002, and 2005 match the reported bleaching events mentioned previously. You can hover
over the lines to see more detailed information about each year.
Chance for Recovery
Corals do have a chance for recovery if the bleaching event does not last too long. Looking at the line graph to the
right, you can see bleaching cases are rapidly increasing over time. Any noticable peaks indicate a bleaching event
while valleys indicate that the corals have a chance for recovery as the bleaching event starts to wane. However, if
the damage to the reef is too severe (the reef was stressed for too long), you can expect high mortality rates with
few corals left to survive in the reef. Mortality rates determined from a combination of bleaching severity, the length
of the bleaching event, and how much energy the coral has stored before bleaching (
source).
Beyond The Data
Beyond the dataset shown across these two charts, there is a massive bleaching event currently going on from the year
of
2016. NOAA has declared this a global bleaching event due to record breaking ocean temperatures around the
world. Due to the extent of the bleaching event, corals have had almost no time for recovery and reefs are rapidly
declining. Because bleaching events are occuring so often in the last few years, the future is bleak for reefs if action
is not taken immediately (
source).
Coral Bleaching Simulator
After reading about coral bleaching, you can now use the following simulator to simulate how coral bleaching takes place
in the ocean that surrounds us. What happens if the temperature gets too hot? Too cold? What if the tide gets too low,
what happens to the coral then? Find out below!
What Life is Like
This simulator demonstrates what life is like for coral in differing ocean conditions. When corals become stressed by changes in conditions such as temperature, tide level, or sunlight, they begin to expel the zooxanthellae algae in their tissues. When all of this algae has disappeared, the coral becomes completely bleached and vulnerable to disease.
Although, even if a bleaching event occurs, there is still a chance of recovery if the stress-caused bleaching is not severe. Unfortunately though, if the loss of algae is prolonged and the stress continues without a chance for the zooxanthellae to return to the coral's tissues, the coral will eventually die (source).
The Ocean's Temperature
The types of water that corals live in is very small as coral reefs are generally confined to tropical and semi-tropical waters due to very strict environmental restrictions. The temperature within these waters plays an additional role in how well coral thrive and grow (source).
Coral grow and prosper the best within temperatures that are between 23° and 29° C. Not only are coral healthy and growing at a consistent rate, but the fish also have plenty of coral to use as food, shelter, or recruitment habitat. Coral are at their brightest in this temperature range as well due to the algae in their tissues.
When starting the simulation, you will see both coral and fish populate across the grid.
Water temperatures that are slightly outside the optimal zone, coral become stressed, but not enough to completely expel all of the zooxanthellae algae in their tissues. They also turn a lighter color due to some of the algae missing in their tissues. During their stressed state, coral do not grow and they become more susceptible to disease. The fish communities that depend on the coral are not affected at this stage.
See what happens when you change the ocean temperature to only slightly cold or slightly hot.
If the temperatures drop below 18° C or rise above 40° C, coral become bleached and die very quickly if the water temperature is not returned to normal in a very short time span. A decline in the coral population also causes declines in genetic and species diversity. Coral bleaching events are known to lead to significant shifts in fish communities who depend on coral who migrate elsewhere to find healthier coral (source).
See what happens when you change the ocean temperature to really cold or really hot!
The Ocean's Tides
Shifts in tides also contribute to coral bleaching. When there are extreme low tides, corals that are exposed to air can become bleached very quickly due to overexposure to the sun. When outside air temperatures are also high, high solar irradiance has a large impact on shallow-water corals and contributes to bleaching as well (source).
Temperature Controls
Here you can change the ocean temperature all in one place!
Current water temperature is between 23° C and 29° C
The Future
Why Does This Matter?
Protecting coral reefs matters not only to the local communities who depend on them, but the global population as a whole
too.
On a local level, coral reefs help protect the ecosystems and coastlines from damage during a storm or heavy winds. They
prevent the coastline from eroding and washing away shelters of those who live near the water. Additionally, the reefs
are a huge economic asset to the communities who depend on them because they not only provide food for those communities' families,
but they also provide jobs to those who sell the marine life that they catch. Without the reefs, marine life would disappear, as
their shelter would be missing, thus putting those who use the reefs as an income and food source at risk. Reefs also bring
in billions of dollars annually for tourism and allows those countries’ economies to thrive.
But what if you don’t live in a community that depends on coral reefs? Reefs are still important to those that don’t
depend on them! Despite coral reefs making up less than 1 percent of the world’s species population, they have extremely
high biodiversity. Scientists think the reefs could potentially hold treatments or cures for viruses and even cancer!
But if the reefs disappear, they won’t ever be able to confirm or deny their hypotheses.
Healthy Corals
=
Marine Life
Income
Medical Advancements
Regardless of where you are in the world, if the reefs disappear, they will leave an impact on your life.
After viewing and simulating what coral bleaching is all about, you may be wondering how you can help. Coral bleaching
can be caused by a number of things, as discussed earlier, but climate change is the number one cause.
Reducing your carbon footprint is important in helping to reduce greenhouse gases in the atmosphere, which are ultimately responsible
for the rise in temperatures both on land and in the ocean. Try the following tips to help reduce your carbon footprint:
Driving. When possible, try to walk, bike, or take public transportation to reduce the release of carbon
emissions from driving a car alone.
Household. If you leave a room, turn off the lights! Only use the heat or air conditioner when you absolutely
need to. By making these changes, you reduce your energy consumption of CO2 emissions which in turns reduces
your carbon footprint.
Reuse or recycle. The creation of new materials costs energy, so try to reuse items when you can. Recycling
also helps because less CO2 emissions are released when the product is recycled to be turned into something else
rather than creating a new product from unused materials.
Food. If you eat a lot of meat, try to cut it down a bit. By reducing your consumption of meat, you
help reduce the amount of fossil fuel-based fertilizers that have to be used.
Support and use clean energy. If you can, use energy sources that don’t require fossil fuels, such as
wind or solar energy. It not only reduces your carbon footprint, but it reduces others too!
