<h2 class="pgc-h-arrow-right" > oasis in the ocean</h2>
Coral reefs in shallow tropical and subtropical waters at 30 degrees north-south latitude are among nature's most spectacular and wondrous "creations".
The beauty of the reef lies in its color and diversity. Corals are made up of polyps, as well as a wide variety of strange marine animals that swim, crawl and glide around corals. It is home to many flora and fauna. A quarter of the world's 500,000 marine animals are said to live in coral reef waters. In addition, some fish living in the ocean will use coral reefs as a place to spawn and raise juveniles.
The largest and most beautiful coral reef in the world is Australia's Great Barrier Reef. It stretches from the Strait of Torre in the north to the vicinity of Fraser Island in the south, with a total length of more than 2,000 kilometers. It is less than 2 kilometers to the north and more than 150 kilometers to the south. It consists of more than 2,900 islands and reefs with a total area of 207,700 square kilometers. The builder of this huge "project" turned out to be a coelenterate with a diameter of only a few millimeters, the polyp.
Corals belonging to marine invertebrates have only a few millimeters of thin layers on the surface of the coral group, but the carbonate bones that various reef-building corals and other reef-building organisms continue to secrete and accumulate eventually form carbonate structures up to a thickness of up to 2,000 kilometers and a length of up to 2,000 kilometers, which is difficult for any other organism on the earth's surface to match.
The secret of polyps' ability to continuously multiply and secrete calcareous bones lies in the mutual benefit and symbiosis of polyps and a huge number of single-celled plants: corals provide shelter for zooxanthellae; xanthophyllum uses the metabolites of corals to photosynthesize and synthesize organic matter, thereby accelerating the organic carbon cycle and the precipitation of calcium ions to form the "bones" of corals. Some organic matter is secreted into the yellow algae in the form of mucus, becoming a nutrient for other marine small organisms to survive. It can be said that the rich ecosystem of the ocean is formed on the basis of polyps.
In addition to providing nutrients to polyps and forming coral "bones", zooxanthellae also provide corals with colorful colors. Because zooxanthellae themselves contain a variety of pigments, they always coexist with corals under normal water temperature conditions, thus giving corals a beautiful color of blue, red or yellow.
Coral reefs have a major role for coastal residents, they can not only resist the wind and waves to invade the coast, but also provide human beings with marine aquatic products and marine new drug materials.
Unfortunately, the massive exploitation of tropical coasts, overfishing of seafood, water pollution and global climate change caused by human factors in the past 20 years have led to severe damage to many coral reefs and facing a desperate situation.
<h2 class="pgc-h-arrow-right" > disappearing reef</h2>
As early as the 1990s, it was discovered that coral reefs were undergoing increasingly significant large-scale changes, but there is much debate about why, particularly in evaluating the impact of global climate change on coral reefs. In 1994, a panel of 15 experts studying the impacts of climate change on coral reefs also reported that the main pressure on the survival of coral reefs comes from human activities and that the threat of climate change to coral reefs is still far away.
From mid-1997 to the end of 1998, the largest throbbing coral (and soft coral, brick, sponge, etc.) of stony corals (as well as soft corals, bricks, sponges, etc.) occurred worldwide in more than 40 years (due to the loss of cogenerated zooxanthellae, the corals were transparent and showed coral white bones) and death events, including corals with a depth of 40 meters and corals that grew for 1000 years (according to which some people judged) The loss rate of coral reefs in the Indian Ocean was as high as 50%, and shallow waters and branching corals were particularly affected.
Before that, people began to pay attention to climate change. As early as 1998, the world's highest temperature and the strongest El Niño-La Niño phenomenon occurred, resulting in an abnormal increase in sea surface temperature that year. The timing of coral reef bleaching events and high temperatures is so coincidental that we have to wonder if there is an inevitable connection between them. As a result, the idea that climate change does affect coral reefs is gradually being accepted by researchers.
If the mass coral bleaching deaths in 1998 were just an accident, or even a once-in-a-millennium extreme event, the frequent coral bleaching incidents in various seas in recent years have sounded the alarm. In the summer of 2015, anomalous high temperatures and large-scale coral bleaching events re-occurred in the Coral Reef Area of the Caribbean Sea. More than 50% of coral bleaching occurs in coral reef areas, and half of them die quickly. Coral reef bleaching has also occurred in other parts of the world. Moreover, with the development of global warming, coral bleaching events in 1998 and 2015 have become more frequent and the degree of bleaching has become more and more serious. A range of evidence suggests that global warming has been identified as the biggest threat to coral reefs in the future.
Although human activities can also adversely affect coral reefs, the impacts are far less than that of climate change on coral reefs. According to a 2000 statistical report by the Global Coral Reef Monitoring Network (GCRMN), the 1998 bleaching event resulted in a global coral reef loss of 16% (although partially bleached reefs recovered to varying degrees in the ensuing time).
< h2 class="pgc-h-arrow-right" > fatal temperature</h2>
The impacts of global climate change on coral reefs include global warming of seawater caused by increased greenhouse gases, ocean acidification, rising sea levels, and increased storm frequency and intensity, with the first two factors being the most influential. This is because reef-building corals are tropical communities and temperature is an extremely important ecological factor for them. The sea temperature in the growth area is 18-29 ° C, the optimal temperature is 18-20 ° C, 16-17 ° C, the coral stops eating, and 13 ° C is the lethal temperature. Too high or too low is not conducive to the growth or survival of reef-building corals.
The results show that the impact of global warming on coral reefs is mainly manifested in two aspects: due to the rise of low temperature in winter, the frequency of cold tide is reduced, the growth conditions of reef-building corals in high latitudes have been improved, and even the trend of expansion to high latitudes has emerged, for example, the only coral reef area in the tropical northern edge of China's Leizhou Peninsula in the southwest of China, the natural coral recovery process has occurred in the past 10 years, on the other hand, due to the abnormal increase in sea surface temperature in summer, the situation has intensified due to the abnormal increase in the sea surface temperature in summer, resulting in the escape of symbiotic zooxanthellae and the increase in coral reef bleaching and death events. Coral bleaching represents the immediate response of corals to environmental pressures such as abnormal increases in sea surface temperatures, and coral bleaching is also the earliest sign of coral reefs under the impact of global warming.
Albino corals that lose symbiotic algae will no longer be able to obtain the most important energy for life from the products of symbiotic algae photosynthesis and will not be able to survive. The temperature threshold that causes coral bleaching is that the surface water temperature exceeds the local summer maximum temperature by 1 °C, which is roughly around 30 °C, with slightly higher sea areas at low latitudes and slightly lower sea areas at higher latitudes. Considering that the increase in atmospheric greenhouse gases caused by human activities in the last century has led to an increase in the global average temperature of about 0.6 °C and a rise in water temperature in tropical coral reef areas by about 0.5 °C, the global average surface temperature is expected to increase by 1.4 to 5.8 °C between 1990 and 2100. Even if all greenhouse gas emissions were stopped now, global average surface temperatures would almost certainly rise by at least 1°C by the end of the century, and coral bleaching would inevitably become more frequent and severe. If the average global temperature rises above 2°C, corals are expected to not have enough time to recover between bleaching and coral reefs will disappear in many shallow seas of the world.
This means that without strong global climate policies, bundles of coral reefs are expected to inevitably become the first ecosystems to disappear as a result of global climate change this century.
The magnitude of the impact of global warming on coral reefs is that the adaptive genetic changes of corals and their symbiotic algae to global warming cannot be kept pace with climate change. In the past, scientists believed that polyps may have an adaptive mechanism to tolerate and keep up with future SST increases by altering the combination of symbiotic algae (the heat-intolerant C-type to the more heat-tolerant D-type).
But the scientists found that corals in reef areas that were severely affected by bleaching and death from rising sea temperatures contained more heat-tolerant D-type symbiotic zooxanthellae, and they also did not observe lasting changes in coral-zooxanthellae partnerships before and after major bleaching events. Observational records over the past 20 years have shown that coral thermal pressure thresholds are relatively stable and do not move upwards as temperatures rise. Taking a step back, even if this adaptation does exist, the rate of genetic change will still not keep up with the frequency of bleaching events.
< h2 class="pgc-h-arrow-right" > acid-changing environment</h2>
The impact of global climate change on coral reefs is also reflected in ocean acidification. Ocean acidification, the increase in the concentration of C02 in the global atmosphere, leads to acidification of surface seawater (more precisely, weakening of the ocean's micro-alkali state) and changes in the ocean carbonate system. This process can seriously affect the calcification process of marine calcareous organisms such as coral reefs. Now that the global average pH of surface seawater has been reduced by 0.1 units compared with before the Industrial Revolution, the C02 level of the surface ocean is expected to reach twice the pre-industrial level (560 ppm) in the next 40 to 50 years, and the pH of seawater will be reduced by another 0.2 units. An increase in CO: concentration and concentration of hydrogen ions in surface seawater will lead to an increase in dissolved bicarbonate and a decrease in carbonate in seawater.
Dissolved CO: and bicarbonate in seawater can be used for photosynthesis, favoring seagrass and some marine algae. However, reduced carbonate ion concentrations reduce the ability of many reef-building organisms to form calcium carbonate bones. Scientists predict that the calcification rate of almost all tropical and cold-water corals will decrease by 20 to 50 percent by 2050. Under extreme conditions, some species of corals will completely lose their bones and they will not be able to build or serve the reef. There is evidence that coral growth rates are now 15% lower than before the Industrial Revolution, possibly due to ocean acidification, ocean warming or other factors.
Reduced calcification rates can lead to slower coral growth and slower coral bone formation, weakening their ability to resist erosion, storm damage, and predators. Eventually coral reefs will be transformed from net construction to net loss, and the biogeographic functions that "keep up" with expected sea level rise through reef construction will be weakened or lost.
Satellite monitoring by the U.S. Oceanic and Atmospheric Administration (NOAA) reveals that tropical oceans are warming at a faster rate in the past 10 years, and it is expected that there will be only 8 to 10 years left to reverse the tide, because the atmospheric CO, reaching 450 ppm, the sea water will become more acidic, and the effort to save coral reefs will become very difficult.
For example, researchers have found that some coral reef organisms have suddenly disappeared, such as the Dutch Antilles, a colorful echinoderm sea lily.
The echinoderm sea serpent tail of Cape Floridii,
Sea snakes from Western Australian reefs are gone.
Suspected that these animals are the equivalent of canaries or land frogs where miners are testing gas underground, they may have warning significance as a harbinger of more extinctions due to climate change.
<h2 class="pgc-h-arrow-right" > race to protect coral reefs</h2>
Since the 1990s, countries have undertaken a series of conservation actions to protect coral reefs from becoming the first ecosystems on Earth to disappear. The U.S. plans to reach 20 percent of the coral reef's full protected area by 2025. Australia expanded the area of fully protected areas within the Great Barrier Reef Reserve from 5 per cent in 1981 to 33 per cent...
In July 2018, at the 11th International Coral Reef Symposium held in Florida, USA, 3,500 scientists and managers proposed that in order to limit the adverse effects of climate change on coral reefs, countries must take concerted action to reduce greenhouse gas emissions, avoid atmospheric carbon dioxide concentrations exceeding 450-500 ppm, reduce the dynamic impact of human activities such as water pollution, harmful fishing and habitat destruction, and strive to improve and maintain the resilience of coral reefs. Enhance the resilience of coral reefs to survive the pressures of climate change, and win important recovery time for coral reefs to cope with climate change; at the same time, increase research and financial investment, such as the protection of coral reef areas during severe bleaching events.
In 1989, some scientists noted that the future of tropical shallow-water ecosystems such as coral reefs "depends on the results of competition between two speeds": the rate at which tropical shallow-water ecosystems continue to decline and disappear;
Decades have passed, and the race continues.