Global Warming and Coral Reefs

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Great Barrier Reef
The Earth's atmosphere and oceans are warming due primarily to greenhouse gases derived from human activities and this having a profound impact on coral and reefs. As temperatures rise, mass coral bleaching events and infectious disease outbreaks are becoming more frequent. Additionally, carbon dioxide absorbed into the ocean from the atmosphere has already begun to reduce calcification rates in reef-building and reef-associated organisms by altering seawater chemistry through decreases in pH. This process is called ocean acidification. Contributing factors that increase greenhouse gases in the atmosphere include burning fossil fuels for heat and energy, producing some industrial products, raising livestock, fertilizing crops, and deforestation. [Source: NOAA]

Elizabeth Weise wrote in USA Today: Coral are vital to the health of the oceans. Although they cover only 0.2 percent of the ocean floor, they are home to at least a quarter of all marine species. They provide safety for juvenile fish and are home to the small organisms and fish which provide food for larger fish. Scientists estimate that the reefs account for 25 percent of fish caught in developing countries. [Source: Elizabeth Weise, USA Today, April 7, 2022]

Coral reefs can survive within only a relatively narrow temperature band. The coral that build them get much of their food from algae living in their tissues. When the seawater is too warm, the coral’s stress response is to expel algae, causing the coral to turn white. The process is called coral bleaching, and if it lasts too long, the coral can starve — turning a thriving ecosystem into a cemetery of dead shells.

A report released in 2021 showed that almost 15 percent of the planet's reefs have vanished since 2009, primarily because of climate change. “They’re being cooked to death,” said Dutton, a MacArthur Genius Award winner, who studies the deep history of the oceans. “The frequency at which we’re seeing these bleaching events is astounding to those of us who study them,” she said. “It’s going to have a huge domino effect on marine systems and on humans.” Waters off Australia's northeast coast face more frequent and severe marine heatwaves, environmental group Climate Council said in 2022, after sea surface temperatures there rose to around 2-4 degrees Celsius above average.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 ; Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium ; MarineBio ; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Wikipedia article Wikipedia ; Coral Reef Alliance ; Global Coral reef Alliance ;Global Coral Reef Monitoring Network

How Climate Change Affects Coral Reefs?

20110307-NOAA coral bleaching_100.jpg
bleached coral
Climate change will affect coral reef ecosystems, through sea level rise, changes to the frequency and intensity of tropical storms, and altered ocean circulation patterns. When combined, all of these impacts dramatically alter ecosystem function, as well as the goods and services coral reef ecosystems provide to people around the globe. [Source: NOAA]

Increased greenhouse gases from human activities result in climate change and ocean acidification. Climate change = ocean change. The world's ocean is a massive sink that absorbs carbon dioxide (CO2). Although this has slowed global warming, it is also changing ocean chemistry.

Climate change dramatically affects coral reef ecosystems Climate change leads to: 1) A warming ocean: causes thermal stress that contributes to coral bleaching and infectious disease. 2) Sea level rise: may lead to increases in sedimentation for reefs located near land-based sources of sediment. Sedimentation runoff can lead to the smothering of coral. 3) Changes in storm patterns: leads to stronger and more frequent storms that can cause the destruction of coral reefs. 4)Changes in precipitation: increased runoff of freshwater, sediment, and land-based pollutants contribute to algal blooms and cause murky water conditions that reduce light. 5) Altered ocean currents: leads to changes in connectivity and temperature regimes that contribute to lack of food for corals and hampers dispersal of coral larvae. 6) Ocean acidification (a result of increased CO2): causes a reduction in pH levels which decreases coral growth and structural integrity.

Affects of Ocean Acidification on Coral

Global warming is causing ocean temperatures as well as air temperatures to rise. An increase of carbon dioxide in seawater cause acidification in the oceans The mixture of carbon dioxide and seawater creates carbonic acid, the weak acid in carbonated drinks. The increased acidity reduces the abundance of carbonate ions and other chemicals necessary to form calcium carbonate used make sea shells and coral skeletons. To get an idea what acid can due to shells remember back to high school chemistry classes when acid was added to calcium carbonate, causing it to fizz.

Corals build reefs of calcium carbonate using carbonate ions drawn from the surrounding water. But when seawater becomes more acidic, fall, these ions become depleted and the corals start to run out of material they use to build their reefs homes. Scientists have predicted that if carbon dioxide levels double, the reef-building powers of the world's corals could fall by up to 80 percent. If they can't rebuild quickly enough to match natural processes of decay and erosion, reefs cold start to disappear. [Source: Ed Yong, Discover magazine, November 16, 2008]

Selina Ward, a researcher at the University of Queensland, has been studying coral reproduction on Heron Island, has found through her research that lower pH leads to declines in fertilization, in larval development, and also in settlement — the stage at which the coral larvae drop out of the water column, attach themselves to something solid, and start producing new colonies. "And if any of those steps doesn't work, you're not going to get replacement corals coming into your system," Ward said. [Source: Elizabeth Kolbert, National Geographic, April 2011]

Affects of Ocean Acidification on Coral Reefs

Elizabeth Kolbert wrote in National Geographic, “Ocean acidification adds yet another threat, one that may be less immediate but ultimately more devastating to hard, reef-building corals. It undermines their basic, ancient structure — the stony skeleton that's secreted by millions upon millions of coral polyps over thousands of years....To make calcium carbonate, corals need two ingredients: calcium ions and carbonate ions. Acids react with carbonate ions, in effect tying them up. So as atmospheric CO2 levels rise, carbonate ions become scarcer in the water, and corals have to expend more energy to collect them. Under lab conditions coral skeleton growth has been shown to decline pretty much linearly as the carbonate concentration drops off.[Source: Elizabeth Kolbert, National Geographic, April 2011]

Slow growth may not matter much in the lab. Out in the ocean, though, reefs are constantly being picked at by other organisms, both large and small. (When I went snorkeling off One Tree Island, I could hear parrotfish chomping away at the reef.) "A reef is like a city," said Ove Hoegh-Guldberg, who used to direct the One Tree Island Research Station and now heads the Global Change Institute at Australia's University of Queensland. "You've got construction firms and you've got demolition firms. By restricting the building materials that go to the construction firms, you tip the balance toward destruction, which is going on all the time, even on a healthy reef. In the end you wind up with a city that destroys itself."

By comparing measurements made in the 1970s with those taken more recently, Caldeira's team found that at one location on the northern tip of the reef, calcification had declined by 40 percent. (The team was at One Tree to repeat this study at the southern tip of the reef.) A different team using a different method has found that the growth of Porites corals, which form massive, boulderlike clumps, declined 14 percent on the Great Barrier Reef between 1990 and 2005.

Ocean acidification seems to affect corals' ability to produce new colonies as well. Corals can, in effect, clone themselves, and an entire colony is likely to be made up of genetically identical polyps. But once a year, in summer, many species of coral also engage in "mass spawning," a kind of synchronized group sex. Each polyp produces a beadlike pink sac that contains both eggs and sperm. On the night of the spawning all the polyps release their sacs into the water. So many sacs are bobbing around that the waves seem to be covered in a veil of mauve.

Once a reef can no longer grow fast enough to keep up with erosion, this community will crumble. "Coral reefs will lose their ecological functionality," Jack Silverman, a member of Caldeira's team at One Tree, told me. "They won't be able to maintain their framework. And if you don't have a building, where are the tenants going to live?" That moment could come by 2050. Under the business-as-usual emissions scenario, CO2 concentrations in the atmosphere will be roughly double what they were in preindustrial times. Many experiments suggest that coral reefs will then start to disintegrate.”

Climate Change Could Be "Catastrophic" for Coral Reefs

In 2022, United Nations warned climate change will result in coral bleaching that will be "catastrophic" for reefs, and potentially, the marine life that live around them and study said the negatively impact of climate change will "overwhelm" reefs, and almost none of them will be able to escape a grave scenario. [Source: Li Cohen, CBS News, February 3, 2022]

CBS News reported:“The latest study, published in PLOS Climate in February 2022 focused on thermal refugia, areas of coral reefs that can maintain the temperatures that coral reefs need to survive, even as nearby ocean temperatures increase. Presently, about 84 percent of reefs are thermal refugia and have had enough time to recover between heat waves that bleach, and kill, coral reefs. Once the planet hits 1.5°C of warming, researchers said, just 0.2 percent of Earth's thermal refugia will have enough time to recover between extreme heat events, and more than 90 percent of those reefs will suffer "an intolerable level of thermal stress." At 2°C, researchers found, no thermal refugia will remain, and all coral reefs will be exposed and vulnerable.

“This study was published the same day that other researchers concluded that marine heat is the "new normal" for oceans. The only areas researchers believe might be able to survive the 1.5° threshold are small regions in Polynesia and the Coral Triangle where lower rates of warming are anticipated. But even those regions would no longer be suitable if Earth hits 2°C of warming. "Our finding reinforces the stark reality that there is no safe limit of global warming for coral reefs," lead author of the study Adele Dixon said in a statement.

“And the world may just be years away from watching this unfold. The U.N.'s Intergovernmental Panel on Climate Change (IPCC) said in August 2022 that the world is likely to hit the 1.5°C of warming threshold in the early 2030s. As the IPCC explained, global warming of 1.5°C will result in more frequent and more intense extreme heat events. It usually takes coral reefs about a decade to grow back and be fully functional again after a severe coral bleaching event, but under the predicted climate scenarios, they will not have enough time to recover.

Reefs Could Perish by 2100 Experts Warn

In July 2009, Reuters reported: Increasingly acidic oceans and warming water temperatures due to carbon dioxide emissions could kill off the world's ocean reefs by the end of this century, experts told a meeting in London, saying the predicted pace of emissions means a level of 450 parts per million of carbon dioxide (CO2) in the atmosphere will be reached by 2050, putting corals on a path to extinction in the following decades. The experts were comprised of two dozen coral reef specialists and climate change experts representing universities, government research offices and the Intergovernmental Panel on Climate Change. [Source: Michael Kahn, Reuters, July 6, 2009]

"The kitchen is on fire and it's spreading around the house," Alex Rogers of the Zoological Society of London and the International Program on the State of the Ocean, said in a statement. "If we act quickly and decisively we may be able to put it out before the damage becomes irreversible."

"If CO2 is allowed to reach 450 ppm, as is currently widely regarded as being the most optimistic threshold target for world leaders to agree at Copenhagen, we will have put the world's reefs on a path to major degradation and ultimate extinction," John Veron, the former chief scientist of the Australian Institute of Marine Science, told the meeting. "Such a catastrophe poses a dire threat to the future wellbeing of all humanity."

The scientists agreed that governments should strive for a level of 320 parts per million of carbon dioxide, saying 360 was a breaking point for reefs to survive. At the current level of 387 parts per million of carbon dioxide, reefs are in serious decline, they said. This will have a future knock-effect that threatens other marine and coastal ecosystems.

Corals Survive Acid Oceans by Switching to Their Soft-Bodied Form

In 2008, Maoz Fine and Dan Tchernov from the Interuniversity Institute of Marine Science, Israel, announced that have found that some corals survive acid sewater by switching to soft-bodied mode. Ed Yong wrote in Discover magazine: They grew some fragments form two European coral species under normal Mediterrenean conditions, and others in water slightly more acidic, by a mere 0.7 pH units. Those that spent a month in the acidic tank were quickly transformed. The skeleton dissolved and the colony split apart. The exposed and solitary polyps, looking like little sea anemones, still remained attached to rocky surfaces. [Source: Ed Yong, Discover magazine, November 16, 2008]

“Even without their protective skeletons, they survived for over a year and seemed to be going about business as usual. They thrived, they reproduced normally and they still kept the symbiotic algae that allow them to produce energy through photosynthesis. And when they were put back in normal conditions, they readily gave up their independence and re-formed both colonies and hard shells.

Fine and Tchernov's findings suggest that corals may be able to survive upcoming climate changes by adopting soft-bodied, free-living lifestyles. And there is evidence that they have used this trick before. The hard shells of coral reefs fossilise easily, but the fossil record still has large gaps where no reefs are found. These may represent periods of time when corals were biding their time in their soft-bodied phase instead. Reference: Fine and Tchenov. 2007. Scleractinian coral species survive and recover from decalcification. Science 315: 1811.

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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