Coral reefs and climate change


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This presentation introduces two of the main threats that climate change poses to the survival of coral reefs: ocean acidification and bleaching events due to global warming.

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  • Coral reefs and climate change

    1. 1. Coral reefs: how they are impacted by climate change, and why it matters.
    2. 2. What are coral reefs ? Coral reefs are underwater calcium carbonate formations secreted by corals. Corals are quite remarkable. They are constituted of colonies of genetically identical invertebrates, with an exoskeleton. “Most corals obtain the majority of their energy and nutrients from photosynthetic unicellular algae that live within the coral's tissue called zooxanthella” (Source: Wikipedia). A coral reef thus associates animal, vegetal and mineral elements. Florida Keys National Marine Sanctuary (CC – NOAA - Commander William Harrigan, NOAA Corps (ret.))
    3. 3. CO2 emissions and ocean acidification Oceans absorb a large quantity of human activity related CO2 emissions. This causes the formation of carbonic acid, which increases the level of acidity of oceans. Oceans have absorbed about 30 % of CO2 through direct chemical exchange. “Since 1750, the pH of the ocean’s surface has dropped by 0.1, a 30 percent change in acidity.” (Source: NASA - CarbonCycle/page5.php)
    4. 4. Ocean acidification: a serious threat to corals Because the exoskeleton of coral polyps are based of calcium carbonates, it is very sensitive to level of acidity levels. The 2012 World Bank report “Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided” states: “If atmospheric CO2 reaches 450 ppm, coral reef growth around the world is expected to slow down considerably and at 550 ppm reefs are expected to start to dissolve. (…) A CO2 level of below 350 ppm appears to be required for the long-term survival of coral reefs (…).” The currently level of acidity is about 390 ppm. Source: World Bank. 2012. Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided. Washington, DC. © World Bank, pp. 24-26
    5. 5. Possible impact of acidification on corals Most CO2 emissions projections imply that increasingly threaten the survival of coral reefs. Diagram from: World Bank. 2012. Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided. Washington, DC. © World Bank., p. 26 (Based on Hare et al. 2011; Rogelj et al. 2010; Schaeffer et al. 2012.)
    6. 6. What is coral reefs bleaching? To learn more about the phenomenon of coral bleaching, we invite you to watch a short Youtube video from the ClimateXchange network:
    7. 7. Increased risks of bleaching events Climate change is likely to induce more frequent episode of unusual temperature variations, which are the cause of bleaching events. “In fact, frequency of bleaching events under global warming in even a 2°C world has been projected to exceed the ability of coral reefs to recover.” (Source: Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided. Washington, DC. © World Bank,p.52)
    8. 8. Scenarios Current climate change scenarios are a call for action. Even with global warming limited to two degrees Celsius, most coral reefs are at risk of definitive disappearance. Detail of an infographic by: World Bank/ CC BY-NC-ND license
    9. 9. A world without corals? “Why should we care about coral reefs anyway? Beyond their clear aesthetic values, they are hotspots of biodiversity. But, they actually support over 500 million people worldwide, and about 30 million people depend entirely on coral reefs because they actually live on them.” Dr. Janice Lough - “Turn Down the Heat” week 3 lecture. Photo: Margaret Wertheim. CC Attribution 2.0 Generic License.
    10. 10. Losing corals reefs would be dramatic for marine ecosystems and humans who depend on it. Yet, corals are already under threat and even limited global warming of 2 degree Celsius could jeopardize most coral reefs future. We must act now, through our individual action, consumption, communication or vote, to rapidly reduce CO2 emissions!
    11. 11. Coral reef ecosystem at Palmyra Atoll National Wildlife Refuge Creative Commons: Jim Maragos/U.S. Fish and Wildlife Service Source: Flickr
    12. 12. To learn more about coral reefs and climate change: US National Oceanic and Atmospheric Administration – Coral reef conservation programme Center for Ocean Solutions World Bank. 2012. Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided. Washington, DC. © World Bank. The international Panel on Climate Change – Fifth Assessment Report (AR5) – Work Group 1