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How climate change affects corals

Tag icon Education
  • Bleached reef patch in Okinawa, Japan. (The Ocean Agency/XL Catlin Seaview Survey)
  • (Climate Central)
  • The Greenhouse Effect (IPCC)
  • A bleached staghorn coral (The Ocean Agency/XL Catlin Seaview Survey)
The world is warming due to the emission of heat-trapping greenhouse gases such as carbon dioxide and methane. Since the beginning of industrialization 270 years ago, humans have emitted more than 2,200 gigatons of carbon dioxide.

This has been done through the burning of coal, oil, and gas; the transforming of wooded areas into farmland; the construction of roads, buildings, and cities; and through the production of all the goods that we love to consume. It’s pretty easy to ignore the rising carbon dioxide concentration in the atmosphere since we can neither smell, see, nor taste it. However, once released, carbon dioxide and all the other greenhouse gases warm the earth by trapping the earth’s longwave radiation that was meant to be reflected into space. 

In a broader sense, a high concentration of greenhouse gases in the atmosphere works the same way an actual greenhouse does. Shortwave sunlight passes through its roof panels, but when the earth’s surface reflects this incoming energy as longwave radiation, it cannot leave the greenhouse. It is trapped instead and warms the air in the greenhouse, or in the troposphere, which is the lowest layer of Earth’ atmosphere, where most of the warming is happening. And The Greenhouse Effect is accelerating. Collectively, the last five years have been the warmest years in modern records. Global temperatures in 2018 were 0.83 degrees Celsius (1.5 degrees Fahrenheit) warmer than the 1951 to 1980 mean, making it the fourth hottest year, ranking behind those of 2016, 2017 and 2015. (NASA)


The ocean absorbs most of the heat  

Most of the additional heat trapped by the greenhouse gases in the troposphere is absorbed by the world’s oceans, which are also warming as a result. Recent scientific studies on ocean heat content show that ocean warming started in the 1950s and has accelerated in the decades after 1991 just as the greenhouse gas emissions did. Warming oceans contribute globally to increases in rainfall intensity, rising sea levels, declining oxygen levels, declines in ice sheets and glaciers and the destruction of coral reefs. Corals are especially susceptible, because most of the ocean heat is stored in the upper 2000 meters of the water column, right were tropical corals live. (IUCN


Corals like it warm, but not hot

Corals usually like it warm. They thrive in tropical and semi-tropical waters and prefer water temperatures between 23 and 29 degree Celsius (73° – 84° Fahrenheit). Some can even tolerate temperature as high as 40 degree Celsius (104° Fahrenheit) for a very short time. If it gets warmer, most coral species and their photosynthetic algae, which live in the corals’ tissue, experience heat stress. The coral host and its algae rely heavily on each other. While the coral offers a safe place to live and fosters the algae’s photosynthesis with their waste products, the tiny algae in turn share their photosynthetically derived products with their coral. The algae also color the coral.

If this relationship is disturbed by stress, such as rising ocean temperature, pollution or overexposure to sunlight, corals expel their algae, causing the corals to turn completely white, which is why this process is called “coral bleaching”. With the algae gone, the coral loses its main food source and is thus susceptible to diseases. Bleaching does not mean that the corals are dead. They might be able to recover, if the water cools down after a short time. Nevertheless, corals suffer during bleaching events and might not be able to grow or reproduce shortly afterwards. As bleaching events are happening globally and are increasing in severity and frequency, the cooling down of seawater temperatures after a short time is getting rarer and as a consequence, more and more corals die.


The importance of limiting global warming

In 2015 the world’s nations agreed to limit global warming to 1.5 – 2°C above pre-industrial levels, a goal also known as the Paris Climate Agreement. So far, many governments are falling well short of their commitments to reduce their emissions and the world is on track for more than 3°C warming by the end of the century.

Most of the tropical corals will not survive such a temperature increase. In its special report “Global Warming of 1.5°C” , the Intergovernmental Panel on Climate Change (IPCC) emphasizes the importance of limiting the warming to 1.5°C or even less. “Reduced rates of change enhance the ability of natural systems to adapt,” the scientists state. This is particularly important for corals and other sessile organisms, which cannot move and therefore escape the heat. In regard to coral reefs the scientists say: “Multiple lines of evidence indicate that the majority of warmer water coral reefs that exist today (70-90%) will largely disappear when global warming exceeds 1.5°C.” Hence, reducing global carbon emission is the key for giving global reefs a future. It takes us all to reach this goal, not only for our coral reefs, but for the health of our planet and the well-being of future generations.

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