Corals – witnesses to the climate emergency

They are the archives of the oceans. Corals are a great indicator of how much human activities affect our oceans. Funded by the Franco-German fellowship program “Make Our Planet Great Again,” researchers in the U Bremen Research Alliance are studying the extent of global warming in tropical waters.

The forearm-thick whitish drill core held by Dr. Henry Wu of the Leibniz Centre for Tropical Marine Research (ZMT) has come a long way. It originates from a stony coral from the coastal region off Rotuma, an island in the Republic of Fiji, more than 15,000 kilometers from Bremen. The oldest corals being examined by the paleo-climatologist are more than 100,000 years old. In the course of their lives, they have accumulated a vast amount of information.

Corals grow on average a few millimeters per year. They thrive best in clean water and live up to 50 meters below the surface of the sea, where sunrays can still reach them. Just like the growth rings of trees, the micro samples from their calcareous skeleton tell of changing environmental conditions: temperature fluctuations, the amount of rainfall, ocean acidification, and salinity – and they do so with month-to-month precision.

Wu is using these archives of the ocean within the context of his five-year research project. “Climate has always been changing naturally. We want to know: How profound were these changes? What impact has industrialization had since the beginning of the 19th century?” says the researcher. “If we know the past, we can better predict the future.”

The 40-year-old has named the project OASIS. The acronym stands for “Witnesses to the Climate Emergency: Ocean acidification crisis and global warming observations from tropical corals.” The title also has a literal meaning: “For me, coral reefs are like oases in the desert: They are places full of life.” Nowhere in the oceans do so many different species exist as in tropical coral reefs – it is estimated that there are one million. Not only are they a significant ecosystem, but they are among the most beautiful and spectacular habitats on earth.

The oceans absorb about 90 percent of the excess heat generated as the Earth becomes warmer due to climate change. They also absorb around one-third of the greenhouse gas carbon dioxide (CO2). Excess CO2 reacts with water to form carbonic acid, lowering the pH of seawater. The more acidic environment makes it more difficult for calcifying organisms such as some plankton species, mussels, and corals to build their calcium carbonate skeleton. These relationships are well known. However, what is less known is how ocean acidification affects the tropics in practice; there is a lack of long-term observations.

The extent to which the pH of seawater has changed can be determined by analyzing boron isotopes in coral drill cores. Boron is a natural component of seawater, and corals absorb it as they form their calcium carbonate skeleton. The pH value determines the ratio of boron isotopes incorporated into the coral skeleton. However, the researchers want to determine not only the pH changes before and since the Industrial Revolution, but also the associated changes in sea surface temperature and water chemistry. This is happening worldwide in regions of the Atlantic, Pacific, and Indian Oceans. Research locations include Indonesia, the Andaman Islands in India, Fiji, Cuba, and Costa Rica.

In recent times, corals have never been exposed to similar stresses. Today’s increased water temperatures are causing coral bleaching and the death of corals. “The scale and speed at which they are dying is unprecedented,” Henry Wu points out. “It is depressing to witness.” If the corals die, this has far-reaching negative consequences for the entire ecosystem with its flora and fauna. Yet, as ocean acidification progresses, there will also be a few individual winners. Certain stony corals are more robust, they are more likely to adapt. “Diversity is decreasing, though; there’s no doubt about that.”

So, there is no doubt that climate change is happening, even though some people are still arguing against it. These people can convince themselves of the opposite with him, Henry Wu emphasizes. “I have Gigabytes of data proving the warming of the oceans caused by humans.”

IDW- Informationsdienst Wissenschaft, 22 July 2021. Full article.

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