Climate memory of corals
Records of sea temperatures have been kept since the middle of the 19th century. The climate data stored in corals goes much further back. This helps us to understand current extreme weather phenomena, such as the ongoing drought in Australia.
The light grey coral drill core, which Miriam Pfeiffer fetches from the cupboard in her office at the Institute of Geosciences, looks like a slim column of concrete. It has nothing in common with the colourful, finely-branched structures in the ocean, for which corals are well known. This is because the colony-forming cnidarians occur in many different species and forms. Pfeiffer's core comes from stony corals of the genus Porites, which grow slowly, mostly in solid structures, and can become very large and old.
For the past 20 years, the Kiel professor of palaeontology and historical geology has investigated current and fossilised coral reefs in connection with climate change, because the corals are a kind of living climate archive. They can record the temperature, just like trees with their annual growth rings. “With corals, the annual growth rings are on top of each other,” explained Pfeiffer, who reinforces the priority research area Kiel Marine Science (KMS). The corals grow approximately one centimetre per year, and - depending on the water temperature - incorporate different amounts of trace elements in their calcium carbonate skeletons. Based on the analysis of these trace elements, and in combination with further studies, we can draw conclusions regarding the water temperature. In contrast with the experimentally-measured sea temperatures, the climate data stored in corals has special advantages: it goes much further back in time, is available in one place, and is less prone to measurement error. “When we work with living corals, we can easily investigate the last 400 years. Only then can we say a phenomenon always recurs, or it occurs once in a while and then not again.” Fossilised corals enable an even further look back into climate history - up to 5000 years ago. They provide important comparative data, for example about natural climatic and environmental variability, and thereby help to place the currently-observed environmental changes in the context of natural fluctuations, and evaluate them accordingly.
The research of Pfeiffer's working group centres around the Indian Ocean and its special climate phenomenon, the Indian Ocean Dipole, a natural anomaly in sea surface temperatures in the western and eastern Indian Ocean. For example, it causes the extreme drought in Australia. “During the course of the 20th century, the Indian Ocean warmed up the fastest of all the ocean basins, and strongly follows global climate warming,” reported the scientist. This warming strengthens the dipole in the Indian Ocean, which is caused by temperature differences between the western and the eastern parts of the Indian Ocean.
Similar to El Niño and La Niña in the Pacific, the dipole also goes through different phases in a cycle of three to eight years, in which the water temperature changes drastically. In the current positive phase, the temperature is higher in the western Indian Ocean and lower in the eastern part, which causes droughts in East Asia and Australia, and on the other hand, torrential rain in parts of India and East Africa. Pfeiffer said “2019 was the most extreme Indian Ocean Dipole ever recorded, at least in terms of the temperature magnitude. In December 2019, this led to torrential rains and flooding in East Africa, as well as extreme dryness and the devastating bush fires in Australia.”
On the one hand, corals are victims of climate change, because rising sea temperatures damage the reefs so badly, that some of them will never recover. At the same time, corals also contribute significantly to determining the climate of past centuries, and thus understanding the processes of climate change. “With the temperatures recorded by the corals, I can calculate the temperature gradients of the Indian Ocean, and thus understand why this dipole in the Indian Ocean has strengthened,” said Pfeiffer. Among other things, this data will be used to further refine the climate models.
Author: Kerstin Nees
Dying coral reefs
Together with other factors (ocean acidification, environmental pollution), the increase in sea temperatures puts the survival of coral reefs at risk. The frequency and intensity of the mass death of corals has reached previously unheard of proportions in the past three decades, and is a direct consequence of global warming. Even at an almost-inevitable global warming of 1.5 degrees, in 2018 the Intergovernmental Panel on Climate Change (IPCC) predicted coral losses of 70 to 90 percent. At an increase of 2 degrees, 99 percent of coral reefs would probably be lost (according to the IPCC Special Report: Global Warming of 1.5°C). Corals do possess a certain degree of resilience and adaptability. But if heat waves follow one another ever faster, they find it difficult to recover. “Even reefs such as the Great Barrier Reef off Australia may disappear completely,” emphasised the Kiel coral researcher Miriam Pfeiffer.
Although coral reefs only cover about 0.1 percent of the global oceans, they provide a habitat for more than 25 percent of known fish, and are therefore an important source of food for the local population. They are also an important economic factor in tourism for many countries, and serve as coastal protection. If we had to replace the coastal protection function of the coral reefs with artificial breakwaters, this would cost a great deal of money. Pfeiffer said “Even if it seems so exotic and far away to us: half of the world's population lives in the tropics. Around 275 million people live directly in the sphere of influence of a coral reef.” (ne)