What Can Coral Reefs Teach Us About Climate Change of the Past?

Written by: Sofia Perez

Edited by: Ayla Sage

One of the hallmarks of any modern-day discussion about coral reefs is dread: dread about the future, dread about the present, and dread about all that we don’t know. Ultimately, we are left in a position marked by ignorance and guilt. Coral reefs are home to approximately a quarter of marine life and buffer the unwanted effects of climate change in the process. They take good care of us, so shouldn’t we take good care of them? But what if the tables were turned? What if rather than treat coral reefs as an enigma, we treat them as a teacher?

In fact, much like trees, corals are living history books thick with information on past climates. Yet the reason why coral reefs are especially good at this goes beyond merely existing before humans could measure climate indicators. In reality, what makes corals so good at keeping track of everything is their calcified skeletons which build up over time. These layers give us crucial information that can then be used to predict future trends. 

In fact, corals have the potential to record several centuries worth of highly detailed environmental information in the form of chemical proxies. These proxies form as their skeletons are calcified at approximately isotopic equilibrium with the surrounding water. Within each individual band, chemical compositions reflect the chemistry of the seawater at the time the band was calcified. 

So like most old people and objects, coral reefs are full of wisdom. All we have to do is listen. 

Yet as always, corals still go above and beyond. In fact, not only can the skeletons of these century-old creatures give us information on the chemistry of seawater, but also data regarding day & year length, Earth’s rotation, and diurnal cycles. For example, the coral skeletons formed in summer have a different density to those formed in winter. So in theory, coral reefs are some of the oldest astronomers and historians on the planet.

Fig. 1: Coral reefs have documented the climate of the past in their aragonite skeletons. Ungaro, Francesco. n.d. Accessed July 2, 2021. https://www.pexels.com/photo/brown-and-black-coral-reef-in-water-3635910/.

But there’s more. These scleractinian over-achievers couldn’t just stop at documenting the ocean chemistry and movement of the Earth. They can also tell us about significant events on land, such as heavy rain and floods. Generally, this is possible based on the transport of sediment from land into the sea. For example, during a flood, the sediment results in cloudier sea water, thus preventing efficient photosynthesis. Without photosynthesis working at its optimal light levels, growth of these reefs is stunted. This leads to observable changes in the growth and color of the coral reef affected. 

As for heavy rain, it can be understood through the ratio of heavy and light oxygen within each growth band. Light and heavy oxygen refer to two different oxygen isotopes which will affect the readiness of water to evaporate. Water with light oxygen evaporates more readily than water with heavy oxygen. Therefore, a heavier rainfall and higher temperature leads to a higher concentration of light oxygen in the ocean and vice versa. In addition to this, a balance between strontium and calcium can also act as an indicator for temperature at the time the skeleton was calcified. 

Similar to sources for paleoclimatology (i.e. the study of climate prior to the availability of recorded data), reefs offer an incredibly useful tool for reconstructing past climates and helping scientists to better understand the evolution of the atmosphere, biosphere, and cryosphere. This helps us to understand how the climate changes without human interference and how to create mitigation strategies accordingly. The creation of these strategies is crucial, as our financial, agricultural, and even immigration systems will be drastically impacted by the direction climate change takes in the future. 

Indeed, coral reefs and other aquatic ecosystems are huge economic assets and, lesser known to most, extremely useful indicators of the past. Thus, while one of the hallmarks of modern-day discussion about coral reefs is dread, perhaps some of the answers can be found within the reefs themselves. From the patterns of storms and floods to the orbit of the Earth, our ancient underwater friends have plenty to share. 

Sources:

“How Can Corals Teach Us about Climate?” 2018. National Centers for Environmental Information (NCEI). August 14, 2018. https://www.ncei.noaa.gov/news/how-can-corals-teach-us-about-climate.

Lemmons, Richard. 2021. “The Purpose of Paleoclimatology – Ecological Footprints.” Climate Policy Watcher. June 6, 2021. https://www.climate-policy-watcher.org/ecological-footprints/the-purpose-of-paleoclimatology.html.

“Paleoclimatology: Climate Close-Up.” 2005. Earthobservatory.nasa.gov. December 23, 2005. https://earthobservatory.nasa.gov/features/Paleoclimatology_CloseUp/paleoclimatology_closeup_2.php.

“Picture Climate: How We Can Learn from Corals | National Centers for Environmental Information (NCEI) Formerly Known as National Climatic Data Center (NCDC).” n.d. http://Www.ncdc.noaa.gov. https://www.ncdc.noaa.gov/news/picture-climate-how-we-can-learn-corals.

Society, National Geographic. 2019. “Paleoclimatology.” National Geographic Society. November 25, 2019. https://www.nationalgeographic.org/encyclopedia/paleoclimatology-RL/.

Ungaro, Francesco. n.d. Accessed July 2, 2021. https://www.pexels.com/photo/brown-and-black-coral-reef-in-water-3635910/

Wilkin, Jack. 2018. “Corals Can Tell Us about Climate Change in the Geological Past, Jack Wilkin Reports.” Theecologist.org. August 4, 2018. https://theecologist.org/2018/oct/04/corals-can-tell-us-about-climate-change-geological-past-jack-wilkin-reports.

Woodward, Ben. 2017. “Paleoclimatology.” http://Www.youtube.com. March 6, 2017. https://www.youtube.com/watch?v=xyPbWO169RY.

Cover Photo: Caption — “An image captured by a scanning electron microscope reveals the growth aragonite crystals to form the layers of the Porities coral skeleton. Photo by Mayandi Sivaguru/ Bruce Fouke Lab/Carl R. Woese Institute for Genomic Biology/University of Illinois“, Citation — “https://www.accuweather.com/en/weather-news/barrier-reef-corals-help-scientists-calibrate-ancient-climate-records/328368”

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