Losing Home #1: Coral Reefs

This is article is part of a series exploring the ramifications of climate change on the physical habitat of coral reefs, and the consequences for the various dependent communities. 

Written by Evan Quinter

Edited by Matthew Tietbohl

Figure 1. Depicts mass coral bleaching at Heron Island in Feb. 2016.© Ocean Agency / XL Catlin Seaview Survey / Richard Vevers

When we try to understand how coral reefs will respond to global climate change, it feels daunting to consider the infinite-like web of entangled ecological factors. There’s a reason why Dr. Vale Robert May, the Professor of Everything, recognizes that, “ecology is not rocket science – it’s much harder than that.” In Losing Home, we’ll explore climate change’s impacts on the physical reef habitat and the consequent effects on the various realms dependent on coral reef systems. The first edition will center on the literal basis for the reef ecosystem, coral.

Scleractinia coral, also known as hard or stony coral, build their skeleton from calcium carbonate, creating three-dimensional structures that provide a living-scaffolding for reef organisms. Although stony coral reside in a wide range of depths, shallow water reefs are particularly vulnerable to climate change due to their proximity to the coast and the surface. This is evident with rising sea level rise, a global phenomenon predicted to rise anywhere from 0.3 to 2.5 meters over the next 80 years. For coral, a marine organism reliant on photosynthetic algae for a majority of its energy, sea surface heightening will submerge coral deep enough to cause a potential loss of coral growth. Sea level rise, coupled with a greater severity of storm events, will also lead to increased wave energy that reduces the production of community carbonate structure. Starved by rising seas and battered by storms, coral face major obstacles to their physical integrity. But as the ocean retains ever-increasing levels of greenhouse gases, coral reefs face two particularly detrimental consequences. 

The first stems from the continual uptake of CO2 into the ocean, resulting in higher levels of acidity in the water. Known as ocean acidification (OA), more acidic waters hinder coral’s ability to build their carbonate structure by impeding skeletal thickening. These damaged coral cannot maintain optimal calcifying rates under OA conditions, resulting in increased levels of coral bioerosion.  

Figure 2. Models the skeletal growth model of Porites coral, a process that OA weakens. Modified from Mollica, et al. 2018

Along with ocean acidification, greenhouse gases trapped in the atmosphere raise global sea surface temperatures. The warming of the oceans causes issues for many marine species, but this is particularly apparent with coral due to a process called bleaching. Warmer waters require coral to request more energy from their symbiotic algae, stressing them to the point where they are expelled from the coral skeleton. When the algae are ousted, the coral lose their apparent coloration, revealing their white skeletal structure underneath. Bleaching events leave coral weakened and starved, increasing their susceptibility to disease and potential death if they cannot recover.

It is important to note that these are not independent factors, but interconnected forces that operate in tandem to influence coral health. We can already see the destructive results, especially in the Great Barrier Reef (GBR), where since 1990, coral calcification rates have dropped 11.4%, a reduction unseen in the past 400 years. In the central and south GBR regions, a 27 year decline in coral reef growth has culminated into 50.7% loss of coral cover.  The above factors are only some of the natural factors resulting from climate change, we haven’t explored the various anthropogenic stressors to coral reefs that compound on an already stressful environment. The impacts of climate change are so disastrous that some coral display “survival traits” in preparation for a mass extinction event.

Figure 3. Maps the mean coral cover of the GBR averaged from 1985 -2012, with points showing direction of growth. The center and south regions illustrate the majority of coral loss. Modified from De’ath, et al. 2012

When we look at the overarching picture of coral reef health for the next century, it’s bleak. Then adding on our guilt knowing these factors all influenced by our actions on the environment, it can feel quite depressing and even debilitating. But I encourage us all to see this as the positive, as the solution to preserve coral reefs; we’re the ones who caused it, we’re the ones who can stop it. Through collective actions in our local and global communities, we can begin to divest from fossil fuels and progress towards a cleaner future, reducing our impact on coral reefs along the way. There’s still time, and any action towards coral preservation is a step to preserving our home.

To find out more about corals physical responses to climate change, check out the links below:

Frontiers | Coral Reef Ecosystems under Climate Change and Ocean Acidification | Marine Science (frontiersin.org)

Coral Bleaching – YouTube

Chasing Coral | FULL FEATURE | Netflix – YouTube

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