Coral, in the 21st Century

adam dhalla
8 min readSep 26, 2020

The sun beat down upon the sand as I ungracefully exited the rental car and began the descent into the bay. I readily identified two species of birds waddling around the parking lot — both introduced species; a Zebra Dove from southeast Asia, and a Red-Crested Cardinal from South America.

My blurry picture that I took five years ago, age ten, of a Red-Crested Cardinal.

My first introduction to a real, live, coral reef, was not unique. It was an experience shared by many, more, perhaps hundreds, on the exact day that I went some four years ago. It was an overcrowded tourist bonanza, and wasn’t exactly the most untouched reef.

I was at Hanauma Bay State Park, on the island of Oahu Hawaii. I was 11.

Putting on my reef shoes bought hastily from an ABC Store the night before, we descended the small, winding road down to the main beach. I spotted a few more species on the way down — a Common Mynah there, a brief encounter with a mongoose, and a Brown Anole (an unsuspecting lizard with a brightly colored sack below the mouth called a dewlap). All non-native species.

It was a relief to find an untaken shady spot to settle down. My mom began to set the blankets while my dad and I grabbed the snorkels, and after a good three minutes of adjustment, cautiously waded into the water.

Trying to find a place with the least amount of people, we descend into the water. It was the polar opposite of graceful— I, to be frank, was scared out of my wits at the thought of encountering a stonefish or a blue-ringed octopus or a sea urchin or any remotely threatening creature, all the while half-drowning through my precariously adjusted breathing tube.

But, between the moments of sputtering, I had my first introduction to a coral reef. Despite everything, I was enthralled. Moorish Idols swam past me, a parrotfish gnawed at coral, and triggerfishes floated around. I distinctly remember an entire school of silver fish swimming around me reflecting like coins in the sun.

Slowly, I did not want to get out of the water fearing I would miss something. Because there was always something happening, like New York at rush hour, minus the creepy subway encounters.

Coral reefs are, perhaps tied with rainforests, the most densely populated and biodiverse places on the planet. They occur in highest frequency between latitudes of 30° north and 30° south, in highest concentrations in the Caribbean, southeast Asia, and eastern Africa.

Despite making up roughly 1% of the sea floor, reefs harbor 25% of all marine species.

Photo by David Clode on Unsplash

The most single revelatory three minutes for me was the first time I put on scuba gear and dived in a coral reef. Sir David Attenborough

Without human intervention, this interplay between species has created some of the most resilient ecosystems to exist, where disease has a hard time spreading through populations because of the sheer variance in genetic data. Coral bleaching events were expected but infrequent, and were generally survivable.

The importance for coral reefs is not just social or ecological — but economic. Coral reefs pump, conservatively, billions of dollars into the global economy annually, through tourism and fish stocks. It is estimated that coral reefs account for 20-25% of the fish caught by developing nations.

Bleached coral. Photo by Francesco Ungaro on Unsplash

But the reign of coral seems to be coming to an end. Increased bleaching events that have ravaged the world’s coral reefs, such as the alarmingly frequent bleaching of the Great Barrier Reef, are predicted to become regular. A study published in Nature in 2019 showed a loss of 89% in larval recruitment (the process of young coral larvae undergoing settlement and joining an adult population) in corals compared to historical levels. Let’s take this time to look at some of the largest threats to coral today.

Warming waters

Average Global Sea Surface Temperature, 1880–2015. Plotted as an anomaly (+ anomaly means higher than baseline, here the 1971–2000 avg.) NOAA, 2016

The ocean, for the past thousands of rather ecologically stable years, has acted as an important balancer for the global environment. Just because of its sheer size, encompassing 70% of the world’s surface, the ocean acts as a failsafe; a carbon sink — absorbing excess heat or carbon dioxide in the atmosphere without a noticeable change in ocean composition (because of the sheer vastness). The fact that this is beginning to change speaks volumes about the massive impact that humanity has caused on the biosphere since the industrial revolution.

Research by NOAA has estimated on average, that the upper few meters of the ocean have increased in temperature at a rate of 0.13°C per decade for the past 100 years, and are predicted, by IPCC estimates, to rise between 1°C and 4°C by the turn of the century⁷.

This is worrying in part of the huge factor temperature has in the survival of coral, and specifically, in the chances of it bleaching.

Coral, in its healthy state, is occupied by microscopic organisms called zooxanthellae that have evolved to live symbiotically with coral. The zooxanthellae, which thrive through photosynthesis, pass on much needed minerals and, for lack of better words, food, to the coral polyps themselves, and the coral in turn protect the zooxanthellae from predation.

Coral polyps by Derek Keats / CC BY 2.0. Inside these polyps are the zooxanthellae.

There are multiple hypotheses for the exact reason why heat puts so much stress on the relationship between zooxanthellae and coral. Two leading ideas persist — one, that simply, zooxanthellae has a hard thermal tolerance limit, or two, that in high temperatures, zooxanthellae begin to release chemicals that are toxic for the coral polyps themselves.

Either way, in high temperatures, coral flushes it’s branches of zooxanthellae, leaving only a white, bony, skeleton, behind, that cannot feed itself and is especially vulnerable to disease. If temperatures return to normal soon enough, coral may regain its zooxanthellae, but more often than not, it perishes.

One of the largest coral bleaching episodes in history, the 2005 Caribbean coral bleaching event, was destructive in both that it occurred during a large heat wave, but was followed by an unprecedented epidemic of coral disease that flowed rapidly through masses of weak coral. In some areas, such as the Virgin Islands, 90% of all coral reported bleaching, and 60% later died.

This, and the rapidly warming oceans, make increased coral bleaching due to raised temperatures a real and existential threat to every coral reef globally.

Ocean acidification

Often called ‘Climate Change’s Evil Twin’, ocean acidification, or the lowering of the ocean’s average pH, goes hand in hand with rising carbon emissions.

Atmospheric CO² and ocean pH as a function of time. This data, mirrored in other oceanography stations around the world, was collection just a few hundred kilometers north of Hanauma Bay at he the ALOHA research center. pCO² is the partial pressure of carbon dioxide per μatm, or microatmospheres, a unit of pressure. Go figure.

The key takeaway from the above graph is the consistent lowering of the ocean’s pH (getting more acidic — or more exactly, becoming basic; saltwater is naturally slightly basic). Although the decimal points might seem trivial, remember that the pH scale is logarithmic — a pH of 6 is 10x more acidic than a pH of 7.

This pH rise is a direct consequence of the ocean’s ability to absorb CO². It absorbs CO² both by diffusion and by biological intake through photosynthesis by phytoplankton and algae.

Despite the gargantuan nature of the problem, ocean acidification was only recently brought into light, in the late 90’s and early 2000’s. One of the first (or possibly the first) experiment on pH and coral survival was done at the artificial ocean in Biosphere 2, an unsuccessful project funded by billionaire Edward Bass in the 80’s and 90’s meant to simulate an artificial earthlike environment that could exist on Mars. It is now owned by the University of Arizona.

the back of Biosphere 2 by kanu101 / CC BY 2.0

The series of experiments, done by Columbia University at the turn of the century, is arguably the most important discovery in oceanography the past few decades. It showed that, contradicting past beliefs, that coral’s pH tolerance was not a clean cut limit — coral growth declined as a gradient towards a limit.

Proof that oceanic acidity slows or, when high enough, inhibits coral (and shell) growth abound, for example the landmark studies at One Tree Island in the Great Barrier Reef and the carbon-emitting volcanic vents in Castello Aragonese off of Italy.

Castella Aragonese by Mannafly // CC BY-SA 4.0. The waters surrounding the island are carbon-rich due to the volcanic vents in the area. The area surrounding the vents become a helpful ‘time machine’ into what high-carbon oceanic environments could look like in the future.

This video explains the science behind ocean acidification.

tl;dr when CO² is introduced into water, it produces H2CO3 (carbonic acid), which then dissolves into HCO3- (bicarbonate) and CO3²- (carbonate). Coral (as well as every shelled animal) requires carbonate to build it’s calcium carbonate (CaCO3) “shell”. When more CO² is added to water, the density of carbonate lessens and bicarbonate rises. This restricts the amount of calcium carbonate that coral can produce (because of a lack of carbonate, the ‘main ingredient’), leading to slow growth, and at worst, coral can begin to dissolve.

Recent estimates say that if we continue down this path, we can expect the ocean’s mean pH to be around 7.7 at the turn of the century — and although recent research has found that coral polyps themselves show signs of adaptation to lower pH environments, coral reefs will be a thing of the past.

Plastic pollution

Plastic pollution carries a plethora of threats to coral. First, the most obvious, the physical damage caused by plastic-coral contact, but more subtly, increased disease rates within populations.

Plastic debris, plastic bags especially, are perfect carriers of germs and viruses. When caught on coral, plastic raises the disease risk. Overall, the disease risk for corals when plastic debris is present rises from 4% to 89%.

Lastly, plastic pollution threatens most the creatures that are integral to reef health, such as fish that limit the growth of algae on corals.


Overfishing can have a multi-pronged effect on coral reefs. Fishing gear (and worse, bottom trawling) can entangle and destroy coral or even whole colonies of coral.

A fishing trawler. by Kumaravel / CC BY 2.0

The unsustainable removal of large amounts of fish, especially herbivorous fish, can cause reef colonies to be overridden with algae. When large populations of young fish are caught, entire generations can be devastated, tipping the precarious balance that is the reef ecosystem.

There are a plethora of other threats to coral reefs, such as pathogens from chemicals and sound pollution, but these are the main threats.

The importance of coral cannot be understated. We must rapidly reduce emissions to combat both ocean acidification and warming. Some predict coral biodiversity to plummet by 2050. With rapid enough action, we can shift. The technology to shift to renewable energy already exists. The science exists. The action must come from below

Adam Dhalla is a high school student out of Vancouver, British Columbia. He is fascinated with the outdoor world, and is currently learning about emerging technologies for an environmental purpose. To keep up,

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adam dhalla

17 y old learning about machine learning, as well as a lifelong naturalist. Climate activist in Vancouver. Writer. Visit me @