Coral reefs support a quarter of marine species but their existence and all life they support is threatened. Hundreds of millions of years of evolution, whereby corals developed complex symbiotic relationships with various species, is suddenly disrupted by human-caused stressors. In the last 20 years, the once varicolored marine oases have sustained a barrage of bleaching events that have destroyed thousands of hectares of an environment that is home to the most biodiverse ecosystem on earth. With humanity failing to cut its carbon emissions by enough to save our planet, global warming in the next 20 years will likely push reefs to mass extinction.

Curbing carbon emissions and reducing our carbon footprint is the most direct and achievable approach to limiting global warming’s threats on our oceans. However, other factors are involved in the death of corals. Researchers at NYU Abu Dhabi have uncovered insights into our seas that could provide the first step toward predicting upcoming coral bleaching events, and it could help save our oceans. 

In his time at NYUAD, John Burt has witnessed several of these severe coral bleaching events. Burt, an associate professor of biology, studies the Arabian Gulf, home to inhabitants that have shown an amazing resilience to hotter sea temperatures. But now their amazing genetic traits forged in one of the warmest bodies of water on earth are increasingly unable to cope with global warming. 

“There was a severe bleaching event in 1996, another in 1998, followed by a decade of relative calm when coral communities recovered,” Burt explains. “The next relatively impactful event occurred as back-to-back events in 2010, 2011, and 2012, although these were relatively modest in terms of coral loss. Then we had 2017, which was among the hottest years on record, and when we lost almost three quarters of corals from Abu Dhabi reefs,” he said.

Those were the years when ocean temperatures in the Gulf spiked, causing the once vibrant underwater paradise beaming with life to fade into a chalky white color – the harbinger of coral death.

Corals can survive a bleaching event and recover as an ecosystem in 10 to 15 years if conditions remain benign, but their recovery progress is reset with every bleaching event. Although the severity varies, the frequency of these destructive die-offs is robbing the slow-growing corals of time needed for recovery, diminishing their chances of occupying the thousands of hectares across the Gulf they once inhabited just a decade ago.

But what if the scientists could predict when these bleaching events were to occur? Burt, working with Francesco Paparella, associate professor of mathematics, believes they have created a model that can accurately predict the onset of these anthropogenic disasters within a two-week window.

The duo collected weather data from the last decade and tracked how climatic conditions correlated to bleaching events. Running a model that looked at wind speed and direction, they found that long stretches of windless days in summer typically resulted in bleaching events in the southern Gulf. Those summers with frequent, modest winds that went just slightly above breeze conditions were almost exclusively immune from bleaching.

“The wind blowing over the surface is just like you’re blowing on a coffee,” Burt said. “It’s causing evaporative cooling, and when you have winds that are 4 meters per second and above, it’s enough to pull energy out of the system to the extent that you won’t get bleaching."

The summer shamal winds provide that respite. The weather phenomena typically occurs when a high-pressure zone over the Mediterranean causes air to tumble down over the Gulf as it races toward the low-pressure zone caused by monsoons in the Indian Ocean. A good shamal can pull 350 watts of energy per square meter out of the water, more than it takes to power a window-mounted air-conditioning unit in most homes.

Although monsoon-dependent, the use of accurate weather forecasts help scientists predict coral bleaching events with relatively high accuracy even two weeks ahead of time. Burt and Paparella say investment is needed for the development in predictive modeling, but it’s well within reach. That would give governments enough time to act by trying to limit the other stressors that could exacerbate the event. Some of those stressors don’t affect corals directly, but could influence the species with which it shares a deep symbiotic relationship.

Understanding coral reefs and their behavior not only requires research on the species and their environment, it also demands a deeper understanding of an ancient yet precarious relationship corals have developed with phytoplankton, the single-celled marine critters responsible for all life underwater.

Marriage of Convenience

Alone, phytoplankton provide half of the world’s oxygen, as much as all the grasslands, forests, and every photosynthetic terrestrial being. Despite their role as the singular foundational being for all life in the water and, some may argue, on earth, little is known about these microscopic, mysterious organisms whose collapse would almost certainly spell complete marine extinction.    

“My lab is trying to understand these organisms, how they survive and what kind of factors allow them to flourish or die. And that comes down to symbiosis. There are no organisms on earth that live in isolation; every living being must get something from someone else to survive,” said Shady Amin, assistant professor of biology.

From bacteria to whales, the marriage of organisms allows species’ specialization, whereby specific species evolve to do singular tasks more efficiently. In the case of corals, the demise of the group of phytoplankton that live within their tissues is synonymous with their own. In fact, the ghostly appearance of corals bleached is nothing more than the disappearance of algae — phytoplankton that symbiotically exist with corals.

But in calmer conditions, their relationship is a convenient, if not harmonious, one. Corals extend tentacles in the ocean in search of food drifting in the water. But corals are poor hunters, and their hauls scarcely satisfy their energy needs. So instead of digesting whatever scraps corals find drifting in ocean currents, they send those digested nutrients to the algae living inside the coral tissues. The algae, using energy from the sun, convert those nutrients and carbon dioxide into sugars and other essential compounds, in a process called carbon fixation, that they send back to the corals to sustain growth. 

Although phytoplankton symbionts are happy to exist in many hosts willing to provide them with what they need to photosynthesize, corals are more particular in selecting their partner.

Corals prefer specific types of algae to achieve certain tasks. Some algae are believed to be more adapted to warmer conditions, so corals exposed to warmer waters, like ours in the Gulf, will reject advances from other algae to specifically welcome a type it detects is capable of benefiting it. It’s a complex relationship that has developed through millions of years of evolution, allowing each of these coexisting species to evolve traits that make their partners stronger.

Despite their importance, science still knows very little about their existence, let alone their relationship with corals. Amin is hesitant to make any predictions on how this fundamental species will fare in unprecedented climatic conditions. Burt shares that sentiment about corals as well, but he says the outlook is not promising. Specifically, the marvel of co-evolution that has occurred over millions of years is now being compromised in just the two centuries since the industrial revolution.

Burt said that we are on track to reach 700 parts per million of CO2 by the end of the century. The last time that amount of CO2 was believed to be in the atmosphere was 55 million years ago, and it caused massive die-off in our oceans. 

“Now we have put more CO2 in the environment at a rate faster than we’ve seen in hundreds of thousands of years. We’re chemically engineering the world’s environment. We don’t know exactly what’s going to happen to the world’s oceans with climate change, but we know it’s not good,” Burt said.