Scientists remember the years between 2014 and 2017 as a particularly bad time for coral reefs. Elevated temperatures fueled by an El Niño climate pattern harmed about three-quarters of the world’s reefs in both hemispheres, forcing corals to release their life-sustaining zooxanthellae and turning them ghostly white in a process known as coral bleaching.
About 30 per cent of the world’s corals died as a result of this bleaching. Others have yet to fully recover.
And now, at a time when global temperatures are higher than ever since the industrial era began due to human-driven climate change, forecasters predict another El Niño will kick off later this year.
If they are correct, this El Niño could further escalate global temperatures, causing significant damage to both terrestrial and marine ecosystems, including the world’s coral reefs, most of which are already struggling to cope with environmental stressors such as pollution, overfishing and warming water.
So, what’s the likelihood of this happening?
An El Niño is forecast for 2023 — but it’s not certain
For the past two and a half years, the world has been experiencing the opposite of an El Niño: a La Niña climate pattern, a condition that generally brings cooler sea and atmospheric temperatures.
But according to the US National Oceanic and Atmospheric Administration (NOAA), the climate is expected to transition to a neutral state by May 2023, and then possibly move into an El Niño phase, a period characterised by warmer sea conditions. The shift between La Niña and El Niño is part of a naturally occurring process known as the El Niño Southern Oscillation (ENSO), but this pattern is unfurling in a world destabilised by climate change.
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The coral is the basis of the reef, it’s the foundation of everything else. So without it, reefs either don’t exist or look completely different. If people are relying on them for food or for tourism, then people can lose that ecosystem service, along with the loss of the ecosystem.
Danielle Claar, marine ecologist, Washington Department of Natural Resources
The probability of an El Niño occurring this year is under debate. NOAA has suggested that there is about a 50 per cent probability of an El Niño developing by the Northern Hemisphere’s autumn months. The UK’s Meteorological Office says there is a 60-70 per cent probability that an El Niño will develop around July this year, and the Potsdam Institute for Climate Impact Research (PIK) in Germany says the probability of a moderate to severe El Niño this year is close to 90 per cent.
One thing that’s certain is that even if an El Niño doesn’t arrive in 2023, it won’t be too long before one does. The current La Niña has been going on for more than two years already, and they typically switch with El Niños every three to seven years, on average.
What happens during an El Niño?
El Niño climate patterns develop in the eastern tropical Pacific Ocean. That’s because “there’s more ocean coverage with less continent in that area of the Earth compared to anywhere else,” said Peter Houk, a coral reef and fisheries ecologist at the University of Guam. “The sun penetrates the water [in the eastern Pacific] and that sets off the process.”
In normal conditions, trade winds blow from the Americas toward Asia, pushing warm water to the western side of the Pacific Ocean, while a process called upwelling brings cooler water from the deep ocean to the surface of the eastern Pacific. But when warm surface water builds up in the eastern equatorial Pacific, an El Niño will develop.
El Niños occur when there is a weakening of the trade winds that blow from east to west, and a reduction in the upwelling processes that bring cold, nutrient-rich waters to the surface off the western coast of South America. This causes a large mass of warm water to accumulate in the eastern and central Pacific.
Other parts of the ocean will warm up as well. For instance, a lack of trade winds blowing from the Americas to Asia means that warm water will move toward the Americas, warming up the Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea. El Niños also weaken the mid-year westerly winds over the Indian Ocean that usually cool the sea surface, so the water warms up there as well. Elevated temperatures across the global ocean can lead to mass coral bleaching events like those between 2014 and 2017.
What happens in the ocean also impacts the weather conditions on land, making some parts of the world drier and others wetter. The El Niño forecast for this year could generate a wide range of terrestrial impacts, including droughts and fires, in certain parts of the world.
For example, eastern Australia might experience hot, dry weather; the Indian monsoon might be suppressed; and parts of Africa might get thrown into drought, said Adam Scaife, the head of long-range prediction at the UK Meteorological Office and professor at Exeter University. There is also a risk of drought in the northern part of South America, which might increase fire risk for the Amazon, he added. Other parts of the world might get increased rainfall, such as the southern United States.
We could surpass 1.5°C of warming
Among the most serious potential global impacts of an El Niño is the prospect of tipping the Earth over the 1.5° Celsius (2.7° Fahrenheit) threshold for climate change-driven warming established under the Paris Agreement. Human-induced climate change has already led to a rise in temperature of 1.2°C (2.2°F) above pre-industrial levels.
But if an El Niño develops later this year and it becomes a moderate to severe event, it could increase global temperatures to the point where the world would actually exceed 1.5°C above pre-industrial levels. But this would happen in 2024, following the peak of the El Niño, Scaife said.
“When there’s an El Niño, you get extra heat released from the ocean,” Scaife told Mongabay. “The temperature on the Tropical Pacific goes up by typically one or two [degrees], sometimes three degrees, in places. That doesn’t sound like very much, but the amount of heat that’s produced and released into the atmosphere is enormous, and that spreads all the way around the tropics over the following weeks and months, following the peak of El Niño in December, January time. That means that the following year, so 2024 in this case, globally, will be warmer than it would be if we had no El Niño.”
However, the surpassing of the 1.5°C threshold would only be temporary — at least for now.
“That’s not to say that we’ve broken the Paris Agreement; there is this subtle difference,” Scaife said. “What we’re talking about here is the first single year above that level … but when the global temperature eclipses that level, it’s going to catch people’s attention. It’s very likely that it will be an El Niño year [when we surpass 1.5°C], even if it’s not 2024.”
Research suggests that when the world breaches that figure, multiple tipping points could be triggered, including the mass die-off of tropical coral reefs, the collapse of ice sheets, and the thawing of boreal permafrost. Yet Ludescher of PIK says it’s unlikely these tipping points will be triggered during a short period above 1.5°C.
“One year might be more manageable than a constant state,” Ludescher said. “But in the not-too-far future we could have 1.5 as the usual temperature.”
On the other hand, Ludescher said he believes a temporary period above 1.5°C could result in more extreme weather events. In other words, think: droughts, fires, flooding, and yes, coral bleaching.
How would an El Niño affect marine ecosystems?
The next El Niño will likely have a profound effect on the ocean, and corals are top of mind for many experts simply because they’re so vulnerable to warming and already so embattled.
“One of the reasons we are really, really concerned about the possibility of an El Niño this year is because many coral reefs are still recovering from the last mega El Niño — the really prolonged, intense El Niño that occurred in 2015 and 2016,” Julia Baum, a marine biologist at the University of Victoria in Canada, told Mongabay. “[That event] caused mass coral bleaching and mortality events on reefs around the world, and coral reefs take decades to recover from those types of events.”
For instance, Baum said that Christmas Island in the central Pacific, or Kiritimati, only has about 10 per cent of its coral reefs left after the 2015-2016 El Niño bleached and killed the other 90 per cent.
“I shudder to think what will happen to those reefs if another heat wave is unleashed on them this soon,” she said.
Danielle Claar, a marine ecologist at Washington state’s Department of Natural Resources (DNR), said widespread coral bleaching would bring “knock-on effects” for all of the organisms that depend upon reefs.
“The coral is the basis of the reef, it’s the foundation of everything else,” Claar told Mongabay. “So without it, reefs either don’t exist or look completely different. If people are relying on them for food or for tourism, then people can lose that ecosystem service, along with the loss of the ecosystem.”
Many coral reef species are slow-growing and may need a decade or more to recover. But ongoing warming and extreme events like marine heat waves may not give corals the opportunity to recuperate, according to experts.
The impacts of an upcoming El Niño would be experienced on top of warmer-than-normal ocean conditions fueled by climate change. A recent analysis found that 2022 was the hottest year on record for the ocean.
Houk of the University of Guam said he expects the next El Niño, whether it happens this year or another, will not only generate widespread coral bleaching, but could lead to outbreaks of crown-of-thorns starfish (Acanthaster cf. solaris), a species that preys upon coral. Scientists have observed that bleaching and invasions of crown-of-thorns starfish often go hand in hand, Houk said.
“If we look back at data, time and time again, you see a starfish event, and then back it up with a bleaching event,” Houk told Mongabay. “And one plus [the other] is terrible.”
Another known impact of an El Niño phenomenon is the disruption of the marine food chain and fisheries in the eastern Pacific through the weakening of the upwelling that brings nutrients to the surface waters off the coast of South America. This process decreases the amount of phytoplankton in the area, which can affect numerous species, including anchovies, small fish that form the basis of one of the most important fisheries in the world.
These impacts could be even more pronounced as the world struggles with the effects of climate change and other human-driven stressors like overfishing.
A forthcoming El Niño could also wreak havoc on kelp forests in parts of the ocean that warm up in response to this climate pattern, said Baum of the University of Victoria. For instance, she said kelp forests in the coastal waters of the Pacific Northwest were “wiped out” by a marine heat wave known as the “Blob” that materialised between 2014 and 2016. While many factors likely contributed to the formation of the Blob, Baum said it was partially driven by El Niño.
“Kelp forests are the second most vulnerable ecosystem marine ecosystem behind coral reefs,” Baum said. “Depending on how prolonged this [forecast] El Niño becomes, and if it starts to interact with other oceanographic factors that cause prolonged warming and warming in temperate areas, then kelp forests are certainly a candidate for another ecosystem to be concerned about.”
Claar of Washington’s DNR said kelp can perish when high surface temperatures cause it to “cook.”
“Kelp forests have most of their biomass up at the very tippy top of the water,” Claar said. “And of course, that’s the area of the water that gets the warmest.”
Many kelp forests, important habitats for a variety of species, are already under threat from rising sea temperatures, overfishing, pollution and other human activities.
Not all coral reefs may be impacted
While an El Niño would likely trigger coral bleaching in many parts of the world, it doesn’t mean that all corals in affected areas would die.
“We did a study during the last major El Niño, and we found some really unique and exciting corals that were able to actually start recovering their symbionts, even though it was still warm,” Claar said. “Some of my colleagues are still working on figuring out how that worked, if that continues to work, and what that might mean for other areas. It’s really promising, but also with the caveat that it’s not common.”
Houk said he anticipates the “spatial footprint” of coral bleaching to continue to grow with each forthcoming El Niño, but that bleaching won’t occur uniformly across a region. Some corals will not bleach, and this could be due to a range of other oceanographic factors, such as regional upwelling events that bring in cooler water.
“Not every El Niño will affect every pixel of the ocean,” Houk said.
He also said that well-managed reefs, such as those safeguarded in marine protected areas (MPAs) — especially those within networks of interconnected MPAs — will be able to recover more quickly from bleaching events.
“When the next El Niño comes, whether it be in August or January or whatever … there’s no doubt about it, the corals will bleach and it will probably be the largest [event], even more widespread than the last ones,” Houk said. “But I think we’re starting to learn more and more about factors that offer resistance and recovery to this.”
This story was published with permission from Mongabay.com.
