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An adversely changing climate and rising sea surface temperatures (SST) have exposed the Great Barrier Reef (GBR) to its highest temperature in more than 400 years and pose an existential threat to the world heritage site.
Scientists in their recent assessment have found that the ocean temperatures at the reef are hottest in the past 407 years and threaten the ecosystem that is larger than the size of Italy.
Samples analysed from the centuries old corals in the reef that register records of temperatures within their skeleton and climate models helped scientists in confirming that the reefs are in danger more than ever.
The research led by Benjamin Henley, University of Wollongong (UOW) Honorary Fellow and University of Melbourne Lecturer, is significant in the light of the world heritage committee not listing the reef under the sites that are officially ‘in danger’.
The reef has already experienced five mass bleaching events within a decade alone.
Published in the Nature journal, the study titled Highest ocean heat in four centuries places Great Barrier Reef in danger finds that all the mass coral bleaching events recorded between 2016 and 2024 were driven by high SST —which is not possible without humans having a role to play in it.
“Climate model analysis confirms that human influence on the climate system is responsible for the rapid warming in recent decades,” the study noted.
The highest temperatures experienced by the reef by far is the ongoing mass coral bleaching event where the SST were recorded to be 1.73 degrees Celsius above mean temperature.
In the past, the high SSTs during the mass bleaching event were registered in 2016, the mean temperature was 1.5 degrees Celsius above the pre-1900s.
While in 2017, the temperatures were 1.54 degrees higher. In 2020 and 2022, the recordings were at 1.53 and 1.46 degrees Celsius respectively. These temperatures were recorded between January-March.
Scientists at the Australian Institute of Marine Science have termed the ongoing mass bleaching event as the most widespread and extreme so far.
Coral bleaching occurs when the heat stress causes separation of dinoflagellates - a single cell organism with two flagella, found widely in marine plankton and fresh water.
The phenomenon also occurs due to low salinity, cold waters or pollution, regional and global mass bleaching events, the study noted. Although corals can recover from bleaching, long term exposure and high intensity can lead to its death.
The study noted that the first modern evidence of the mass coral bleaching at the reef was observed in the 1980s, but the events were not widespread and less severe in nature compared to the ones recorded in the 21st century.
In their research, scientists drilled the centuries-old corals from 22 locations and used stress bands in their skeletal structure to find evidence of mass coral bleaching during the pre-1980s and during the 1877-78 El Niño. The stress bands suggest that the coral bleaching did not occur during the 1800s and even most of the 1900s.
But as oceans warmed around the 19th century at the advent of industrial revolution, the phenomenon became more damaging for the reef.
The scientists reconstructed the record of temperatures starting from 1618 and compared them with the modern recordings of the ocean temperatures.
They concluded that climate change and coral bleaching together have deteriorated the integrity of the reef that possesses World Heritage Status.
Scientists warn that even limiting global warming to 1.5 degrees Celsius in accordance with the Paris Agreement can result in loss of about 70-90 per cent of the corals on the reef existing today.
If all current international mitigation commitments are implemented, global mean surface temperature is still estimated to increase in the coming decades, with estimates varying between 1.9 °C and 3.2 °C above pre-industrial levels by the end of this century. Global warming above 2 °C would have disastrous consequences for coral ecosystems, and the hundreds of millions of people who currently depend on them, the study observed.
It argued that if the situation persists, corals of the future will possess a different community structure than its recent past with far less diverse coral species.
Henley, who is also the lead author of the study told DTE that repeated mass bleaching and mortality events are likely to lead to a simplification of the reef ecosystem.
"We believe that this would have a profound negative impact on the diversity of marine life which relies on the complex structures of the reef system," he said.
It also expressed concern that though coral adaptation and acclimatisation are the only practical conservation solutions for the reef, the model projections show that the rates of adaptation are not matching the pace of global warming and over 1.5 degrees Celsius can be catastrophic for coral reefs.
Henley added that different coral species have different thermal stress thresholds. For example, Acropora species are known to be quite sensitive to thermal stress, but species of the genus Porites are more resistant.
“The evidence presented in our study indicates that the GBR is in danger. Given this, it is conceivable that UNESCO may in the future reconsider its determination that the iconic GBR is not in danger,” the study stated.