Highest ocean heat in four centuries places Great Barrier Reef in danger

The Great Barrier Reef, a World Heritage site, is facing increasing threats, including mass coral bleaching, declining calcification rates, cyclones, and overshipping. Coral bleaching, triggered by heat stress, results in the breakdown of the symbiosis between corals and their symbiotic dinoflagellates. While localized bleaching can occur, widespread coral mortality is strongly linked to rising SSTs associated with global warming. The first documented mass bleaching events on the GBR occurred in the 1980s, but these were less severe than those in the 21st century. While some pre-1980 bleaching evidence exists, severe mass bleaching appears to have been infrequent before the late 20th century. The frequency and severity of mass bleaching events have dramatically increased with rising ocean temperatures, impacting the health and survival of coral reefs. The study aims to understand the long-term context of recent high SSTs in the GBR region and assess the extent of anthropogenic influence on this warming trend, ultimately clarifying the threats to the GBR ecosystem.

Previous studies using coral skeletal records have attempted to reconstruct past SSTs in the GBR region, but these have been limited by temporal resolution and spatial coverage. Early studies, based on limited data, suggested that SSTs in the early 2000s were not unusually high compared to the past three centuries. However, these studies predate the recent surge in SSTs. The current research addresses these limitations by incorporating a wider network of coral data and more advanced analysis techniques to provide a more comprehensive understanding of long-term SST trends.

This study combines a network of 22 coral Sr/Ca and δ¹⁸O paleothermometer series from the Coral Sea region to reconstruct January–March SSTs from 1618 to 2024. The reconstruction is calibrated against instrumental SST data (ERSSTv5 and HadISST1.1). The researchers assess the statistical significance of the reconstructed SSTs, considering uncertainty bounds and performing various sensitivity analyses to validate the findings. To investigate the anthropogenic influence on SST warming, the study utilizes climate model simulations from CMIP6, comparing historical simulations (including anthropogenic forcing) with historical-natural simulations (excluding anthropogenic forcing). This approach allows the researchers to quantify the contribution of human activities to the observed warming trend. The analysis focuses on January-March SSTs to minimize noise related to temporal resolution and maximize focus on the seasonal bleaching period.

The study's key findings are: (1) The January–March Coral Sea SST extremes in 2024, 2017, and 2020 were the warmest in 400 years, exceeding the 95th percentile uncertainty limit of pre-1900 maxima. (2) The 2016, 2004, and 2022 events also exceeded the 90th percentile limit. (3) Climate model analysis confirms that human influence on the climate system is responsible for the rapid warming in recent decades. The observed warming trend since 1900 is 0.09 °C per decade (ERSSTv5 data) and 0.12 °C per decade for 1960–2024. (4) The five warmest January–March periods between 1618 and 2024 occurred after 1900, with the 23 warmest all occurring after 1900 and the 20 warmest occurring after 1950. (5) The anthropogenic warming signal in the Coral Sea significantly exceeds natural variability, increasing rapidly after 1960. (6) The observed SSTs for 2016-2024 are warmer than any in historical-natural climate model simulations.

The findings demonstrate unequivocally that the recent unprecedentedly high SSTs in the Coral Sea are primarily driven by anthropogenic climate change. The extreme heat events coincide with mass coral bleaching on the GBR, highlighting the direct link between human-induced warming and reef degradation. The study's focus on the Coral Sea region and use of CMIP6 simulations allows for a robust attribution of warming to human activities. While the study acknowledges limitations in the spatial resolution of climate models and the availability of multi-century coral data within the GBR itself, the results provide compelling evidence of the severe threat posed by climate change. The findings underscore the urgent need for mitigation strategies to reduce greenhouse gas emissions and protect the GBR ecosystem.

This study provides robust evidence linking unprecedentedly high sea surface temperatures in the Coral Sea to anthropogenic climate change, directly impacting the Great Barrier Reef through mass coral bleaching. The findings highlight the urgent need for climate change mitigation and adaptation strategies to protect this vital ecosystem. Future research should focus on improving the spatial resolution of climate models, collecting more high-resolution temperature proxy data within the GBR, and exploring coral adaptation and acclimatization mechanisms.

The study acknowledges several limitations: Potential biases in instrumental SST datasets, especially before 1945, could affect the reconstruction accuracy. The use of coral δ¹⁸O records for SST reconstruction may be influenced by changes in the coral δ¹⁸O–SST relationship and non-SST changes in seawater δ¹⁸O. The limited availability of multi-century coral data within the GBR itself limits the spatial representation of reconstructed low-frequency variability. The use of some annual-resolution records to reconstruct seasonal SSTAs could introduce bias. The coarse resolution of global climate models does not fully capture small-scale processes impacting local SST variations.