Scientists have reconstructed month by month climate conditions on Australia’s Great Barrier Reef from around 200 years ago by analysing the shell of a giant clam, revealing that summers were cooler than today while winter temperatures were remarkably similar.
The research focused on a giant clam (Tridacna gigas) collected from the lagoon at Jiigurru also known as the Lizard Island Group, in the northern Great Barrier Reef. The findings provide a detailed record of past ocean temperatures, rainfall patterns and climate variability before widespread industrial era warming.
Giant clams are the world’s largest living bivalves. They can grow up to 1.2 metres long, weigh more than 200 kilograms and live for over a century. As they grow their shells develop microscopic daily growth lines and broader annual bands, preserving a record of environmental conditions throughout their lives.
The study was conducted in partnership with the Walmbaar Aboriginal Corporation and Nguurruumungu Indigenous Corporation and published in the journal Global and Planetary Change.
Researchers examined a 58 centimetre clam shell collected from Jiigurru, a culturally significant area for Dingaal and Ngurrumungu Aboriginal communities. Archaeological evidence from shell midden sites indicates that Aboriginal people have occupied the area for at least 6,500 years.
Radiocarbon dating showed that the clam lived between approximately 1785 and 1827, near the end of a preindustrial cooling period known as the Little Ice Age. Researchers counted almost 10,900 daily growth lines under a microscope, indicating that the clam lived for around 30 years.
To reconstruct past climate conditions, the team drilled 784 tiny holes into the shell and analysed the chemical composition of the resulting powder samples. Oxygen isotopes helped researchers estimate historical sea temperatures, while carbon isotopes provided clues about rainfall patterns.
The reconstruction showed that average temperatures around Jiigurru approximately 225 years ago were between 0.6°C and 0.9°C cooler than those recorded in the same lagoon between 1995 and 2015.
The difference was most pronounced during summer, when temperatures were estimated to be 0.9°C to 1.2°C cooler than today. Winter temperatures, however, were almost unchanged compared with the modern observations.
The shell also preserved evidence of seasonal rainfall changes and climate variability. Carbon isotope measurements revealed a recurring pattern of wetter and drier periods, while temperature variations showed repeated cycles lasting two to three years.
These cycles were consistent with local El Niño-like and La Niña like conditions. Warmer and drier El Niño like episodes were more frequent towards the end of the 18th century. In the early 19th century, cooler and wetter La Niña like conditions became more common, including the strongest events recorded in the shell.
Researchers said the detailed monthly record provides insights that annual averages alone cannot capture, including seasonal temperature differences, alternating wet and dry periods, and changes in climate from one year to the next.
Evidence from other parts of the Southern Hemisphere suggests that the findings fit into a broader pattern of climate changes during the same period. Coral records from the Great Barrier Reef and northwestern Australia indicate cooler seas or shifts in ocean conditions, while Antarctic ice and glacier records point to cooling and changes in atmospheric circulation.
Other climate archives also suggest more frequent La Niña like conditions in the early 1800s. Coral records from the Galápagos Islands, cave records from northern Peru and lake levels in southeastern Australia provide additional evidence of changes in rainfall and ocean circulation.
However, the cooling was not uniform across the Southern Hemisphere. The temperature decline reconstructed at Jiigurru was smaller than estimates from many Northern Hemisphere records, highlighting the importance of collecting climate data from different regions.
Scientists said giant clam shells could help fill gaps in historical climate records, particularly in tropical oceans of the Southern Hemisphere, where detailed observations from before industrial era warming remain limited.
Further analysis of similar shells from reefs across the Indo Pacific could help researchers understand how historical climate changes varied between locations and improve knowledge of long term ocean and rainfall patterns.