Ocean temperature record unlocked from Fijian coral reveals highest warmth in over six centuries

The ocean warmth in the Fijian archipelago is at its highest for the past 653 years, according to a ground-breaking new coral record that provides further evidence for unprecedented warming in the western Pacific Ocean.

An international team of earth and climate scientists spanning three continents, co-led by the University of Leicester, have published in the journal Science Advances an annually resolved 627 year-long coral record, based on a coral geochemical record from the southwestern tropical Pacific, as well as 26 years of instrumental data. The region plays a pivotal role in regulating global climate patterns via its association with the long-term Pacific climate swings.

It is the longest continuous coral Strontium/Calcium-sea surface temperature reconstruction to date, offering a distinct and unprecedented perspective on long-term Pacific climate swings from a key location to characterise ocean temperature gradients. 

The scientists used the giant coral Diploastrea heliopora colony in Fiji to provide the data for the new reconstruction. The team collected a more than 2m long core from this 3-6mm per year slow growing colony by scuba diving and analysed its skeletal chemical composition. These unique and long-lived massive corals have stories locked in their chemical makeup about the long-term climatic and environmental changes that have shaped the reef and the giants themselves over many centuries. They are vital archives of past climate variability across the tropics, often providing a direct link to human storylines of past experiences of climate variations. 

Co-lead of the study Jens Zinke is a Professor for Palaeobiology at the University of Leicester School of Geography, Geology and the Environment and Royal Society Wolfson Fellow, whose research examines massive coral colonies from the Indo-Pacific Oceans to track environmental and climate change over the last centuries into the modern day. 

Professor Zinke said: “As corals grow, they record the chemical composition of the ocean around them in their skeletons, and this happens day-in, day-out over centuries. Those corals act like a living temperature sensor by changing the ratio of skeletal Strontium over Calcium with lower ratios being related to higher temperatures. 

“By comparing our reconstruction with instrumental data, we uncover unprecedented warmth in recent years and showcase the impacts of ongoing global warming on decadal Pacific climate dynamics. The study shows that the present ocean warmth in the Fijian archipelago is highest for the past 653 years, and persistent synchronous western and central Pacific warming rates could adversely impact Pacific Island ecosystems and nations.”

The Pacific Ocean is a major driver of climate variability, affecting human activities and natural ecosystems worldwide across timescales ranging from years to decades and beyond. Tropical Pacific decadal variability plays a significant role in modulating the El Niño Southern Oscillation, the major climate phenomenon affecting global climate. Changes in Pacific Ocean temperature gradients lead to major shifts in rainfall producing areas across South Pacific Islands and adjacent areas and play a major role in the development of cyclones impacting the western and southwestern Pacific Islands. These ocean changes drive atmospheric responses that have a strong impact on marine and terrestrial ecosystems and people that depend on them.   

Since the late 19th century, human activities have led to a rise in ocean temperatures, triggering marine heatwaves that have devastated coastal ecosystems. These heatwaves have caused widespread coral bleaching, often leading to the mass mortality of coral reefs. 

The long-term increase in temperature is modulated at interannual and decadal time scales, but our understanding of this decadal and centennial Pacific climate variability over the last millennium has been limited due to the sparseness, shortness, and lack of continuous high-resolution instrumental ocean records.

Lead author Juan P. D’Olivo from Universidad Nacional Autónoma de México said: “The instrumental record from our oceans is simply too short to distinguish the full influence of anthropogenic warming and its interplay with decadal to centennial climate variability.”

Professor Zinke said: “Our work sheds a new light on the interplay between ocean temperature trends across the tropical and subtropical Pacific Oceans over six centuries, revealing how late 20th century warming has become increasingly synchronized between these regions that normally oppose each other, creating important temperature gradients that are the heart of major shifts in rainfall centres across the past few centuries. 

“The recent synchronous warming across the tropical and subtropical Pacific lead to lower temperature gradients with significant impacts on Pacific Islands nations and circum Indo-Pacific continental climate through shifting drought and flood cycles.

“Rapid reductions in greenhouse gas emissions remain the most important action for humanity to combat ongoing ocean warming and associated changes in atmospheric circulation, otherwise we will find ecosystems and ourselves in uncharted climate territory, requiring rapid adaptations to the relentless march of climate change.”  

• ‘Sr/Ca-SST reconstruction from Fiji extending to ~1370 CE reveals insights into the Interdecadal Pacific Oscillation’ is published in the journal Science Advances, DOI: 10.1126/sciadv.ado5107 
• The study was a collaboration between the Universidad Nacional Autónoma de México, Universidad Autónoma de Baja California, the University of Leicester, the University of New South Wales, Monash University, Université de Bordeaux, Helmholtz Centre Hereon and Johannes Gutenberg-Universität Mainz.