The oceans are getting warmer. This not only poses major challenges for marine life, but also has an impact on the global climate. A further warming of the southwest Pacific would be particularly critical, but as the core of a 600-year-old coral off Fiji shows, we are heading exactly towards this. A unique climate archive is stored inside it, dating back to the year 1340 – and showing an ominous trend.
Ocean temperatures are not only essential for marine ecosystems, but also for the regional and global climate. For example, the southwest Pacific plays a central role in the El Niño weather phenomenon. If the ocean surface there warms up too much, a chain of processes is set in motion that then leads, for example, to heavy rainfall on the west coast of South America and drought in Indonesia and Papua New Guinea. Understanding the long-term temperature development in the southwest Pacific is therefore of great interest to science.
A Methuselah coral as a contemporary witness
In order to be able to place the current temperature developments in a larger context, researchers led by Juan D’Olivo from the National Autonomous University of Mexico have now resorted to a very special climate archive: an ancient honeycomb coral (Diploastrea heliopora) off the coast of the Fiji archipelago. Although corals of this type only grow three to six millimeters per year, they can reach enormous lifespans – in the case of the Fiji specimen, an impressive 627 years. And everything that the coral has experienced during this long period is stored inside its huge, dome-shaped calcareous skeleton. Its “memories” also include the surface temperatures that prevailed in the individual years of its existence.
To reconstruct the development of these temperatures over the past centuries, D’Olivo and his team extracted a two-meter-long core from the coral skeleton and analyzed the ratio of strontium to calcium in each annual layer. More calcium than strontium indicates warmer water temperatures, more strontium than calcium indicates cooler temperatures. By supplementing the temperature data obtained with scientific measurements from the past decades, the team was able to stretch the coral’s climate archive to a span of 653 years and to extend it to the entire southwest Pacific.
Current warming is unprecedented
The measurements showed that the current sea temperature on the Fiji archipelago is the highest in the past 653 years. Although there was a notable warm period between 1370 and 1553, when the sea around Fiji was almost as hot as it is today, this is still not comparable to current developments, as D’Olivo and his colleagues report. “The warming of the Pacific over the past century, which is largely attributed to human-caused global warming, represents a significant deviation from the natural variability of previous centuries,” the team said.
“While some parts of the Pacific were warmer in the past, others experienced one or two cooler decades at the same time, and vice versa, this relationship is now breaking down. Warming has become increasingly synchronized throughout the tropical and subtropical Pacific,” the researchers explain. In the future, this holistic, unprecedented warming of the Pacific could cause unprecedented changes in the climate system. D’Olivo and his colleagues warn, for example, of extreme droughts and increased heavy rainfall in countries bordering the Pacific. The warming southwest Pacific could also be felt in Europe, albeit to a much milder extent: for example in the form of a wetter start to winter and a colder end to winter.
Source: Johannes Gutenberg University Mainz; Article: Science Advances, doi: 10.1126/sciadv.ado5107