A study focused on the evidence preserved in speleothems in a coastal cave by an international team of scientists, suggests that over three million years ago, at a time when the Earth was two to three degrees Celsius warmer than the pre-industrial era, sea level was nearly 16 meters higher than the present day. These findings represent significant implications for understanding and predicting the rate of current-day sea level rise in a warming climate.
The team including Professor Yemane Asmerom and Sr. Research Scientist Victor Polyak from The University of New Mexico, the University of South Florida, Universitat de les Illes Balears and Columbia University, published their findings in the recent edition of the journal Nature. The scientists said that the in-depth analysis of deposits from Artà Cave on the island of Mallorca in the western Mediterranean Sea produced sea levels that can serve as a target for future studies focusing on ice sheet stability, ice sheet model calibrations and projections of future sea-level rise.
Melting ice sheets result in sea level rises like in Greenland and Antarctica. However, how much and how fast sea level will rise during warming is a question scientists have strived to find out. According to USF Ph.D. student Oana Dumitru, the lead author, who did the majority of her dating work at UNM under the guidance of Asmerom and Polyak, reconstructing ice sheet melts and sea-level changes during past periods when the climate was naturally warmer than today, provides an Earth’s scale laboratory experiment.
Polyak said, “Constraining models for sea-level rise due to increased warming critically depends on actual measurements of past sea level. This study provides very robust measurements of sea level heights during the Pliocene.”
USF Department of Geosciences Professor Bogdan Onac said, “We can use the knowledge gained from past warm periods to tune ice sheet models that are then used to predict future ice sheet response to current global warming”.
The project focused on cave deposits called phreatic overgrowths on speleothems that form in coastal caves at the interface between brackish water and cave air every time the ancient caves were flooded by rising sea levels. Professor Joan J. Fornós of Universitat de les Illes Balears notes that the water table in the Artà Cave, which is located within 100 meters of the coast and was in the past, equal to the sea level.
The scientists discovered, investigated, and interpreted six of the geologic formations found at elevations of 22.5 to 32 meters above current sea level. Careful selection and laboratory analyses of 70 samples resulted in ages ranging from 4.4 to 3.3 million years old BP (Before Present), indicated that the cave deposits were formed during the Pliocene epoch. The ages were determined in UNM’s Radiogenic Isotope Laboratory using uranium-lead radiometric dating.
Asmerom explained, “This was a unique convergence between an ideally-suited natural setting worked out by the team of cave scientists and the technical developments we have achieved over the years in our lab at The University of New Mexico. Judicious investments in instrumentation and techniques result in these kinds of high-impact dividends.”
Columbia University Assistant Professor Jacky Austermann, a member of the research team noted, “Sea level changes at Artà Cave can be caused by the melting and growing of ice sheets or by uplift or subsidence of the island itself”.
She employed numerical and statistical models to carefully analyze how much uplift or subsidence might have occurred since the Pliocene and subtracted this from the elevation of the formations they investigated.
One key interval of specific interest during the Pliocene is the mid-Piacenzian Warm Period which was around 3.264 to 3.025 million years ago when temperatures were 2 to 3º Celsius higher than pre-industrial levels.
Onac says, “The interval also marks the last time the Earth’s atmospheric CO2 was as high as today, providing important clues about what the future holds in the face of current anthropogenic warming,”
This study determined that during this period, global mean sea level was as high as 16.2 meters (with an uncertainty range of 5.6 to 19.2 meters) above present. This implies that even if atmospheric CO2 stabilizes around current levels, the global mean sea level would still probably rise at least that high, if not higher, the scientists concluded. In fact, it is expected to rise higher because of the increase in the volume of the oceans due to rising temperature.
Dumitru said, “Considering the present-day melt patterns, this extent of sea level rise would most likely be caused by a collapse of both Greenland and the West Antarctic ice sheets”.
The authors also measured sea level and found it to be 23.5 meters higher than present about four million years ago during the Pliocene Climatic Optimum when global mean temperatures were nearly 4°C higher than pre-industrial levels.
Asmerom summarized, “This is a possible scenario, if active and aggressive reduction in green house gases into the atmosphere is not undertaken”.