Four million years ago, rising temperatures and global ice loss may have pushed sea levels as high as 16 meters (52 feet) above current levels, according to new research that scientists say will help us understand and predict changing coastlines in the face of climate change.
Describing their work in the journal Nature, scientists discovered and analyzed six geological formations in Artà Cave on the island of Mallorca in the western Mediterranean Sea. Seventy samples of phreatic overgrowths were taken from speleothems, mineral deposits that form on stalagmites and stalactites, found between 22.5 and 32 meters (74-105 feet) above modern sea levels. Though the cave is located 100 meters (330 feet) from the coastline, analysis of the deposits suggests they were formed from oceanic buildup over the years as sea levels rose, flooding the cave between 3 and 4 million years ago.
Sea levels rose during an era known as the mid-Piacenzian when temperatures were 2 to 3°C (3.6-5.4°F) higher than pre-industrial levels, marking "the last time the Earth's atmospheric CO2 was as high as today”. At their highest, sea levels may have been more than 23 meters (74 feet) above current levels during the Pliocene Climatic Optimum, an era 4 million years ago when global mean temperatures were up to 4°C (7.2°F) higher than pre-industrial levels.
"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," said researcher Jacky Austermann in a statement.
Altogether, the data can help to inform what our planet may look like as increasing temperatures and melting ice caps contribute to global sea-level rise. Reconstructing ice sheet and sea-level changes from the past when the climate was warmer than now provides insight for future predictions.
"We can use 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," said study co-author Bogdan Onac.
Even if atmospheric CO2 stabilizes at current levels, the authors note that global sea levels will probably still rise to previously seen levels, if not higher. The data indicate that ice sheets are “very sensitive” to warming and provide important calibration targets for future ice sheet models.
"Constraining models for sea-level rise due to increased warming critically depends on actual measurements of past sea level," said study author Victor Polyak. "This study provides very robust measurements of sea-level heights during the Pliocene."
Though the volume and rate of ice melt remain a mystery, the authors believe their work will allow future predictions to accurately measure sea-level rise in the face of human-caused climate change.