by Mary Caperton Morton Wednesday, April 19, 2017
During the last interglacial period, between 129,000 and 116,000 years ago, global sea levels were 6 to 9 meters higher than at present. Scientists have long wondered how global atmospheric and ocean temperatures then compared to modern times, but efforts to reconstruct such temperatures have often fallen short. In a new study, researchers who compiled past records of sea-surface temperatures (SSTs) have revealed that SSTs during the last interglacial were similar to modern day temperatures. But, the similarity doesn’t necessarily predict a future surge in sea levels.
“At the height of the last interglacial, a lot of Earth would have looked familiar” to us, says Jeremy Hoffman, a paleoclimatologist at the Science Museum of Virginia in Richmond and lead author of the new study in Science. “We would have recognized a lot about the natural world, but beachfront property would have had a much different location.”
To reconstruct a record of SSTs during the last interglacial, Hoffman and his colleagues compiled more than 100 previously published records for 83 globally-distributed marine sediment cores, mining information from nearly 40 years of study. “The main contribution of [this] study is that they pulled together a large number of records across the entire time interval of the last interglacial,” says Lorraine Lisiecki, a paleoceanographer at the University of California, Santa Barbara, who was not involved in the new study. “Because they put together so many of these records, they are able to see consistent patterns between different locations and regions on the planet, which is really helpful for comparing with global climate models.”
The team found that at the onset of the last interglacial, roughly 129,000 years ago, global SSTs matched the global average between the years 1870 and 1889. By 125,000 years ago, global SSTs had increased by half a degree Celsius, making them similar to SSTs reached between 1995 and 2014.
“A lot of these patterns had been suggested at individual sites before, but it’s very helpful to see them all together in a global framework,” Lisiecki says.
The high-resolution dataset also revealed spatial and temporal changes in sea-surface temperature variations between different ocean basins. For example, it showed that the North Atlantic was colder relative to the South Atlantic, suggesting a “bipolar seesaw,” in which overall circulation is reduced, creating colder conditions in the north while allowing more heat to build up in the south. “This tells us that the process of redistribution of heat through the ocean played a large role in the expression of the last interglacial period,” Hoffman says. “Previous research assumed temperature changes were happening at the same time around the world, which we know from present climate change is not how it actually works,” he says.
So, if global SSTs during the last interglacial were similar to today, why aren’t sea levels higher now too? The answer lies in long-term variations in Earth’s orbit around the sun, Lisiecki says. During the last interglacial, the planet’s orbit was more elliptical, bringing more solar energy and increased seasonality to the poles, especially during the summer months. “The higher latitudes were warmer than they are today, due to the increased amount of sunlight they received during the summer,” Lisiecki says. This would have led to increased melting from ice sheets in Greenland and the Arctic, while disruptions in oceanic circulation in the Southern Hemisphere would have allowed heat to build up, facilitating more melting of the Antarctic ice sheets. “There are still a lot of very active research questions here: Just when did sea level get higher than present [during the last interglaciation]? What were the exact sources? What played the bigger role: Greenland or Antarctica?” Hoffman says.
“Our paper does not make predictions about future sea-level change” based on what happened during the last interglacial, Hoffman says. “But it does point out that natural climate change during the last interglacial drove these sorts of changes over thousands of years, and what we’re doing [today] is driving change over the course of decades to centuries.”
© 2008-2021. All rights reserved. Any copying, redistribution or retransmission of any of the contents of this service without the expressed written permission of the American Geosciences Institute is expressly prohibited. Click here for all copyright requests.