Throughout Earth’s geologic history, the sea level has risen and fallen numerous times, under the influence of cooling and warming conditions, as well as tectonic plate movements. At times in the past there was no ice at the poles, and the sea surface was several hundred meters higher than it is now. In contrast, when ice covered the planet the sea level was more than a hundred meters lower. These changes are part of Earth’s natural glacial cycles but they are of special interest to climate scientists today as we consider the potential consequences of the current global warming.
A team of researchers, led by scientists from Victoria University of Wellington (NZ) and Birmingham (UK) have recently published the first record of Antarctic Ocean temperatures over the past 45 million years, which offers important insights into future sea level changes. They used a combination of evidence from molecular fossils and machine learning in order to track changes in sea temperature.
The molecular fossils came from core samples obtained during ocean drilling projects. Within these cores are the fossilized remains of lipid molecules that were produced by archaea – single-celled organisms that are similar to bacteria. The lipid molecules were part of the organism’s outer membrane and the composition of these molecules changed in response to differences in sea temperature. By studying these changes, scientists can draw conclusions about the ancient sea temperature which would have surrounded the archaea when they were alive.
While the use of molecular fossils is familiar to paleoclimatologists, the current researchers went a step further and used machine learning to refine the technique. This gave them the first record to date of changing Antarctic sea temperatures throughout the past 45 million years, which represents much of the Cenozoic Period.
The findings, published in Nature Geoscience, enable scientists to pinpoint much more accurately the historic temperatures which caused ice sheets to grow and shrink during that period. This is critical to our understanding of the possible consequences of the current global warming, as the loss of ice sheets and the retreat of glaciers in the Antarctic could mean a sea level rise of up to 50 m.
“The record we’ve produced offers a much more robust overview of fluctuating Antarctic temperatures and how these relate to changes in the amount of ice, and the topography of Antarctica, over this period and paves the way for improved estimates of future events,” explained study lead author Dr. James Bendle.
The link between CO2, sea-surface temperatures and the amount of ice on Antarctica is clearly evidenced in the study’s data from the last 45 million years. But one surprising finding was that ocean cooling did not always correspond to increases in Antarctic ice. This applies specifically to a period of 1 million years of ocean cooling that occurred between 25 and 24 million years ago. “We show that this is likely related to tectonic subsidence and the influx of relatively warm ocean water in the Ross Sea region,” said Dr. Bendle.
“We can see that ice in Antarctica is currently changing, not least with the loss of some ice-shelves and cracks appearing recently in the Thwaites Glacier, one of the largest glaciers in the region. This new study of Earth’s past is one of the clearest indications yet that humans continue to produce CO2 levels for which we can expect major ice loss at the Antarctic margins, and global sea-level rise over the coming decades and centuries.”
In fact, the researchers say their results suggest we are nearing a ‘tipping point’ where ocean warming, caused by atmospheric CO2, will lead to catastrophic rises in sea levels because of melting ice sheets. They plan to continue to apply biomarker and machine learning approaches to reconstruct the climatic evolution of Antarctica and understand the implications for future warming and sea-level rise.