As surface temperatures rise, ice sheets are melting at record rates and sea levels are rising—but perhaps not as quickly as predicted.
To predict sea level changes, scientists look to Earth’s distant past, when climate conditions were similar to today, and investigate how the planet’s ice sheets responded then to warmer temperatures brought on by increased carbon dioxide in the atmosphere.
In a recently published study in the journal Geology, Ph.D. students Matthew Winnick and Jeremy Caves at Stanford School of Earth, Energy, and Environmental Sciences explored these very old conditions and found that sea level might not have risen as much as previously thought—and thus may not rise as fast as predicted now.
To better understand global sea level rise, Winnick and Caves analyzed the middle Pliocene warm period, the last time in Earth’s history, approximately 3 million years ago, when carbon dioxide levels in the atmosphere were close to their present values (350-450 parts per million).
“The Pliocene is an important analogue for today’s planet not only because of the related greenhouse gas concentrations, but because the continents were roughly where they are today, meaning ocean and climate circulation patterns are comparable,” explains Winnick.
These similarities are why the Intergovernmental Panel on Climate Change (IPCC), the group responsible for global sea level rise projections, focuses on the mid-Pliocene warm period to inform their computer models.
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Previous studies of the mid-Pliocene warm period used oxygen isotope records to determine the volume of Earth’s ice sheets and, by proxy, sea level. Effectively, the oxygen isotope records act as a fingerprint of Earth’s ice sheets. By combining the fingerprint with models of ice sheet meltwater, many previous researchers thought that sea level was likely 82 feet to 98 feet (25 meters to 30 meters) higher during the Pliocene.
