Earth without ice caps: Sea shells reveal 66 million years of CO2 levels

In a new study, scientists have estimated carbon dioxide levels from the past 66 million years, using two methods analyzing tiny organisms found in sediment cores from the deep seafloor, and found a consistent picture of the evolution of the ocean-atmosphere carbon dioxide levels. 

Co-authored by Yige Zhang, assistant professor in the Department of Oceanography at Texas A&M University, and oceanography graduate student Xiaoqing Liu, the research was conducted by an international team and recently published in in the Annual Review of Earth and Planetary Sciences.

This new environmental reconstruction research shows that twenty-first-century carbon emissions have the potential to return atmospheric carbon dioxide (CO2) to levels not seen since the much warmer climates of Earth’s distant past.

The study found CO2 levels of over 1500 parts per million (ppm) in the atmosphere associated with extreme global warmth 50 million years ago, a climate so warm that no ice existed on the poles, the researchers said.

“For instance, at CO2 levels of around 1500 ppm, last seen about 50 million years ago, it was so warm that we find fossilized alligators in the Arctic,” said James Rae, lead author of the paper and reader in the School of Earth and Environmental Sciences at the University of St. Andrews.

“Societally, we all care about carbon dioxide in the atmosphere, and in paleoclimatology we have these methods to look at CO2 variations in Earth’s history,” Zhang explained. “And in particular, these two-ocean marine-based methods are the two most common and arguably the most accurate. We used those two state-of-the-art methods, analyzed how they compare with each other, and looked at what history they inform us about for the last 66 million years.”

The researchers used two proxy methods to reconstruct carbon dioxide levels during the last 66 million years, also known as the Cenozoic Era: one method using carbon isotopes in alkenones, and another method using boron isotopes in planktic foraminifera – single-celled organisms with calcite shells, common in marine cores.

Zhang collaborated with researchers from the University of St. Andrews and the University of South Hampton, both in the United Kingdom, and the Swiss Federal Institute of Technology in Zürich.

“These data allow us to see how CO2 has changed through Earth’s history, and the impact this has had on climate,” Rae said. “Over the last 66 million years we find that CO2 and global climate go hand in hand, with higher CO2 levels associated with dramatically different climates.”