Trace elements trapped in ancient plankton reveal that atmospheric carbon dioxide levels have been largely stable over the last 2.1 million years. In a study published in Science in June, paleoceanographer Bärbel Hönisch and colleagues at Columbia University examined the remnants of planktonic foraminifera—single-celled creatures with elaborate shells—buried beneath the seafloor off the coast of Africa. The plankton incorporate different forms of boron into their shells, depending on the seawater’s acidity, so each shell serves as a chemical record of the ocean’s pH during its occupant’s brief life. The sea’s acidity, in turn, reflects how much carbon dioxide was present in the atmosphere at the time. By analyzing boron in shells accumulated over more than 2 million years, Hönisch was able to reconstruct in unprecedented detail how atmospheric carbon dioxide levels have changed over time.
As expected, carbon dioxide fluctuated with variations in local temperature, with higher levels corresponding to warmer epochs. But despite major shifts in the climate over the period she studied, Hönisch found that overall concentrations of the gas remained remarkably constant. That makes today’s sky-high readings look even more anomalous. “It really shows how much we have interfered with the environment,” Hönisch says. “This goes way beyond anything that earth has seen in a really long time.” The researchers now want to dig deeper below the seafloor, where plankton have been piling up for some 100 million years, to study times when carbon dioxide levels were as high as they are today.