Thanks largely to our burning of fossil fuels, the average carbon dioxide concentration of the Earth’s atmosphere surged past 400 parts per million in March. It has been a long time since the planet experienced such CO2 levels. How long? It depends on who you ask. Bloomberg says it has been at least a million years. Scientific American says 23 million years ago, at the end of the Oligocene.
This isn’t an example of bad reporting—even scientists argue about this. NASA climate scientist Carmen Boening says we last saw this concentration during the Pliocene (between 2.5 million and 5.3 million years ago). On the very same page, her colleague Charles Miller says 400 ppm is “maybe higher than any time in the last 25 million years.”
Scientists hate “maybes.” So what’s the problem?
During our 200,000-or-so years on this planet, CO2 concentrations have largely stayed below 300 ppm, as ice cores prove.
The problem is ice cores. When water freezes, tiny gas bubbles are trapped in the ice, providing a snapshot of the composition of the planet’s atmosphere at the moment of freezing. Since the ice has piled up over time in places like Greenland and Antarctica, scientists can drill into it to learn about the atmospheres of the past. But there’s a limit to how far back the records go. As of now, the oldest recovered, reliable ice cores stretch back around 800,000 years, during an ice age when the atmosphere was about 185 ppm carbon dioxide. That’s why many of the reports on the current concentration of CO2 say we haven’t seen these levels in at least a million years.
The ice cores may soon go back further. Scientists have recently found samples in Antarctica up to 1.5 million years old, which would extend our records substantially. But they will not get us back to the Pliocene, much less the Oligocene.
Instead, we have to rely on indirect evidence of carbon dioxide concentrations. Scientists call these proxies, but you may as well think of them as clues. The most time-tested proxy comes from the ocean. Foraminifera are tiny, plentiful marine protists whose remains cover the ocean floor. Their shells contain the element boron. By examining what form of boron is in a shell, paleoceanographers can determine with reasonable precision the pH of the ocean when that shell was created.
Ocean pH, as you should know, is largely a function of the amount of carbon dioxide dissolved in the water. Because the air and ocean exchange gases at the sea surface, ocean acidity and the atmospheric carbon dioxide are related. The problem is, they’re not that related. Scientists say that marine proxy records “place constraints” on determining past atmospheric carbon dioxide levels—in other words, they give us an approximate range, but we can’t measure it down to one or two parts per million. So the foraminifera, unfortunately, can’t tell us exactly when the atmosphere last hit 400 ppm of CO2.
There are other proxies out there. Paleobotanists, for example, use plant matter to characterize the ancient atmosphere—they are almost literally reading tea leaves. During eras of low CO2 concentration, leaves contained high numbers of pores, called stomata, to improve gas exchange with the environment. Plants living in high-carbon times have fewer stomata. Advocates of this approach are working on atmospheric CO2 estimates going back hundreds of millions of years, but the technique is still very new and the estimates feature a fairly large range of uncertainty. (The early results suggest that the Earth may have experienced 400 ppm or higher of CO2 as recently as two million years ago.)
This is all slightly academic, of course. The undeniable truth is that we Homo sapiens have never seen concentrations this high. During our 200,000-or-so years on this planet, CO2 concentrations have largely stayed below 300 ppm, as ice cores prove. For humans, this is uncharted territory.