Human activity is pumping carbon dioxide into Earth’s atmosphere at an unprecedented rate. But centennial-scale increases in atmospheric carbon dioxide—albeit significantly smaller—also persisted in the past. These so-called carbon dioxide jumps are tied to the tilt of Earth’s axis, new research suggests. That work has turned up seven previously unknown jumps that occurred between 260,000 and 190,000 years ago.
The concentration of atmospheric carbon dioxide, a potent greenhouse gas, is typically measured in parts per million. A level of 315 parts per million was measured in 1958, when modern records began. Today the value is 420 parts per million. On average, the concentration of atmospheric carbon dioxide has been increasing by roughly 1–3 parts per million per year since the late 1950s. But getting a handle on the long-term record of atmospheric carbon dioxide—that is, changes due to nonanthropogenic sources—means looking beyond the instrumental record, and that’s where ice comes in.
Faint Whiffs of Ancient Atmosphere
Ice cores contain whiffs of Earth’s past atmosphere in tiny bubbles entrained when the ice formed. Etienne Legrain, a paleoclimatologist at the Institut des Géosciences de l’Environnement in Grenoble, France, and his colleagues recently analyzed ice cores collected in Antarctica as part of the European Project for Ice Coring in Antarctica (EPICA) to study ancient records of atmospheric carbon dioxide.
The researchers didn’t simply melt the ice to release and measure the gas, which is a commonly used technique for analyzing ice core records. Carbon dioxide is soluble in water, so it would have been impossible to measure the gas that way, Legrain said. Instead, the researchers used a machine to pulverize samples of ice. “You crush the ice, you release the carbon dioxide, and then you can measure the air you extract,” he said.
“They’re fast enough to make us puzzle about what’s going on.”
The team analyzed 203 measurements of carbon dioxide concentration ranging in age from 260,000 to 190,000 years before present. When they plotted those data, which came in chunks of 100–200 years, they spotted seven unusual events in which the carbon dioxide concentration increased by more than 5 parts per million from one data point to the next. That’s an unusually rapid shift, said Shaun Marcott, an Earth scientist at the University of Wisconsin–Madison who was not involved in the research. “They’re fast enough to make us puzzle about what’s going on.”
Fifteen such carbon dioxide jumps have been noted in other ice core records, and Legrain and his colleagues set out to explore what might be driving these centennial-scale variations.
First, the researchers noted that 18 of the 22 known carbon dioxide jumps in the past 500,000 years occurred when the tilt of Earth’s axis of rotation—known as the planet’s obliquity—exceeded the average value of 23.3°. Earth’s tilt varies over a roughly 41,000-year timescale, primarily because of our planet’s gravitational interactions with the Sun and Jupiter. “All of these jumps occur in a specific astronomical context,” Legrain said.
From Tilt to Vegetation
Earth’s tilt controls how much sunlight falls on different parts of our planet. “Obliquity changes the amount of solar energy that is shared between the poles and the equator,” Legrain said. During periods of higher-than-normal obliquity, higher latitudes receive more solar energy. That change allows more vegetation to grow at higher latitudes and vegetation patterns at lower latitudes to shift. “A lot of carbon is available to be released into the atmosphere, which is not the case in periods of low obliquity,” Legrain said.
But an abrupt shift in Earth’s climate could trigger events such as wildfires or floods that result in the die-off of all that vegetation, Legrain and his team surmised. Such events would release a pulse of carbon into the atmosphere that would manifest in the ice core record as a carbon dioxide jump.
One oft-hypothesized explanation for rapid past climatic shifts is the shutdown of the Atlantic Meridional Overturning Circulation (AMOC). This system of large-scale oceanic currents is responsible for transporting nutrients, redistributing oceanic heat, and modulating precipitation patterns. There’s evidence that AMOC has repeatedly weakened in the past, and some researchers have suggested that it’s completely collapsed at some times as well.
“It’s a pretty novel thing to start to think about whether these rapid changes can be induced or amplified by longer-term changes in the climate system.”
Legrain and the team simulated the collapse of AMOC to evaluate how such an event could affect carbon dioxide levels when Earth’s obliquity is high. The resulting climatic changes released more than 50 gigatons more carbon into the atmosphere over the course of 400 years compared with a low-obliquity scenario, according to Legrain. Atmospheric carbon dioxide concentrations increased by roughly 10 parts per million over that same time period in the high-obliquity scenario, compared with roughly 3 parts per million for the low-obliquity scenario, the team reported in Nature Geoscience.
Because carbon dioxide jumps are defined as short-interval events marked by upticks in carbon dioxide concentration of more than 5 parts per million, only periods of high obliquity should logically register jumps, Legrain said. “Jumps are the result of AMOC shutdown happening in a high-obliquity context.”
These findings makes sense, and furthermore, they’re original, Marcott said. “It’s a pretty novel thing to start to think about whether these rapid changes can be induced or amplified by longer-term changes in the climate system.”
But there’s a critical piece of follow-on work that’s needed, Marcott said. Measuring the relative frequencies of different stable carbon isotopes—that is, 12C and 13C—present in the carbon dioxide would reveal the relative influence of different processes occurring in the atmosphere, the oceans, and the terrestrial biosphere. “That’s the best way to fingerprint, in my mind, where the different sources of the carbon are coming from,” he said.
Such carbon dioxide jumps could very well occur in the future, Legrain said. Our planet’s obliquity is currently high, and there’s evidence that AMOC may be weakening. The uptick in atmospheric carbon dioxide from a carbon dioxide jump would be comparable to about 4 years’ worth of anthropogenic emissions, he said. “It’s not a negligible impact.”
—Katherine Kornei (@KatherineKornei), Science Writer