Of the 1.85 billion billion metric tons of carbon that exist on Earth, 99.8% exists belowground, according to new reports on deep carbon.

The research estimates that human activity annually releases into the atmosphere around 40 to 100 times as much carbon dioxide as does all volcanic activity. That’s also a slightly higher rate of carbon emission than Earth experienced just after the asteroid impact that likely killed the dinosaurs, the researchers found.

Carbon “provides the chemical foundation for life…and it plays a disproportionate role in Earth’s uncertain, changeable climate and environment,” Deep Carbon Observatory (DCO) executive director Robert Hazen said in a statement. Scientists with DCO led the studies on Earth’s carbon that published today in Elements.

“We cannot understand carbon in Earth—we cannot place the changeable surface world in context—without the necessary baseline provided by deep carbon research,” Hazen said.

A Mostly Steady State

“It’s almost like a forensic detective story.” The new reports summarize 10 years of field data collection, lab experiments, and computer modeling of the origin of Earth’s carbon, how it circulates throughout the Earth system, and extreme events that can upset Earth’s carbon balance. The research estimates that Earth holds a total of 1.85 billion gigatons of carbon, although estimates of total carbon content of the core and lower mantle are speculative and likely to change with future research.

“It’s almost like a forensic detective story, putting together lots of bits of evidence using a wide range of techniques to come up with a planetary carbon budget,” DCO volcanologist Marie Edmonds of the University of Cambridge in the United Kingdom said at a 1 October press conference.

More than 90% of the carbon that exists above the crust resides in the deep ocean and marine sediments. The atmosphere contains only 1.4% of all above-surface carbon, mostly in the form of gaseous carbon dioxide (CO 2 ).

With few exceptions over the past 500 million years, Earth has maintained a balanced carbon cycle, returning to the ground about as much carbon as it outgasses. Silicate weathering is the fastest way to return carbon belowground, with smaller contributions from organic carbon burial, ocean crust update, and subduction (see Figure 1).

In the past 500 million years, four volcanic eruptions created large igneous provinces (LIPs) that each released massive quantities of CO 2 over tens of thousands of years. These LIPs caused the above-ground quantity of CO 2 to spike to about 170% of its steady state value, which led to warmer surface conditions, more acidic oceans, and mass extinctions.

Likewise, large impact events, including the Chicxulub impact 65 million years ago, released large quantities of carbon from the subsurface into the atmosphere.

“The Chicxulub event…greatly disrupted the budget of climate-active gases in the atmosphere, leading to short-term abrupt cooling and medium-term strong warming,” DCO scientists Balz Kamber and Joseph Petrus said in a joint statement.

The Volcanic Details

Volcanic regions—including fractures, faults, soil, lakes, mid-ocean ridges, and active vents—outgas 280–360 million metric tons of CO 2 per year through direct venting and diffuse emissions. In a steady state carbon cycle, this is the largest contributor to aboveground carbon. Other varied geologic processes outgas an additional 20–40 million metric tons of CO 2 . Widespread regions like Yellowstone, the East African Rift, and the China’s Tengchong volcanic field can also have significant diffuse CO 2 emissions.

“We have achieved a much more complete picture of volcanic carbon dioxide degassing on Earth, reinforcing the importance of active volcanoes,” said U.S. Geological Survey geologist Cynthia Werner. But the degassing studies also led to the discovery “that the subtle release over large hydrothermal provinces and areas of continental rifting are also dominant regions of planetary outgassing.”

Continuously monitoring outgassing rates and compositions could also serve as a new eruption forecasting tool. At seven active volcanoes—including Italy’s Etna and Stromboli and the United States’ Kīlauea and Redoubt—the ratio of CO 2 to sulfur dioxide changed significantly months or years prior to large eruptions, according to these studies. In some cases, the outgassing change happened before precursor quakes and ground deformation.

Humanity’s Outsized Carbon Impact

By far the largest disruptor of Earth’s steady state carbon cycle is anthropogenic outgassing. The new reports indicate that humans emit around 10 gigatons of CO 2 into the atmosphere each year (see Figure 2). That flux is more than 10 times the rate at which natural geologic processes return it belowground, according to the studies.

The rate of anthropogenic carbon emissions is higher than that from extinction-level impacts and large outpourings of magma and is 40–100 times higher than the emission rate from all natural outgassing phenomena. Researchers noted that Earth is responding to human emissions with all the hallmarks of massive carbon perturbation of the past: hotter surface temperatures, disruptions to the hydrologic cycle, ocean hypoxia and acidification, and mass extinction.

“To secure a sustainable future, it is of utmost importance that we understand Earth’s entire carbon cycle,” Edmonds said in a statement. She added in the press conference, “Earth will rebalance itself, but it will take 100,000 years.”

These reports were released ahead of the Deep Carbon 2019 conference in Washington, D.C.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer