Carbon Conundrum

Trends in atmospheric concentrations and anthropogenic emissions of carbon dioxide. Click here for larger image.
Credit: US DOE

As scientists study the complicated dynamics of a warming planet, they are trying to understand the movement of Earth’s principal greenhouse gas, carbon dioxide, as it meanders through the planet’s atmosphere, biosphere, and geosphere. The easy part is calculating how much net carbon is released by human activities – about 6.6 gigatons (billion metric tons) per year. (For ease in tracking the element as it undergoes chemical changes while moving through ecosystems, measurements of carbon dioxide ignore the mass of the oxygen. Note also that all numbers regarding the carbon cycle are estimates that are more or less accurate.)

The hard part is figuring out what happens to airborne carbon after it enters the ill-understood natural carbon cycle. Carbon dioxide can leave the atmosphere through organic and inorganic processes at or below the planet’s surface. It may be removed through photosynthesis and stored in wood before being incorporated into soil or returned to the atmosphere when the wood decomposes. The oceans absorb lots of carbon dioxide, which is stored in biomass or sediment, or returned to the atmosphere.

While it is impossible to say for sure that an enhanced greenhouse effect due to human activities is causing the ongoing planetary warming, the great majority of climate scientists believes that proposition. And that, in brief, explains the concern about the carbon cycle. A major goal of carbon-cycle studies is to balance this simple equation:

carbon entering atmosphere
carbon leaving atmosphere
net change in carbon content of atmosphere

As you can see from the "global carbon cycle" figure, many natural processes are involved in the planetary movement and transformation of carbon dioxide:



  • Primary production, or photosynthesis (by which green plants store carbon in organic molecules) on land is almost balanced by respiration. A net of 1.5 gigatons of carbon is taken up on land each year, according to figures from the International Panel on Climate Change (IPCC), the United Nations group charged with researching climate change.
  • Dissolution of atmospheric CO2 into seawater stores massive quantities of carbon dioxide in the ocean, some of which ends up in plants through photosynthesis. Although the ocean "exhales" practically the same amount of carbon dioxide, it does take up a net of about 2 gigatons of carbon each year.
  • Sedimentation of animal detritus (primarily shells) removes carbon very slowly from marine environments.
  • Weathering and erosion both gradually return carbon from the lithosphere to the ocean.

    The Human Element

    Global carbon cycle (billion metric tons). Click here for larger image.
    Credit: US DOE

    Added to these natural interactions is the impact of human activities. The largest human sources of atmospheric carbon are combustion of fossil fuels and cement production, which release about 5.5 gigatons per year), according to the IPCC. Changing land use (mainly deforestation) releases a net of about 1.1 gigatons of carbon.

    Fossil-fuel use has risen steadily during the industrial age, as has the CO2 content of the atmosphere, up from 200 to 280 parts per million (ppm) to today’s 370 ppm. Through processes describe above, humans are adding about 1.5 ppm per year.

    The IPCC projects that a doubling of carbon dioxide over pre-industrial levels, expected to occur in about a century, could cause a worldwide warm-up of 2 to 5 degrees Celsius (3.5 to 9 degrees Fahrenheit). Burning ever-increasing amounts of the estimated 4,000 gigatons of carbon stored in fossil fuels could wreak havoc with the climate, notes James Kasting, in the department of geosciences and meteorology at Penn State. "Carbon dioxide may double, and double again," largely due to the increased appetite for fossil fuels. A second doubling, Kasting warns, would produce an increase of 4 to 10 C (7 to 18 F), returning Earth to the climate of the dinosaurs and causing incalculable upheaval of human societies and natural ecosystems.

    Stores and Sinks

    For many years, the key question in carbon-cycle research was the "missing sink," or storage location, for carbon. The carbon books did not balance. The atmospheric carbon dioxide concentration was rising more slowly than expected, given known inputs and outputs. Estimates from the 1980s, for example, showed that about 1.8 gigatons of the carbon entering the atmosphere every year could not be accounted for. It didn’t remain in the atmosphere, but nobody could figure out where it went.

    Among the possible sinks that scientists are investigating is the vast Amazon forest. The system is complex, and Jeffrey Richey, a professor of oceanography at the University of Washington, recently calculated that flooded Amazonian forests might be leaking a substantial amount of carbon that originates in decomposing plants. Instead of being stored in vegetation or soil, this carbon is dissolving in water. Each year, 0.5 gigatons of this waterborne carbon dioxide is outgassed from the rivers and wetlands of Amazonia, according to the study, which used satellite radar to assess ecosystem area.

    "We always suspected it was a big deal, but it wasn’t until the smoking gun of the radar that we were able to prove how big the outgassing really was," says Richey. The study was published in Nature on April 11, 2002.

    If not in Amazon forests, the carbon sink may be in the North. In the December 18, 2001, Proceedings of the National Academy of Sciences, researchers announced the discovery of a large carbon sink in northern-hemisphere forests. Tree growth in these forests was calculated to be storing 0.7 gigatons of carbon per year, about 12 percent of annual industrial carbon releases. While Canada’s forests were losing carbon (probably due to disease and logging), forests in the United States, Europe and Russia were soaking up the element.

    US gas emissions by gas, 1996 (million metric tons carbon equivalent). Click here for larger image.
    Credit: US DOE

    The 0.7 gigaton finding probably represents only a portion of northern hemisphere carbon storage, said Compton Tucker of Goddard Space Flight Center, one author of the report, which used data from NASA’s Earth Science Enterprise. "This is only a piece of the total carbon sink in the north, which may be as large as 2 billion tons."

    If, as scientists currently suspect, most the "missing carbon" is being stored in northern-hemisphere ecosystems, Kenneth Davis, an associate professor of meteorology at Penn State, says the implication is clear: "We need to understand the terrestrial system. What is the cause of the sink? If we can identify why there’s currently a sink, we have the possibility of looking into the future to see how it might change. "

    To pin down the relevant processes, Davis says, carbon-cycle studies must fill in gaps. "We have small-scale ecological methods that give a wonderful description of processes locally," he says, and relatively good knowledge of the global budget. At this point, he says, "a large focus is to bring the two scales together, so the local-process stage meets the global budget." Detailed knowledge of carbon processing at the regional scale, he says, could help policy-makers manage forests to maximize carbon storage and slow global warming. Terrestrial forests may be able to store another 50 to 100 gigatons of carbon globally, he says, about 10 years of fossil fuel burning at current rates. But as climate changes, the terrestrial carbon cycle will also likely change, obscuring the net impact of climate change on carbon stored in land ecosystems. "I think it is important to note the limits on terrestrial storage and the potential for unplanned changes due to climate," says Davis. "The science we are doing is broader than simply storage technology. There is danger that the system may outgas carbon due to climate change, for instance."

    Projected global carbon dioxide emissions. Click here for larger image.
    Credit: US DOE

    Carbon-cycle studies have used a wide range of new techniques to assess carbon flows. Decades ago C.D. Keeling began monitoring the atmospheric concentration of CO2 at Mauna Loa. A global network of similar measurements is now a mainstay of this science. From towers in various habitats scientists have studied the flux of carbon entering and leaving the atmosphere. In water they measure the partial pressure of carbon dioxide (the pressure of CO2 among the many gases in the atmosphere), to calculate the rate of solution and exhalation.

    One new effort, the Large-Scale Biosphere-Atmosphere Experiment in Amazonia, is intended to produce better data on the interaction of land and atmosphere in the Amazon forest. This study, opened in 2000, with NASA’s participation, will use towers and ground-based measurements to "capture the entire cycle of water, nutrients and carbon moving in and out of the Amazon ecosystem," according to NASA.

    The long-term picture of carbon dioxide’s influence on Earth’s climate is uncertain, but it helps to distinguish natural processes from human impacts, Kasting says, "There’s a natural carbon cycle, and a natural greenhouse effect. That’s all well and good, it’s what stabilizes the climate on long time scales, but we are perturbing that cycle in a very large way, and it’s likely to lead to very serious consequences, especially if you project several hundred years in the future."