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The Ins and Outs of the Global Carbon Cycle

Tracking All That Carbon Dioxide Is Harder Than It Looks

illustration of the global carbon cycle SOURCE: NASA Scientists are now worried about the degree to which carbon sinks could shrink, or carbon sources could grow, in response to the rapid increase in anthropogenic CO2 emissions. Above: the global carbon cycle.

On its face, the question seems simple enough: Where exactly does all that carbon dioxide we are spewing into the atmosphere go? Like most matters in climate science, though, it is usually the most innocuous sounding questions that conceal the greatest degree of complexity—and uncertainty.

Having spent the last few decades piecing together the different components of the global carbon puzzle, scientists now have a good idea of how the planet’s natural carbon sinks (or reservoirs) work—primarily these sinks are plants and the oceans. But when it comes to pinpointing the locations of all the sources (areas or organisms which release carbon dioxide to the atmosphere), there remains a lot of ambiguity—mostly because climate change is constantly changing the picture of how the sources work (and it’s usually changing for the worse). Indeed, scientists are only beginning to understand how the behavior of sinks and sources will shift as atmospheric CO2 levels continue to increase. And they do not like what they see. What many scientists are now worried about is the degree to which carbon sinks could shrink, or carbon sources could grow, in response to the rapid increase in anthropogenic CO2 emissions.

The easiest way to think of the global carbon cycle is as the sum total of different reactions—primarily those resulting in the destruction or formation of calcium carbonate or organic matter like carbohydrates—between and within the planet’s major carbon repositories: the ocean and terrestrial biosphere.[1] The ocean is by far the larger one—estimated to hold about 38,000 petagrams (1 petagram equals one trillion grams); the land plants and soils that make up the terrestrial biosphere store only about 2,000 Pg.[2]

Scientists are beginning to come to grips with the realization that many erstwhile sinks, primarily plants and soils, could lose their ability to draw down CO2 in a warming world.

Carbon transport between and among the reservoirs is primarily accomplished via CO2 gas exchange. The two types of processes that affect the flow of the global carbon cycle are “long-term” fluxes—those that operate on the scale of millennia (anything having to do with weathering, or the decomposition of rocks, minerals, and soils)—and “short-term” fluxes—which are driven by natural reactions like photosynthesis and respiration (when plants absorb carbon dioxide directly from the atmosphere, for instance). These help control the concentration of CO2 in the atmosphere.

Together, the ocean and land absorb roughly 2.1 petagrams of carbon every year; that number is the difference between the average amount of fossil fuel emissions produced, about 5.4 Pg of carbon per year, and the growth in atmospheric CO2 concentrations, around 3.3 Pg per year. These numbers are based on measurements taken during the 1980s, one of the most recent decades for which researchers have an estimate of all sources and sinks.

So how do scientists distinguish between the contributions made by each sink? Until the mid-1990s, that question was still very much in doubt. Though researchers knew about the major sinks and sources, they were unsure as to the magnitude of the terrestrial sources—particularly the one associated with tropical deforestation (which, for a long time, they thought accounted for 10 and 50 percent of all fossil fuel emissions). Their calculations seemed to suggest that there was an additional “missing” sink somewhere, but they couldn’t put their finger on it.

In a landmark study published in 1996, Ralph F. Keeling of the Scripps Institution of Oceanography (the son of Charles D. Keeling, the man behind the famous “Keeling Curve”) and colleagues showed that it was possible to determine the difference between the two sinks by examining the partial pressures—what scientists describe as the pressure exerted by each of the constituents of a mixture of gases—of CO2 and O2 in the atmosphere.[3]

Scientists already knew that an increase in atmospheric CO2 corresponded to an equivalent decrease in atmospheric O2 (because of photosynthesis and respiration). Keeling’s breakthrough was demonstrating that the ocean, though a crucial sink (and source) for CO2, did not release much O2 in response to a decrease in atmospheric O2. This meant that any change in atmospheric O2, other than that resulting from fossil fuel use, had to be attributed to a terrestrial source. Working backwards, they were able to quantify the importance of the different sources and sinks.

As most climate scientists will tell you, though, there is always a large degree of uncertainty implicit in these measurements so they could, in reality, be very different. Moreover, because the carbon balance—the difference between the amount of emissions released to the atmosphere and those taken up by sinks—is never static, year-to-year variations can be significant. These can usually be attributed to fluctuations in the response of the terrestrial biosphere to the climate—often because of increased land-use or changing water availability. The oceanic response, by comparison, is typically muted.

These sinks currently absorb around half of all the carbon dioxide emitted through fossil fuel combustion.[4] Around 85 percent of new anthropogenic CO2 ends up in the ocean, where, after slowly dissolving into the surface waters, it gets trapped in the “conveyor belt” (also known as thermohaline circulation), the large-scale movement of currents driven by density gradients in the deep. Almost half of the total amount of anthropogenic CO2 that has been added to the atmosphere since pre-industrial times has gone into the ocean.[5]

Indeed, as I wrote about in a recent column, scientists are beginning to come to grips with the realization that many erstwhile sinks, primarily plants and soils, could lose their ability to draw down CO2 in a warming world—with a worst-case scenario being that they would turn into sources.

Steven W. Running of the University of Montana at Missoula’s College of Forestry crunched the numbers in an article for Science a few months ago and was dismayed by the results.[6] Though several of the 11 land models he ran projected that photosynthesis rates would dramatically increase under conditions of doubled atmospheric CO2 levels, he found that most did not incorporate land-use variations or episodic disturbances like wildfires and insect epidemics.

Those are crucial omissions, he says, since a recent FLUXNET synthesis determined that disturbances often caused sinks to turn into sources. (FLUXNET is a global network of micrometeorological tower sites which measure the exchanges of CO2, water vapor, and energy between the terrestrial biosphere and the atmosphere.) With droughts, wildfires and insect invasions, such as the mountain pine beetle epidemic, on the rise, Running cautions that many important terrestrial sinks could soon become sources.

The ocean, of course, faces its own litany of problems. As I’ve written about in the past, ocean acidification is a major concern—and one that has only become more acute in recent months. A study published last week in the Proceedings of the National Academy of Sciences found that increases in acidity are happening over 10 times faster than previously thought. This is a major problem, as lead author Timothy Wootton of the University of Chicago’s Department of Ecology and Evolution explains, because, as the ocean’s natural carbonate buffering system weakens—the direct result of millions of tons of CO2 lowering the pH of seawater—it will no longer be able to absorb as much atmospheric CO2. And that could spell big trouble for all of us.

One tool that promises to make scientists’ work much easier in the near future is NASA’s Orbiting Carbon Observatory, a satellite that will track the geographic distribution of atmospheric CO2—and thus help pinpoint the exact locations of all the sinks and sources. While not a solution in of itself, the OCO will help researchers refine their models, making them more useful for policymakers, which, in turn, should lead to the creation of more effective, targeted mitigation strategies.

Jeremy Jacquot is a graduate student in marine environmental biology at the University of Southern California and is a contributing writer for The Huffington Post, Discover Magazine, DeSmogBlog, and TreeHugger.


[1] Sarmiento, J. L. & Gruber, N. (2002). Sinks for Anthropogenic Carbon. Physics Today, 30 – 36.

[2] Emerson, S. R. & Hedges, J. I. (2008). Chemical Oceanography and the Marine Carbon Cycle. Cambridge University Press: New York, NY.

[3] Keeling, R. F., Piper, S. C. & Heimann, M. (1996). Global and hemispheric CO2 sinks deduced from changes in atmospheric O2 concentration. Nature, 381, 218 – 221.

[4] Schimel, D. S. et al. (2001). Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems. Nature, 414, 169 – 172.

[5] Emerson, S. R. & Hedges, J. I. (2008). Chemical Oceanography and the Marine Carbon Cycle. Cambridge University Press: New York, NY.

[6] Running, S. W. (2008). Ecosystem Disturbance, Carbon and Climate. Science, 321, 652 – 653.

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