E8 Global Carbon Cycle and Trace Gases

Topic

The global carbon cycle describes the movement of carbon among four main reservoirs: the atmosphere, containing approximately 870 gigatonnes of carbon; the ocean, containing about 38,000 gigatonnes; the terrestrial biosphere, containing around 2,600 gigatonnes; and the geological lithosphere, containing over 60 million gigatonnes. Anthropogenic perturbation—with current emissions approaching 10 gigatonnes of carbon per year—is shifting this global carbon cycle out of the equilibrium it maintained throughout the Holocene, the period spanning the last few millennia during which the climate system remained relatively stable.

The dynamic model of this global carbon cycle couples three distinct processes: the chemistry of dissolved carbon dioxide in the ocean, which equilibrates with various forms of dissolved inorganic carbon in seawater; terrestrial ecosystem photosynthesis and respiration, which remove carbon dioxide from and release it into the atmosphere depending on the season and the amount of active vegetation; and atmospheric methane oxidation, which converts methane into carbon dioxide over time. Together, these three coupled processes determine the amount of carbon moving between the atmosphere, the ocean, and the terrestrial biosphere at any given time.

Of the total anthropogenic perturbation emitted annually, the "airborne fraction" measures the proportion of those emissions that remains in the atmosphere rather than being absorbed by another reservoir in the global carbon cycle; this airborne fraction is approximately 44 percent, meaning that the global carbon cycle's natural sinks—the ocean and the terrestrial biosphere—absorb the remaining 56 percent of the anthropogenic perturbation emitted each year. However, the capacity of these natural sinks to continue absorbing that same proportion of the anthropogenic perturbation may become saturated: if the natural sinks of the global carbon cycle reach saturation, an airborne fraction greater than the current 44 percent would remain permanently in the atmosphere for the same level of emissions.

Two carbon-climate feedbacks are particularly crucial to the global carbon cycle. The first involves permafrost, which holds nearly 1,700 gigatonnes of accumulated carbon: if permafrost thaws, it releases additional carbon dioxide and methane into the atmosphere; this additional carbon dioxide and methane further warm the climate system, which in turn thaws more permafrost, creating a positive feedback loop between carbon released from permafrost and the warming of the climate system. The second carbon-climate feedback occurs in the ocean: the ocean has become more acidic—experiencing a drop of one-tenth of a unit on the acidity scale since pre-industrial times—and this increased acidity can weaken the biological carbon pump, the mechanism by which marine organisms transport organic carbon from the ocean surface to the depths; if the biological carbon pump weakens, the ocean removes less carbon from the atmosphere than it previously did, leaving an even larger airborne fraction in the atmosphere.

Both the permafrost feedback and ocean acidification operate in the same direction: they drive the global carbon cycle further away from the Holocene equilibrium with which this description began, doing so precisely by reducing the capacity of natural sinks to absorb the anthropogenic perturbation, rather than leaving that absorption capacity intact. Thus, the global carbon cycle—which describes the movement of carbon among the atmosphere, the ocean, the terrestrial biosphere, and the geological lithosphere—ultimately responds to anthropogenic perturbation not only through the direct transfer of carbon into the atmosphere but also through feedbacks that amplify that very transfer over time.

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