Understanding the carbon cycle is a key part of understanding the broader climate change issue. But a number of misconceptions floating around the blogosphere confuse basic concepts to argue that climate change is irrelevant because of the short residence time of carbon molecules in the atmosphere and the large overall carbon stock in the environment.
It turns out that while much of the “pulse” of extra CO2 accumulating in the atmosphere would be absorbed over the next century if emissions miraculously were to end today, about 20 percent of that CO2 would remain for at least tens of thousands of years.
The complex global carbon cycle process involves carbon absorption and release by the atmosphere, oceans, soils, and organic matter, and also emissions from anthropogenic fossil fuel combustion and land-use changes. The figure below shows the best estimate of annual carbon fluxes from main sources and sinks.
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|Figure from Oak Ridge National Laboratories (Units in gigatons of carbon).|
At first glance, it may seem that the narrow black arrows representing anthropogenic sources are relatively insignificant, making up only a few percent of the total carbon released to the atmosphere in any given year. To understand why anthropogenic emissions are of concern, it is important to think of the carbon cycle as a balance of sorts; every year around 230 gigatons of carbon dioxide are released to the atmosphere, and around 230 gigatons of carbon dioxide are absorbed by the world’s oceans and biosphere. This balance forms an equilibrium of sorts, with the level of atmospheric carbon dioxide remaining largely unchanged over time. However, anthropogenic emissions throw this process out of kilter, adding a new source of emissions unmatched by additional sinks.
The carbon dioxide record over the past 10,000 years demonstrates this situation: the modern period exhibits a large spike in atmospheric carbon dioxide coincident to the time humans started burning fossil fuels.
|Atmospheric CO2 concentrations over the past 10,000 years. From the IPCC AR4 WG1 SPM.|
Graphing emissions over the modern period against changes in atmospheric concentrations illustrates a clear relationship between emissions and increasing CO2 concentrations.
It is important to note that not all anthropogenic emissions are accumulating in the atmosphere. Indeed, about half of annual CO2 emissions are absorbed by the ocean and vegetation, and this percentage of absorption, called the airborne fraction, is currently the subject of vigorous debate over whether or not it is changing over time. Scientists can model the absorption of anthropogenic carbon by year for different sinks.
|Image from the Global Carbon Project.|
Determining the residence time of carbon dioxide in the atmosphere is a rather complex problem. A common misconception arises from simply looking at the annual carbon flux and the atmospheric stock; after all, with 230 gigatons absorbed by the oceans and land every year, and a total atmospheric stock of 720 gigatons, one might expect the average molecule of CO2 to remain in the atmosphere for only three to four years.
Such an approach poorly frames the issue, however. It is not the residence time of an individual molecule that is relevant. What really matters is just how long it will take for the stock of anthropogenic carbon emissions that has accumulated in the atmosphere to be reabsorbed.
The simplest way to approximate the time it will take to reabsorb the anthropogenic flux is to calculate how long it would take for the atmosphere to revert to preindustrial levels of 280 parts per million if humans could cease emissions immediately. If the current net sink of around 4 gigatons of carbon per year remained constant over time, it would take about 50 years for the atmosphere to return to 280 ppm. However, there is no reason to think that these sinks would remain constant as emissions decrease. Indeed, it is more realistic to anticipate that the net sink would shrink in proportion to the decrease in emissions.
Scientists can approach this problem in a number of different ways. They can use models of carbon sink behavior based on their best knowledge of the physics of ocean carbon absorption and the biosphere. They can also use records of changes in atmospheric carbon dioxide during glacial periods in the distant past to estimate the time it takes for perturbations to settle out.
Using a combination of various methods, researchers have estimated that about 50 percent of the net anthropogenic pulse would be absorbed in the first 50 years, and about 70 percent in the first 100 years. Absorption by sinks slows dramatically after that, with an additional 10 percent or so being removed after 300 years and the remaining 20 percent lasting tens if not hundreds of thousands of years before being removed.
As University of Washington scientist David Archer explains, this “long tail” of absorption means that the mean lifetime of the pulse attributable to anthropogenic emissions is around 30,000 to 35,000 years.
|Figure via Global Warming Art.|
So while a good portion of warming attributable to carbon and other greenhouse gas emissions would be removed from the atmosphere in a few decades if emissions were somehow ceased immediately, about 10 percent will continue warming Earth for eons to come. This 10 percent is significant, because even a small increase in atmospheric greenhouse gases can have a large impact on things like ice sheets and sea level if it persists over the millennia.