|Fig. 1: Diagram demonstrating exchange of carbon between global sources and sinks (Gt/Year). (Courtesy of NASA)|
Concern over climate change permeates many geopolitical domains. In general, this is a result of the many negative implications associated with a dangerously fluctuating climate; politically, economically, and even ethically.
Though the consequences of a warming Earth climate are quite well understood, the actual state of the climate, and the trends of it's many parameters, such as temperature, are not as concrete. This uncertainty is inherent to climate; it originates in the nature of its definition. WMO defines the climate as "the statistical description of the weather in terms of the mean and variability of relevant quantities over periods of several decades".  In other words, knowing everything about the weather for an entire season tells us nothing about the climate, because we need enough data to make statistical observations.
Greenhouse gases represent a widely known, statistically valid, trigger for change in certain climate parameters. Though most would agree that anthropogenic atmospheric carbon dioxide and the global average temperature are positively correlated, it is unlikely that as many can describe the nature of that correlation. In order to better understand climate change in a quantitatively valid way, we must be able to describe the mechanics of processes that have a statistically valid impact on it. To study the effects of anthropogenic carbon dioxide on climate change, one must address what is called the global carbon cycle.
As it turns out, the relationship between anthropogenic carbon and global warming effects is not direct. Carbon in various forms is exchanged between stores, which often behave as both carbon 'sinks' and carbon 'sources', resulting in a complicated network of carbon interactions. All of the carbon cycle connections must be understood in order to intelligently answer crucial questions, such as "Where is the atmospheric Carbon coming from, and how much of it is our fault?"
Since the dawn of the industrial era, Earth's atmospheric carbon dioxide content has increased by up to 38%, currently up to about 385 parts per million (ppm).  Clearly, it can be inferred that there is a connection between human activity, namely fossil fuel burning, and atmospheric carbon dioxide concentrations.
However, it would be ignorant to assume that all anthropogenic atmosphere produced simply diffuses through the atmosphere and stays there. The Earth is, in fact, quite good at managing it's atmospheric carbon content. According to the Congressional Research Service, the carbon dioxide content of the atmosphere has remained relatively constant, at about 280 ppm, for most of human history.  The Earth passively regulates carbon dioxide in the atmosphere through the carbon cycle, which consists of a network 'sinks' and 'sources', constantly exchanging carbon in different forms.
Sources: Carbon dioxide enters the atmosphere through natural or human processes called sources. Two significant examples of carbon sources are (1) plant respiration, which includes plant decay and consumption by other organisms, and (2) interactions between Earth's atmosphere and oceans, the latter of which contains tremendous stores of dissolved carbon dioxide.  The most infamous sources of carbon are, of course, those of human origin. These include all forms of burning, from deforestation to fossil fuel consumption, in addition to cement production. Relative to natural carbon sources, the human contribution is actually very small. For example, Earth's biosphere naturally releases around 120 gigatons (Gt) of carbon dioxide every year, in comparison to the total human contribution of only around 9 Gt per year.  The 9 Gt annual of anthropogenic carbon dioxide emissions is significantly produced through the burning of fossil fuels. In fact, according to calculations based solely on the annual coal, oil, and natural gas consumption rates globally provided by the BP Statistical Review, approximately 3.2 Gt of carbon dioxide are released every year through the burning of fossil fuels.  Here, Human energy consumption is behaving as a catalyst for the exchange of carbon between the geosphere and the atmosphere.
Sinks: On the other hand, there are the carbon sinks, which as the name suggests, act as collectors of atmospheric carbon. Sinks are often also sources, inferring the cyclic nature of global carbon flux. Earth's oceans and biosphere, the largest sources of atmospheric carbon, are also the largest sinks of carbon. The biosphere, through the process of photosynthesis, consumes over 120 Gt of carbon dioxide every year, which implies a net carbon dioxide flux of roughly 2 Gt per year.  The Ocean also acts as a very good carbon sink, removing a net 2 Gt of carbon dioxide from the atmosphere every year.  All in all, Earth's natural sinks are so effective that they remove around 55% of anthropogenic carbon dioxide from the atmosphere, reducing the net human impact from close to 9 Gt to about 4 Gt annually.
Even with the Earth's incredibly efficient natural sinks, the relatively tiny human contribution the carbon sources is enough to tip the balance. This 4 Gt net annual atmospheric carbon increase contributes roughly 60% to the 2.6 Watts per square-meter of human-induced green-house gas warming.  On one hand, we can see that the human contribution to the carbon cycle is minuscule in comparison to tremendous exchanges between Earth's oceans, biosphere, geosphere, and atmosphere, but magnitudes are irrelevant in understanding the problem. Here fluxes, or net exchanges, are king, and the numbers are telling us that net fluxes are increasing alarmingly as a result of the human perturbation.
What's perhaps even more alarming than the current magnitude of the annual net carbon flux is the rate at which it's increasing. The immediate issues are urbanization, development, and population increase. Urban areas contribute around 70% of global carbon dioxide emissions, and the urban population is expected to grow from its current level of about 50% to 70% by the year 2050.  It is well documented that population and GDP are highly correlated with carbon dioxide emission rates.  In the years to come, the human population will significantly increase, more will move into urban environments, and more will be able to afford personal modes of transportation. In short, there is no question that natural carbon dioxide sinks will be pushed to their limits moving forward, because anthropogenic carbon dioxide will increase for many years to come.
The issues associated with increasing atmospheric carbon content extend further than the human contribution. As the climate becomes warmer and land use increases, many large natural carbon stores, such as high latitude and tropical peatlands, will become vulnerable. Deforestation, whether natural or human-caused, acts as negative sink as well as a source of carbon, because the potential for carbon dioxide respiration is removed. In addition to the biosphere's worsening ability to act as a carbon sink, the sink efficiency of the Earth's oceans is also dropping.  This is a result of the oceans' sink effectiveness being a function of atmospheric carbon content. The problem is worsens when one considers the longevity of carbon dioxide in the atmosphere. Atmospheric carbon has an especially long residence time; roughly 20%-35% of today's emissions will remain in the atmosphere for several centuries.
There must be no doubt that anthropogenic carbon dioxide is increasing at rate with which the Earth's natural carbon sinks can not compete.
This conclusion is not obvious, however, unless one approaches the issue of increasing atmospheric carbon dioxide content from a carbon cycle accounting perspective. That is, until all of the natural and human sources, stores, and sinks are considered, as well as their effects on one another, the total anthropogenic human contribution is a quite useless value.
Once we add up all the contributions, we find not only that there is a net increase in the atmospheric carbon content every year, but that the rate of that increase is rising as well. Decreasing carbon sink efficiency, high carbon residency times, and increasing human population and urbanization, are largely to blame for this effect, and none of them are simple issues to address. Ultimately, it is still difficult to say, quantitatively, how the climate will adjust to increasing atmospheric carbon dioxide flux, but the fact remains: humanity is perturbing Earth's carbon cycle in dramatic ways.
© Julian Girard. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. Houghton et al., eds., "An Introduction to Simple Climate Models Used in the IPCC Second Assessment Report," Intergovernmental Panel on Climate Change, February 1997, pp. 9-13.
 J. G. Canadell et al., "The Human Perturbation of the Carbon Cycle," United Nations (UNESCO), November 2009.
 "Statistical Review of World Energy 2015," British Petroleum, June 2015.