The Modern Carbon Cycle

The contemporary global carbon (C) cycling involves the exchanges of C within and between the atmosphere, the oceans, and biosphere. The C may be transferred from one reservoir to another in seconds (e.g., the fixation of atmospheric carbon dioxide (CO2) by photosynthesis) or over millennia [e.g., the accumulation of fossil carbon (coal, oil, gas) through deposition and diagenesis of organic matter (OM)]. The focus of this chapter is on the exchange of CO2 occurring over the scale of months to a few centuries that are important for the cycling of C over years to decades with the focus on human influence starting from Industrial Era (1750). The cycling of C is important because it approximates the flows of energy around the Earth. The increased use of fossil fuels has led to increase in atmospheric concentration of CO2 and methane (CH4), which are the two most important greenhouse gases (GHGs). Addition of GHGs to the atmosphere enhances the greenhouse effect and is the main cause of the global warming. The rate and extent of the warming depend, in part, on changes in global C cycle. The processes responsible for adding C to, and withdrawing it from, the atmosphere are the part of the global C cycling. Some of the processes that add C to the atmosphere such as the combustion of fossil fuels and changes in land use and land management are under direct human control. Similarly human beings can control removal of CO2 through afforestation and/or reforestation as well as restoration of degraded lands . Others, such as the accumulation of carbon in the oceans or on land as a result of changes in global climate are not under direct human control except through controlling rates of greenhouse gas (GHG) emissions and therefore, climatic change. Because CO2 is more important GHG, and is expected to continue to be in the future, understanding the global C cycle is a vital part of managing the global climate. This chapter will address, first, the natural flows of C on the Earth, then the anthropogenic sources of C to the atmosphere and the sinks of carbon on land and in the oceans that have kept the atmospheric accumulation of CO2 lower than it would otherwise have been. Since 1750, the atmospheric concentration of CO2 has increased by ~44% from 278 ± 5 ppm in 1750 to 400.0 ± 0.1 ppm in 2015, corresponding to atmospheric burden of 260 ± 5 Pg C, largely as a result of fossil fuel combustion, but also from changes in land use and management. At the beginning of Industrial Revolution, the emissions of CO2 were from land use and land use change; now the emissions are largely (~90%) from fossil fuels. The decadal annual rates of fossil fuel CO2 emissions increased from 3.1 ± 0.2 Pg C yr−1 in 1960s to 9.3 ± 0.5 Pg C yr−1 for 2006–2015, while land use CO2 emission decreased from 1.5 ± 0.5 Pg C yr−1 to 1.0 ± 0.5 Pg yr−1 over the same period. The total global anthropogenic CO2 emission from 1750 to 2015 is estimated at 600 ± 70 Pg C, of which, fossil fuels and cement production is estimated at 410 ± 20 Pg C and land use change emission at 190 ± 65 Pg C. About 43% of the total anthropogenic CO2 emission or 260 ± 5 Pg C remained in the atmosphere, while ocean and terrestrial ecosystems sinks were 28 and 27%, respectively. The decadal atmospheric CO2 growth increased from 1.7 ± 0.1 Pg C yr−1 in the 1960s to 4.5 ± 0.1 Pg C yr−1 during 2006–2015, with ocean and terrestrial sinks increasing roughly in line with atmospheric increase over the last 50 years. Although there is no clear signal globally of a saturation of land sink strength, there are some indications suggesting that the ocean total CO2 uptake rate may have declined in recent decades.