Birdsey, R., M. A. Mayes, P. Romero-Lankao, R. G. Najjar, S. C. Reed, N. Cavallaro, G. Shrestha, D. J. Hayes, L. Lorenzoni, A. Marsh, K. Tedesco, T. Wirth, and Z. Zhu, 2018: Executive summary. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M.A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 21-40, https://doi.org/10.7930/SOCCR2.2018.ES.
The carbon cycle is changing at a much faster pace than observed at any time in geological history (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide). These changes primarily are attributed to current energy and transportation dependencies on the burning of fossil fuels, which releases previously stable or sequestered carbon. Also contributing to rapid changes in the carbon cycle are cement production and gas flaring, as well as net emissions from forestry, agriculture, and other land uses. The associated rise in atmospheric GHGs is largely responsible for Earth’s increased temperature over the past 100 years. The global mean temperature in 2017 relative to the 1880 to 1920 average has increased by more than 1.25°C in response, as documented in the Climate Science Special Report (USGCRP 2017). Human-induced warming is having significant—usually negative—impacts including more frequent heatwaves, heavy precipitation, and coastal flooding, all of which lead to lost lives, damaged communities, and disrupted ecosystems.
Since SOCCR1, concentrations of atmospheric CO2 and CH4 have been on the rise (see Figure ES.4, this page). From 2007 to 2015, the global rate of increase averaged 2.0 ± 0.1 parts per million (ppm) per year for CO2 and 3.8 ± 0.5 parts per billion (ppb) per year for CH4 (see Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane). Current understanding of the sources and sinks of atmospheric carbon confirms the overwhelming role of human activities, especially fossil fuel combustion, in driving the atmospheric changes in CO2 concentrations (see Ch. 1: Overview of the Global Carbon). In North America, projections suggest that by 2040, total fossil fuel emissions, in terms of total carbon, will range from 1.5 petagrams of carbon (Pg C) to 1.8 Pg C per year, with the United States contributing 80% of this total. Compared to 2015 levels, these projections represent a range from a 12.8% decrease to a 3% increase in absolute emissions of carbon (see Ch. 19: Future of the North American Carbon Cycle).
Globally, land and ocean ecosystems are net sinks of atmospheric carbon, taking up more carbon annually than they release. The most recent estimates suggest that from 2006 to 2015, land ecosystems removed about 3.1 ± 0.9 Pg C per year while the ocean removed 2.3 ± 0.5 Pg C per year. Combined, these removals equal about half the amount of CO2 emitted from fossil fuel combustion and land-use change (see Ch. 1: Overview of the Global Carbon Cycle). However, a range of research suggests the carbon uptake capacity of all these systems may decline in the future, with some reservoirs switching from a net sink to a net source of carbon to the atmosphere.
Figure ES.4: Global Monthly Mean Atmospheric Methane (CH4) and Carbon Dioxide (CO2) Concentrations
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