- Lead Authors:
- Lori Bruhwiler, NOAA Earth System Research Laboratory
- Anna M. Michalak, Carnegie Institution for Science and Stanford University
- Contributing Authors:
- Richard Birdsey, Woods Hole Research Center
- Joshua B. Fisher, NASA Jet Propulsion Laboratory and California Institute of Technology
- Richard A. Houghton, Woods Hole Research Center
- Deborah N. Huntzinger, Northern Arizona University
- John B. Miller, NOAA Earth System Research Laboratory
<b>Bruhwiler</b>, L., A. M. <b>Michalak</b>, R. Birdsey, J. B. Fisher, R. A. Houghton, D. N. Huntzinger, and J. B. Miller, 2018: Chapter 1: Overview of the global carbon cycle. 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. 42-70, https://doi.org/10.7930/SOCCR2.2018.Ch1.
Overview of the Global Carbon Cycle
SUPPORTING EVIDENCE
KEY FINDINGS
Key Finding 1
Atmospheric carbon dioxide (CO2) has increased from a preindustrial abundance of 280 parts per million (ppm) of dry air to over 400 ppm in recent years—an increase of over 40%. As of July 2017, global average CO2 was 406 ppm. Methane (CH4) has increased from a preindustrial abundance of about 700 parts per billion (ppb) of dry air to more than 1,850 ppb as of 2017—an increase of over 160%. The current understanding of the sources and sinks of atmospheric carbon supports the dominant role of human activities, especially fossil fuel combustion, in the rapid rise of atmospheric carbon (very high confidence).
Description of evidence base
Preindustrial concentrations of CO2, CH4, and other trace species are known from measurements of air trapped in ice cores and firn from Greenland and Antarctica (e.g., MacFarling Meure et al., 2006). These measurements show that preindustrial levels of CO2 and CH4 were 280 ppm and 800 ppb, respectively. Contemporary global measurements of CO2 and CH4 are archived and documented at esrl.noaa.gov/gmd/ccgg/trends/global.html. Estimates of cumulative carbon emissions, along with atmospheric observations and estimates of net uptake by ocean or land, show that human emissions dominate the observed increase of CO2 (Tans 2009). Analyses of “bottom-up” estimates of the CH4 budget and atmospheric observations also support a strong role for anthropogenic emissions in the contemporary atmospheric CH4 budget (Saunois et al., 2016).
Major uncertainties
There is a high degree of confidence in the overall increases in CO2 and CH4 since the preindustrial era. Attribution of these increases to anthropogenic emissions or natural emissions is subject to uncertainty (e.g., Saunois et al., 2016; Tans 2009). However, these uncertainties are unlikely to change the central conclusion that anthropogenic emissions have caused the significant increases in CO2 and CH4 since preindustrial times.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Observations clearly show substantial increases in greenhouse gas (GHG) concentrations since preindustrial times resulting from anthropogenic GHG emissions and land-use change.
Summary sentence or paragraph that integrates the above information
For Key Finding 1, there is very high confidence that CO2 and CH4 have increased by over 40% and 160%, respectively, since preindustrial times and that this increase is due to anthropogenic emissions. Uncertainties in natural exchanges among the atmosphere, ocean, and terrestrial biosphere and in anthropogenic emissions are unlikely to change the latter conclusion.
Key Finding 2
In 2011, the total global anthropogenic radiative forcing resulting from major anthropogenic greenhouse gases (not including anthropogenic aerosols) relative to the year 1750 was higher by 2.8 watts per meter squared (W/m2). As of 2017, the National Oceanic and Atmospheric Administration’s Annual Greenhouse Gas Index estimates anthropogenic radiative forcing at 3.1 W/m2, an increase of about 11% since 2011. In 2017, CO2 accounted for 2.0 W/m2 and CH4 accounted for 0.5 W/m2 of the rise since 1750. The global temperature increase in 2016 relative to the 1880 to 1920 average was over +1.25°C, although this warming was partially boosted by the 2015–2016 El Niño. Global temperature, excluding short-term variability, now exceeds +1°C relative to the 1880–1920 mean in response to this increased radiative forcing (Hansen et al., 2017; very high confidence).
Description of evidence base
Global anthropogenic radiative forcing was extensively reviewed in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) (Myhre et al., 2013). The change in radiative forcing since 2011 and the contributions from CO2 and CH4 are based on global observations of radiatively active trace species and computed using empirical expressions derived from atmospheric radiative transfer models. Details are available at esrl.noaa.gov/gmd/aggi/aggi.html. Changes in global average temperature over the last century are based on the Goddard Institute for Space Studies surface temperature analysis (GISTEMP, data.giss.nasa.gov/gistemp; Hansen et al., 2017).
Major uncertainties
The uncertainty of radiative forcing calculations is about 10% (Myhre et al., 2013), including uncertainty of the atmospheric radiative transfer model and the global abundance of trace species. Uncertainty of global average temperature trends is determined by the distribution, type, and length of surface observation sites. The effects of these factors are discussed extensively by Hartmann et al. (2013) and also by Hansen et al. (2010, 2017).
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Observations and models clearly demonstrate that radiative forcing has increased substantially since preindustrial times and that this increase is ongoing, resulting primarily from the observed increase in atmospheric GHG concentrations.
Summary sentence or paragraph that integrates the above information
For Key Finding 2, there is very high confidence in the value of global anthropogenic radiative forcing (2.8 W/m2) and the fact that CO2 accounts for the largest share of anthropogenic forcing, with CH4 accounting for half the remainder. There is very high confidence that this increased radiative forcing has led to global average temperature increases since the preindustrial era.
Key Finding 3
Global fossil fuel emissions of CO2 increased at a rate of about 4% per year from 2000 to 2013, when the rate of increase declined to about 2% per year. In 2014, the growth in global fossil fuel emissions further declined to only 1% per year (Olivier et al., 2016). During 2014, the global economy grew by 3%, implying that global emissions became slightly more uncoupled from economic growth, likely a result of greater efficiency and more reliance on less carbon intensive natural gas and renewable energy sources. Emissions were flat in 2015 and 2016 but increased again in 2017 by an estimated 2.0% (high confidence).
Description of evidence base
Quantification of global fossil fuel emissions relies mainly on energy consumption data collected by multiple international organizations such as the International Energy Agency (IEA), the Carbon Dioxide Information Analysis Center (CDIAC), the United Nations (UN), and the Energy Information Administration (EIA). UN energy statistics are used to estimate the amount of CO2 released by gas flaring, and production statistics are used to quantify emissions from cement production. More details on estimation of global fossil fuel emissions are given by Le Quéré et al. (2016) and Ciais et al. (2013).
Major uncertainties
Uncertainty of global fossil fuel emissions is approximately 5% when expressed as a standard deviation (Le Quéré et al., 2016). This assessment of uncertainties includes the amounts of fuel consumed, the carbon and heat contents of fuels, and the combustion efficiency. Although typically considered as constant in time, the uncertainty expressed as a percentage of total emissions is in reality growing in time, as a higher fraction of total emissions come from emerging economies and developing countries with less sophisticated accounting (Le Quéré et al., 2016; Marland et al., 2009). The majority of the uncertainty is likely to be in the form of systematic errors for individual countries, resulting from biases inherent to their energy statistics and accounting methods (Le Quéré et al., 2016).
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Energy consumption data clearly show that global fossil fuel emissions have grown over the past decades, with only slight decreases in certain individual years.
Summary sentence or paragraph that integrates the above information
For Key Finding 3, there is high confidence that fossil fuel emissions increased at a rate of 4% per year, until recently when they began to slow even as the U.S. economy grew. The slowing of emissions occurred even as the global economy was growing, implying greater reliance on lower carbon–emitting energy sources.
Key Finding 4
Net CO2 uptake by land and ocean removes about half of annually emitted CO2 from the atmosphere, helping to keep concentrations much lower than would be expected if all emitted CO2 remained in the atmosphere. The most recent estimates of net removal by the land, which accounts for inland water emissions of about 1 petagram of carbon (Pg C) per year, indicate that an average of 3.0 ± 0.8 Pg C per year were removed from the atmosphere between 2007 and 2016. Removal by the ocean for the same period was 2.4 ± 0.5 Pg C per year. Unlike CO2, CH4 has an atmospheric chemical sink that nearly balances total global emissions and gives it an atmospheric lifetime of about 9 to 10 years. The magnitude of future land and ocean carbon sinks is uncertain because the responses of the carbon cycle to future changes in climate are uncertain. The sinks may be increased by mitigation activities such as afforestation or improved cropping practices, or they may be decreased by natural and anthropogenic disturbances (high confidence).
Description of evidence base
Using observations of CO2 accumulation in the atmosphere and statistics on fossil fuel and cement production, the total uptake of carbon by the terrestrial ecosystem and the ocean can be resolved as residual. Inland waters are implicitly included in the terrestrial component through this process. The partitioning of the residual uptake between land and ocean is more complicated and requires the use of upscaled quantities such as partial pressure of CO2 (pCO2) measurements in seawater or measurements of atmosphere-land biosphere fluxes to understand contemporary fluxes and their variability. Among these two major sinks, the oceanic sink generally is understood to be better constrained by independent observations. In terms of interannual variability, substantial uncertainty remains for both oceanic and terrestrial sinks. In terms of the cumulative sink, cumulative oceanic uptake is best constrained by interior data for the ocean (e.g., Khatiwala et al., 2009, 2013), while the cumulative land uptake typically is understood as the difference between cumulative emissions and the estimated cumulative oceanic sink. In addition to the more direct data-based constraints, models of oceanic circulation often are used with pCO2 measurements to estimate oceanic fluxes, and inverse modeling techniques also are used to estimate carbon uptake by global land and ocean. Inverse modeling combines information from atmospheric observations, atmospheric transport models, and best-available estimates of carbon fluxes from land and ocean via models and observations. Recent synthesis studies by Le Quéré et al. (2016 and 2017) overview the recent carbon budget. Future uptake by land and ocean is estimated using models of the terrestrial and oceanic carbon cycle coupled to climate simulations (e.g., Friedlingstein et al., 2014).
Major uncertainties
The partitioning of carbon fluxes between land and ocean has significant uncertainty resulting from sparse observational coverage of atmospheric concentration and fluxes. Models of ocean-land carbon exchange must be evaluated against observations of carbon fluxes and storage in ecosystems, but in general there is not enough global coverage. Similarly, large regions that are important for understanding the global carbon budget, such as the tropics and Siberia, are not covered by atmospheric observations. This lack of observational coverage makes accurate estimates of the partition of carbon uptake between global land and ocean difficult to achieve using inverse modeling. Uncertainties in atmospheric transport models add to the problem of sparse observational coverage. Increased observational coverage offered by space-based instruments may improve the situation in the future, assuming technical limitations can be understood and overcome. The future evolution of the carbon cycle, including climate–carbon cycle feedbacks, is highly uncertain (e.g., Friedlingstein et al., 2014), and the use of inverse techniques to understand the carbon budget over recent decades could help to improve simulations of the future carbon budget. Future carbon cycle–climate feedbacks are expected to be positive (Ciais et al., 2013).
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Observations and models clearly demonstrate that about half of annually emitted CO2 is absorbed by the terrestrial biosphere and by oceans. However, the exact partitioning between the land and ocean sinks is somewhat uncertain, while projections of the future of this uptake are highly uncertain.
Summary sentence or paragraph that integrates the above information
For Key Finding 4, there is very high confidence that the land and ocean are absorbing a significant amount of carbon emitted by fossil fuel use. The partitioning of this uptake between the land and ocean is more uncertain. The future evolution of the global carbon cycle is also uncertain.
Key Finding 5
Estimates of the global average temperature response to emissions range from +0.7 to +2.4°C per 1,000 Pg C using an ensemble of climate models, temperature observations, and cumulative emissions (Gillett et al., 2013). The Intergovernmental Panel on Climate Change (IPCC 2013) estimated that to have a 67% chance of limiting the warming to less than 2°C since 1861 to 1880 will require cumulative emissions from all anthropogenic sources to stay below about 1,000 Pg C since that period, meaning that only 221 Pg C equivalent can be emitted from 2017 forward. Current annual global CO2 emissions from fossil fuel combustion and cement production are 10.7 Pg C per year, so this limit could be reached in less than 20 years. This simple estimate, however, has many uncertainties and does not include carbon cycle–climate feedbacks (medium confidence). These conclusions are consistent with the findings of the recent Climate Science Special Report (USGCRP 2017).
Description of evidence base
Cumulative carbon emissions are quantified for Key Finding 5 using energy consumption statistics as described for Key Finding 3. The cumulative emissions required for staying below 2°C are estimated using climate models.
Major uncertainties
There is a range of plausible responses of global temperature to carbon emissions as a result of uncertainty in climate models, especially modeling cloud, aerosol, and carbon cycle feedbacks. In particular, the range of climate model sensitivity to a doubling of CO2 is 1.5 to 4.5°C, suggesting uncertainty in the amount of cumulative carbon emissions that cannot be exceeded to stay below a global temperature increase of no more than 2°C. In addition, some potential carbon cycle–climate feedbacks, such as the effect of carbon emissions from permafrost thaw, are highly uncertain and may significantly lower the cumulative amount of carbon that can be emitted before the 2°C global temperature increase limit is exceeded.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Based on climate models, temperature observations, and inventories of cumulative GHG emissions, it is clear these emissions have resulted in the observed global temperature increase. However, there remains some uncertainty about the exact temperature response to future emissions due to uncertainty about climate feedbacks.
Summary sentence or paragraph that integrates the above information
For Key Finding 5, carbon emissions would have to be slowed and reduced within a few decades to avoid a high probability of global temperature increases that exceed 2°C. Over half the cumulative emissions allowable for a 67% chance to stay below 2°C may already have been emitted, and current emissions rates suggest that emitting the remainder may take as little as 20 to 40 years. There is a medium degree of confidence in the remaining emissions available to keep temperature increases below a given level.
See Full Chapter & References
