finding 1.1 : key-message-1-1

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).

This finding is from chapter 1 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

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 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).

New information and remaining 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: Observations clearly show substantial increases in greenhouse gas (GHG) concentrations since preindustrial times resulting from anthropogenic GHG emissions and land-use change.

This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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