finding 1.2 : key-message-1-2

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).

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

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 Changes in global average temperature over the last century are based on the Goddard Institute for Space Studies surface temperature analysis (GISTEMP,; Hansen et al., 2017).

New information and remaining 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: 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.

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

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