Chapter 1, Key Finding 2

In 2011, the total global anthropogenic radiative forcing resulting from major anthropogenic green- house gases (GHGs, 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 Administra- tion’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 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)