Question i): Roles of external influences
Newly available observed data for the last two centuries were analyzed individually and interpreted, based on observations only and in conjunction with coupled model simulations, to arrive at estimates of the contribution to temperature change by greenhouse gas increases, other anthropogenic influences (including aerosols and land use change), solar forcing, volcanism, and internal climate variability.
Preindustrial periods show considerable variability, yet if considered may make the Paris target slightly more ambitious (by 0-0.2 degrees additional warming prior to the late 19th century; Schurer et al., Nature Climate Change 2017). We also showed in collaboration with the Bern group that the warming trends following the very cold early 19th century was driven by the recovery from two large volcanic eruptions, and that delayed response to this also triggered the glacier advance to about 1850 followed by retreat. (Broennimann et al., Nature Geosc. 2019). The year without a summer would have been almost impossible without volcanic forcing in temperature, and the wet European summer may also have been influenced by circulation change caused by the eruptions (Schurer et al., ERL 2019).