Thank you for the acurate quote. Do they go on to say WHY it is unlikely that it is any force but human-influenced?
The main way this has been done is to model the global climate over the past 100-150 years, and to use only natural (volcanic and solar) forcings in some model runs, and to then include the anthropogenic factors (greenhouse gases and land use change) in other model runs. The only runs that successfully reproduce the general pattern of observed changes are those which include the anthropogenic effects. Natural forcings are too limited in their effect. This is briefly alluded to in the text but not described in the text.
There may be comments that the solar influence has been underestimated due to potential effects on cloud cover. The mechanism for this has not been adequately established and experimentation is ongoing. The current state of knowledge does not indicate that the solar influence is responsible for most of the currently observed warming, but 30-50% of the warming early in the 20th century is attributed to increased solar activity. Note that the study period did not include the early part of the 20th century.