Q:  is the observed warming over the last 50-100 years due to natural climate variability, human influence, or both?

•“…. natural forcing alone is unlikely to explain the recent observed global warming or the observed changes in vertical temperatures structure of the atmosphere.”

•“In light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.”

Figure and quotes from Technical Summary from IPCC Working Group I