There is no doubt for example that over the last 100 years or so human action has significantly increased the atmospheric concentrations of several gases  which are closely related to global temperature It seems likely that these increased concentrations which are set to continue building up in the near future are already affecting global climate but our poor knowledge and understanding of the global heat balance make the current and future situations uncertain What we do know is that atmospheric concentrations of greenhouse gases have fluctuated in close harmony with global temperatures over the past 40000 years