We know very well now how much CO2 fossil fuel burning is putting into the atmosphere. We also know the rate at which atmospheric CO2 is taken up by the ocean and eventually turned into carbonate minerals. Everything else is dwarfed by those two flows. We know the excess ends up in the atmosphere, and we know the greenhouse effect of the CO2 concentration in the atmosphere.
So what is there to doubt? Anybody denying man-made global warming at this point just does not want to acknowledge the truth, for political reasons. Climate science is science. Climate change denial is politics. Politics that is going to make it very difficult for our grandchildren to forgive us.