We have been able to take direct measurements of the sun's energy output using satelites for about thirty years now. In that time there has been no significant change in its output. There are 11 year solar cycles in which the energy output of the sun varies slightly, but the differences between total energy out put between the last three cycles has been extremely minor. Since the start of the industrial revolution human activity has been increasing the amount of several greenhouse gases in the atmosphere. The largest increase in the greenhouse effect has resulted from human activity increasing the amount of CO2 in the atmosphere by about a third through burning fossil fuels and landclearing. The human caused increase in the concentration of greenhouse gases is enough to account for the 0.74 +/- 0.18 degrees of warming the earth has experienced over the past 100 years. So to answer you question, we do know the cause of global warming and it is greenhouse gases. We cannot be 100% certain that they are responsible for all the warming, but their increase is sufficient to cause all the warming that has been experienced.