Attribution and the Hiatus

Temperatures on Earth are rising at the surface and especially for the oceans. But how is it known that this is due to human activities? The main way this question has been addressed in most places is to use climate modeling, where a climate model can be run with and without various forcings and the results compared with observations. In these models, the increase in carbon dioxide is generally assumed. But how is it known that the increased carbon dioxide comes from human activities?