Proving that humans do have a significant impact on climate change is not possible yet because we have way too little data. The warming right now could as well not be caused by humans.
We can actually measure our contribution to climate change directly. So it is not only possible, but measurable.
You measure things that according to a THEORY change the climate. That theory is not proven, since it can't be proven yet, due to lacking data.
You can actually measure the energy imbalance directly...
