Well we know enough about the climate and how it works to be 100% sure that climate change is caused by greenhouse gases. Like in the human body, we don't know everything about climate change (e.g. when exactly some tipping points will be triggered etc.). But the relationship between CO2 and global warming is as basic as the fact that you will gain weight when you eat more calories than your body burns. Just because we don't know everything to the smallest detail doesn't mean that we know nothing at all.