From what I understand, the scientific consensus [1] is not that climate change is completely caused by human activity, for the very reason you mention: it’s complicated. :)
But like you also mention, the consensus is that we are we are at least partially causing it, even if it’s not “completely.” If we’re not causing 100% of the change, but perhaps 50%, I’m not sure if that’s much of a distinction in terms of what policy to change. Do you agree?
[1] https://climate.nasa.gov/scientific-consensus - “Climate-warming trends over the past century are extremely likely due to human activities.” It does not say or imply 100% due to those human activities. The ACS says, “The Earth’s climate is changing in response to increasing concentrations of greenhouse gases (GHGs) and particulate matter in the atmosphere, largely as the result of human activities.” Even there, it’s not saying completely, although it’s saying largely, which definitely has a distinction.