How do we know humanity is causing climate change?
I think we have to distinguish between THE cause and A cause, or even the primary cause. People can spout off about this study or that research and that may have some value but too often it is the product of somebody's politics rather than objective truth. [Money talks very loudly.] There are too many natural causes that shift in time in terms of cause and effect, and much of that cannot be accurately measured. Clearly there have been stretches of time where the various measurements used to describe climate change have risen and fallen, sometimes significantly and unexplainably. I do not believe there is any doubt that mankind has polluted the air and water and that did/does have an effect, but to what extent relative to natural causes remains unknown.
Which does not mean we should do nothing about it, but the programs and policies we adopt ought to be smart and cost-effective without undue negative impact on the people who can afford it least. So much of the climate change politics is based on money, who gets it vs how much good it'll do. It has become a hot-button political issue where no one can really know how much difference a given policy or program will make. Instead, we are rewarding political donors and supporters with huge sums that have not produced results that were worthy of the expenditures. These days truth is in the eye of the beholder, and it is very elusive.