You said " extent and impacts of man's contributions are unknown ". That is not true.
I refer you to this.
https://climate.nasa.gov/scientific-consensus/
and this chart.
https://www.google.com/imgres?imgre...&docid=zqWwkPyOkkIR-M&h=405&w=540#h=405&w=540
CO2 is the earth's most important greenhouse gas and we have raised it's level by around 40% since the start of the industrial revolution by extracting carbon fuels and burning them.
Virtually all of the warming over the last two hundred years has been due to man. It is already becoming catastrophic and this is just the start.
And frankly, to contend that the opinion of literally every climate expert on earth is not significant is absurd. Yes, the validity of science is established by consensus of the experts and peer review by them. The argument saying otherwise is often repeated but always wrong.
BTW, climatologists are by and large physicists, mathematicians and computer experts. It isn't like the fuzzy sciences like psychology.