I'm not a scientist, and neither are 99.?% of the climate deniers. So.... my credentials are no weaker than theirs.
Recently I wrote not all carbon is equal. Scientist can take samples of CO2 and determine if it came from fossil fuel, or a dead rat. Dead rat carbon is what you want to breath, not coal or car gas carbon.
First, we don't make CO2 breathing, my stupid business associates are always saying stop breathing if you want to stop global warming, then they slap their knee. We take it in when we inhale, then out, it's the same, we didn't filter or add to it. OK, got that.
14C is a radioactive isotope of carbon. It is only found in carbon which has been sheltered from galactic rays, below the surface. Carbon that has been cycling around the earth in and out of trees, oceans, grass, rats, that carbon has had the 14C hammered out of it by "the force" or some magic guy in the sky.
So, next time your uncle says they can't prove CO2 comes from human activity, think of C14, and every tailpipe smokestack and kiln is spewing it out. That is pollution. That adds to greenhouse gases, and increases a little tiny teenzey bit the atomic poisons in the sky. The tree that falls and rots, that CO2 is not pollution, it was here before, and absent C14. When they drill down for ice samples, or amber, or mud samples from a thousand years ago, it's all good CO2. Out there on the tundra, the sea, and in balloon 5 miles high know that there is a geek (or his mechanism) with a tiny little butterfly net scooping up CO2, and most of it has our fingerprint on it.
Mauna-Loa Observatory, where the first 400 ppm of CO2 was recorded last week. Temperature follows CO2. CO2, Methane, water vapor and other greenhouse gasses let sun rays through but reflect back infrared rays. This is good, this is why we have the goldielocks atmosphere. But as they increase they make the atmosphere more dense, slowing the heat loss to space, rising the temperatures. Stephen Hawking says we can end up with some regions on earth reaching nearly 200° and a very acidic atmosphere. Oh, what does he know, he never played football.