Start throwing a bag of trash in your back yard every week and see how long people wait until they start saying your back yard is a dump. It seems people think that same idea doesn't hold true for the rest of our environment.
Those who claim man isn't a huge impact on our environment wish to deny the third law of physics. For every action there is an equal and opposite reaction.
I think it is better to be a bit quantitative:
Since 1850, around 400 trillion kilograms of fossil fuels have been burned. This is an easily verifiable number -- the oil and coal industries will gladly calculate it for you. We just count up all the coal and oil we have extracted and used.
https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide
The level of CO2 in the atmosphere has increased from about 300 ppm to 400 ppm. Again - this number is easy to calculate, any undergraduate chemistry lab has the capability to measure CO2 concentrations.
The total mass of the atmosphere is 5 x 10^18 kg. Again this is easy to measure because we know the pressure (15 psi) and temperature and composition of the atmosphere -- and as you learned in high school, PV=nRT.
So the addition of 100 parts per million of 5x10^18 kg = 500 trillion kilograms.
It's very clear that the CO2 emitted from oil and natural gas is a significant fraction of the total change in CO2 over the last 170 years. The contribution of humans simply cannot be negligible.Some notes about obvious simplifications, I made -- which don't matter but will cause people to complain:
1.) When you burn C to produce CO2, the CO2 weighs more than the fuel (because you add the O2 from the atmosphere. The exact ratio depends on the fuel (Coal and natural gas are different). But roughly, this is a factor of 2 or so -- thus 400 trillion tons of fuel produces about 800 trillion tons of CO2.
2.) The 400 ppm number is volumetric (i.e., counting the number of atoms of CO2 vs. atoms of O2 and atoms of N2). CO2 is heavier than either O2 or N2 -- so you should make an adjustment for this mass difference before multiplying 400 ppm * 5e18 kg. This is about a factor of 1.5 or so -- and it goes the other direction, and sort of cancels the factor of 2 or so from above.
3.) We only have really good CO2 measurements (at the same location in Hawaii) since about 1950. So maybe you can go back and calculate the numbers from 1950 until now, instead -- you will get the same result.