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The data showing the increase in CO2 is from human activities is about as rock solid as anything can be.  I take it as a fact.  I can imagine an alternative explanation for the data, meaning the isotope ratios.

The relative enrichment of C12 in the atmosphere and depletion of C14 indicates an old source of carbon enriched in C12.  Fossil fuels are the best explaination unless some other C12 enrichment process besides photosynthesis is discovered.  But that does not prove mankind is responsible for the increase of CO2 in the atmosphere. 

The planet has been warming up since the end of the mini ice age around 1840.  As the ocean warms the solubility of CO2 decreases and the partial pressure of CO2 in the air increases. Perhaps the freshest CO2 in the air is from burning fossil fuel but in the absence of that the CO2 conc. would be the same?  In other words, mankind may be responsible for the C12 enrichment but not the increase.  The concentration of CO2 has been higher in the distant past.  It has also been lower and has increased before mankind was around in significant numbers and burning stuff.

The relevant data isn't only the diminishing C12/C13 ratio or the increasing total atmospheric [CO2], it's these *and* the observation that both trends begin around 1850. Total abundance and isotope changes correlate just as tightly with one another as they do with onset of the industrial revolution.

Also, while it's important to continually seek new, alternative explanations, let's not forget to pay attention to the probabilities those alternatives are correct.

But that does not prove mankind is responsible for the increase of CO2 in the atmosphere.

It's ok to not be convinced of something as long as you could conceivably change your mind if the right data came along later. If you were an oceanographer/geologist/whichever in charge of leading a study on this, what's something you could conceivably discover that would reverse your opinion?

Basically I'm curious about your threshold for believing something. Separately, I'm curious whether your threshold on this is similar to your threshold for believing other things.

I find it more interesting to ponder what could be done about it in any practicable sense.

My answer thus far is "Very little".

If "we" expend significant assets at curtailing CO2, that likely means fewer assets for curtailing "normal" problems.

My approach would be to curtail coal burning for power as expediently as possible.  I'm not sure if that needs funding assistance, loan guarantees, what, but it would be useful overall to reduce coal to the extent possible.

The US has done a pretty good job on air quality, much better than Europe has.

https://www.thelocal.fr/20180810/paris-pollution-like-smoking-183-cigarettes-a-year

https://www.numbeo.com/pollution/compare_cities.jsp?country1=France&city1=Paris&country2=United+States&city2=Los+Angeles%2C+CA

Europe made a shift to favoring Diesel fuel to combat CO2 production and in the bargain ended up with much higher "normal" pollutants.

https://cleantechnica.com/2017/08/11/air-pollution-ranking-32-cities-measure/

My understanding was that the ozone layer is now better sealed over the poles because ozone is a gas and has wafted around sufficiently over the decades, but that this sealing effect comes at the cost of the entire layer being thinner on average.