I don't find it horrifying. Not that long ago, CO2 ppm was twice what it is today. Before that, it was twice that. If we could travel through time, we could drop you in that world and you'd be fine, other than the fact that the world would be an incredibly dense jungle. History shows that we're not particularly close to any dangerously high amount of CO2; we were much closer to a dangerous low amount, since plant respiration fails at around 150 ppm, which would probably mean the extinction of most if not all life on the planet. That's for certain; on the other hand, nobody's able to point to anything particularly bad happening at 400 ppm; the earth has already endured far higher amounts with no problem. Being worried about CO2 is a tenet of a modern religious faith.
Well humans would suffer with 840ppm: e.g. Seppänen etal. 1999. Association of Ventilation Rates and CO2-Concentrations with Health and other Responses in Commercial and Institutional Buildings. Indoor Air 9: 226-252. Discuss sick building syndrome effects fell sharply <800ppm. Publ. CO2 levels are outside; within buildings even now rates >1000 common, as ambient rises so do within-building rates.
Even at our current levels eCO2 is harming our health: Rising CO2, Climate Change, and Public Health: Exploring the Links to Plant Biology https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2649213/ these are direct CO2 effects on plant physiology leading to negative human outcomes.
eCO2 has dozens of negative effect on natural (and engineered systems including concrete chemistry) systems (inc. us). There are no positives that I've seen yet.
Carbon dioxide concentrations dropped from 4,000 parts per million during the Cambrian period about 500 million years ago to as low as 180 parts per million during the Quaternary glaciation of the last two million years:
https://en.m.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_...
I would argue 500 million years ago is not ”not so long ago”. The Cambrian period also didn’t have more than a microbial soil crust living outside of oceans.
