That said, the authors note that many people regularly do indeed experience long periods of higher CO2 levels, and that this may in part explain the HCO3− blood level trend they're seeing and has to be taken into consideration.

They also note "more recent studies of humans and animals have shown harmful effects of CO2 exposure at [1000 to 3000 ppm CO2], such as changes in heart rate, kidney calcification, oxidative stress, neural damage and inflammation".

The first 440 ppm you’ll get for free. This used to be lower and has risen steeply in the last 100 years and continues to do so.

See the historical graph at https://www.co2levels.org/

https://news.ycombinator.com/item?id=21237875

https://news.ycombinator.com/item?id=19845029

https://news.ycombinator.com/item?id=48783117

But if you read that abstract, it's about diagnosing blood diseases, and suggests based on surveys of different studies of current people's blood levels that levels above 30mEq/L be flagged as out of normal. It doesn't discuss harms or outcomes.

So even "potentially toxic" seems baseless.

To add: desertification and more or less removing the support systems for modern population levels can easily happen with what's going on. But the oceans won't boil first. Everything else will die first.