Rising CO₂ levels are reflected in human blood. Scientists don't know what it means

Humans evolved in an atmosphere containing roughly 200–300 parts per million (ppm) of carbon dioxide (CO₂). Today, that figure sits above 420 ppm, higher than at any point in the history of our species.

source https://medicalxpress.com/news/2026-03-human-blood-scientists-dont.html