There is a study (<a href="https://href.li/?https://ehp.niehs.nih.gov/doi/10.1289/ehp.1104789" rel="nofollow">https://href.li/?https://ehp.niehs.nih.gov/doi/10.1289/ehp.1...</a>), conducted in a double-blind manner on 22 volunteers, which seems to demonstrate a decrease in some cognitive abilities, particularly decision-making, above a concentration of 600 ppm of CO2. For this reason, it is recommended to ventilate workspaces in order to bring carbon dioxide levels back to acceptable levels present in the external atmosphere, which is currently at 420 ppm.
Unlike pollution from fine dust and toxic substances, CO2 has a very convenient property: the value is almost uniform worldwide, with slight variations depending on the area in which the measurement is taken. For this reason, the reference value is taken at the top of Mauna Kea, in the Hawaiian Islands.
This IPCC graph (<a href="https://href.li/?https://www.ipcc-data.org/observ/ddc_co2.html" rel="nofollow">https://href.li/?https://www.ipcc-data.org/observ/ddc_co2.ht...</a>) shows predictions, with various optimistic and pessimistic scenarios, of the increase in CO2, linked to human activity, starting from the 313 ppm recorded in 1958 at Mauna Kea. I have only added an orange line at 600 ppm to highlight the very close moment when there will likely be too much CO2 everywhere to ensure optimal functioning of our brains.
We can reasonably assume that finding a solution to reduce carbon dioxide emissions is a significant intellectual effort, since we have not yet been able to find one.
This means that from 2055 onwards, if we have not already found a solution to reduce carbon dioxide emissions and if we have not already started to reverse course, we may no longer be able to make the cognitive effort necessary to solve the emissions problem. And gradually, we will not even be able to solve simpler problems.
Life on Earth will probably continue, since it has endured much higher concentrations of carbon dioxide. But our planet will probably no longer be compatible with human intelligence, which has evolved over the last couple of million years at lower CO2 concentrations. And it will not have time to adapt, precisely because of the extraordinary speed with which we are absurdly continuing to emit carbon dioxide into the atmosphere.
It is difficult to prove that cognitive problems are not already occurring.