If allowed to soar unchecked, greenhouse-gas emissions will interfere with people’s ability to think.

Carbon dioxide released by the burning of fossil fuels isn’t just bad for the planet — it’s also bad for your brain.

Scientists know that humans’ cognition declines as carbon dioxide levels rise in the air they breathe. Kristopher Karnauskas at the University of Colorado Boulder and his colleagues analysed how indoor CO 2 levels might increase under various emissions scenarios.

If greenhouse gases continue to rise unabated, atmospheric CO 2 levels will increase from roughly 410 parts per million to 930 parts per million by 2100. Indoor CO 2 levels track outdoor CO 2 levels, which means that in a typical school classroom, where people are also breathing out CO 2 , the level could reach 1,400 parts per million, the team concluded.

Extrapolation from other studies suggests that, at that level, scores on a test of basic decision-making could be 25% lower than they are today, and scores on a test that assesses more complicated strategic thinking could be 50% lower. Better building ventilation would not help much, because outdoor CO 2 levels would be so high.

Cutting greenhouse-gas emissions is the best way to stave off this slow-thinking future.