September is usually a month when the atmospheric concentrations of carbon dioxide (CO2) are at their lowest levels in the northern hemisphere. That’s because plants suck up a lot of the gas as they grow during the summer. But this year, the level of the greenhouse gas has remained stubbornly above the symbolic “red line” of 400 parts per million. This has caused scientists to predict that CO2 levels will not return to environment-friendly levels "ever again for the indefinite future.”