During the glacial–interglacial cycles of the past million years, natural processes have varied CO2 by approximately 100 ppm (from 180 ppm to 280 ppm).
Durante os ciclos glaciais–interglaciais dos pasados millóns de anos, os procesos naturais fixeron variar o nivel de CO2 en aproximadamente 100 ppm (desde 180 ppm a 280 ppm).