Earth's climate has fluctuated between cold and warm periods for millions of years. During the so-called "lukewarm interglacials"—warm phases between 800,000 and 430,000 years ago—atmospheric CO2 concentrations were only around 240 to 260 ppm (parts per million, i.e., molecules per 1 million molecules of air). Later interglacials reached values of 280 to 300 ppm.