Carbon dioxide levels today are higher than at any point in at least the past 800,000 years. In fact, the last time the atmospheric CO2 amounts were this high was more than 3 million years ago, when temperature was 2°- 3°C higher than during the pre-industrial era, and sea level was 15–25 meters higher than today.
Carbon dioxide is a greenhouse gas: a gas that absorbs heat. Without this natural greenhouse gas, Earth’s average annual temperature would be below freezing instead of close to 60°F. But increases in greenhouse gases have tipped the Earth’s energy budget out of balance, trapping additional heat and raising Earth’s average temperature.
If global energy demand continues to grow and to be met mostly with fossil fuels, atmospheric carbon dioxide will likely exceed 900 ppm by the end of this century.