Today, however, it’s the direct input of carbon dioxide into the atmosphere from burning fossil fuels that’s responsible for changing Earth’s atmospheric composition over the last century, rather than climate feedbacks from the ocean or land caused by Milankovitch cycles.
Since the beginning of the Industrial Age, the concentration of carbon dioxide in Earth’s atmosphere has increased 50 percent, from about 280 ppm to 412 ppm (update: 421 ppm in 2023).
Scientists know with a high degree of certainty this carbon dioxide is primarily due to human activities because carbon produced by burning fossil fuels leaves a distinct “fingerprint” that instruments can measure. Since 1850, Earth’s global average temperature has increased by over 1 degree Celsius (1.8 degrees Fahrenheit). Furthermore, recent scientific assessments show that Earth is expected to warm another half a degree Celsius (almost a degree Fahrenheit) as soon as 2030.
This relatively rapid warming of our climate due to human activities is happening in addition to the very slow changes to climate caused by Milankovitch cycles. Climate models indicate any forcing of Earth’s climate due to Milankovitch cycles is overwhelmed when human activities cause the concentration of carbon dioxide in Earth’s atmosphere to exceed about 350 ppm.
