But Milankovitch cycles can’t explain all climate change that’s occurred over the past 2.5 million years or so. And more importantly, they cannot account for the current period of rapid warming Earth has experienced since the pre-Industrial period (the period between 1850 and 1900), and particularly since the mid-20th century. Scientists are confident Earth’s recent warming is primarily due to human activities — specifically, the direct input of carbon dioxide into Earth’s atmosphere from burning fossil fuels.So how do we know Milankovitch cycles aren’t to blame?First, Milankovitch cycles operate on long time scales, ranging from tens of thousands to hundreds of thousands of years. In contrast, Earth’s current warming has taken place over time scales of decades to centuries. Over the last 150 years, Milankovitch cycles have not changed the amount of solar energy absorbed by Earth very much. In fact, NASA satellite observations show that over the last 40 years, solar radiation has actually decreased somewhat.Second, Milankovitch cycles are just one factor that may contribute to climate change, both past and present. Even for Ice Age cycles, changes in the extent of ice sheets and atmospheric carbon dioxide have played important roles in driving the degree of temperature fluctuations over the last several million years.The extent of ice sheets, for example, affects how much of the Sun’s incoming energy is reflected back to space, and in turn, Earth’s temperature.Then there’s carbon dioxide. During past glacial cycles, the concentration of carbon dioxide in our atmosphere fluctuated from about 180 parts per million (ppm) to 280 ppm as part of Milankovitch cycle-driven changes to Earth’s climate. These fluctuations provided an important feedback to the total change in Earth’s climate that took place during those cycles.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.
