evidence: Atmospheric CO2 levels of between 280 and 350 parts per million created the climate that let humanity build and feed the modern world. The farther we get from those levels, the more we run the risk of disturbing that balance. According to NASA, the amount of carbon dioxide (CO2) in Earth’s atmosphere was about 416 parts per million (ppm) in April 2021.1 This level has been rising for 200 years—a worrying sign for the planet, since CO2 is a powerful heat-trapping greenhouse gas. Climate experts warn that humanity must drastically lower its CO2 emissions to avoid the most catastrophic consequences of climate change. But if we could choose a different level of CO2 in the air, what number would we pick? The first thing to know is that our species arose in a world with much less CO2, says Noelle Selin, Associate Professor in the MIT Institute for Data, Systems and Society and the Department of Earth, Atmospheric and Planetary Sciences. As humanity evolved over the past several hundred thousand years, atmospheric CO2 cycled between about 200 and 300 ppm. The preindustrial level of CO2—the amount in the air a few centuries ago, before humans began to burn CO2-producing fuels like coal and oil at an industrial scale—was about 280 ppm. Selin says a good argument could be made that 280 is the ideal level of CO2 for human life, since it creates temperature ranges that are comfortable for the human body and allowed civilization to grow. "The changes that we've seen since then just haven't happened on the timescale that you could evolve changes in humans."