Andrew Cull

CO₂ is just one small piece in a planetary-scale jigsaw puzzle, assembling itself over millions of years. Ultimately, the world's climates are determined by the agonisingly slow tectonic dance of the continental and oceanic crustal plates. They set the long-term patterns for the shape of the continents, the seaways, future ocean circulation and cloud albedo, while solar irradiance and orbital cycles are the trigger points. Atmospheric CO₂ and global surface temperature are not locked together either, a story revealed in the geological record. Far from causing a mass collapse, past high-carbon epochs created the richest expansions of biosphere diversity in Earth’s history. During the early Paleozoic era (538 to 251 million years ago), starting with the Cambrian explosion, atmospheric CO₂ levels were often 10 to 15 times higher than today. During these evolutionary peaks, atmospheric CO₂ was not measured in hundreds of parts per million, but in the thousands. Changes did not happen over a few decades then either—they took tens of thousands of years. A modest rise in CO₂ levels going into the modern industrial era is not a sign of future runaway warming; it is a bounce-back during a far longer global decline in atmospheric CO₂. This brief pulse has stimulated global vegetation growth and leaf area expansion, verified by NASA satellite observations. Our current geological epoch—the Quaternary—sits near historical low levels for atmospheric CO₂ concentrations, a severely cold epoch that has locked the world in a 2.58-million-year ice age regime. This cold spell continues today. During the Last Glacial Maximum 26,000 to 20,000 years ago, ice sheets miles deep blanketed North America, northern Europe, and Siberia. CO₂ fell to approximately 180 ppm, flirting dangerously with the 150 ppm threshold where land plants cease photosynthetic activity. The glaciers began to melt and retreat rapidly around 20,000 to 15,000 years ago, with most ice disappearing from major landmasses by 10,000 years ago. Even then, sea levels rose by only 1.3 cm a year for around 9,000 years, an invisible pace of change. By contrast, today's CO₂ hovers around 425 ppm, rising and falling through the years with the natural rhythms of the seasons. When we look at Earth’s full story, CO₂ does not pose a danger to climate, but is an essential building block for the biological world. IMAGE: What 540 million years of geological history says: Willis Eschenbach.