Atmospheric carbon dioxide trends over geological time
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Geological Time Scale of Atmospheric Carbon Dioxide Trends
Atmospheric carbon dioxide (CO₂) levels have changed dramatically over Earth’s geological history, closely linked to shifts in climate, tectonic activity, and biological evolution 24510.
Phanerozoic CO₂ Fluctuations: 570 Million Years to Present
Over the last 570 million years (the Phanerozoic Eon), CO₂ levels were generally high during the early Paleozoic and Mesozoic eras, sometimes exceeding several thousand parts per million (ppm). These high levels corresponded with warm “greenhouse” climates. In contrast, CO₂ dropped during the Permo-Carboniferous and late Cenozoic, aligning with cooler “icehouse” periods and the expansion of polar ice 45610.
Cenozoic Era: Declining CO₂ and Climate Cooling
During the Cenozoic (the last 66 million years), CO₂ concentrations were much higher than today, peaking around 1600 ppm about 51 million years ago. This period was marked by a much warmer Earth, with no permanent ice on Antarctica or Greenland 12. Around 34 million years ago, CO₂ dropped to about 720 ppm, coinciding with the onset of Antarctic glaciation. By 32 million years ago, levels fell further to 550 ppm, and have generally remained below this threshold since, supporting the development of modern grasslands and deserts 12.
Miocene to Present: Stabilization and Recent Increases
From the early Miocene (about 24 million years ago), atmospheric CO₂ stabilized below 500 ppm, with occasional short-term reductions during rapid cooling events around 15 and 3 million years ago 127. For the last 800,000 years, ice core records show CO₂ fluctuated between 180–280 ppm, with lower values during glacial maxima and higher values during interglacials 389. Since the Industrial Revolution, human activity has driven CO₂ from pre-industrial levels of about 280 ppm to over 400 ppm today, exceeding any natural levels seen in at least the last 23 million years 237.
Mechanisms Driving Long-Term CO₂ Changes
Long-term CO₂ trends are controlled by a balance between volcanic and metamorphic outgassing, silicate weathering, and carbon burial. Weathering of silicate rocks acts as a negative feedback, stabilizing atmospheric CO₂ over millions of years 148. Plate tectonics, seafloor spreading, and subduction cycles also drive periodic fluctuations in CO₂, with evidence for 26-million-year cycles linked to oceanic crustal processes .
Short-Term Variability and Abrupt CO₂ Releases
On shorter timescales, abrupt CO₂ increases have occurred during both glacial and interglacial periods, often linked to disruptions in ocean circulation and rapid climate changes 89. These events, while brief compared to geological trends, can have significant impacts on global climate.
Conclusion
Atmospheric CO₂ has varied widely over geological time, with high levels during warm “greenhouse” periods and lower levels during “icehouse” intervals. The long-term decline in CO₂ over the Cenozoic has been crucial for the development of modern climates and ecosystems. Today’s rapid, human-driven increase in CO₂ is unprecedented in at least the last 23 million years, highlighting the importance of understanding these long-term trends for predicting future climate change 27.
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