Mass Extinctions and Their Relationship With Atmospheric Carbon Dioxide Concentration: Implications for Earth's Future

Industrialization has raised the concentration of carbon dioxide (CO2) in Earth's atmosphere by half since 1770, posing a risk from ocean acidification to global biodiversity, including phytoplankton that synthesize approximately (similar to) 50% of planetary oxygen. This risk is estimated here from the fossil record and implications for our energy and economic future are explored. Over the last 534 million years (Myr), 50 extinction events present as peaks of genus loss-and-recovery cycles, each spanning similar to 3-40 Myr. Atmospheric CO2 concentration oscillates with percent genus loss, leading in phase by similar to 4 Myr and sharing harmonic periodicities at similar to 10, 26 and 63 Myr. Over the last 210 Myr, where data resolution is highest, biodiversity loss is correlated with atmospheric CO2 concentration, but not with long-term global temperature nor with marginal radiative forcing of temperature by atmospheric CO2. The end-Cretaceous extinction of the dinosaurs is anomalous, occurring during a 20-million year depression in atmospheric CO2 concentration and rising global temperature. Today's atmospheric CO2 concentration, similar to 421 parts per million by volume (ppmv), corresponds in the most recent marine fossil record to a biodiversity loss of 6.39%, implying that contemporary anthropogenic CO2 emissions are killing ocean life now. The United Nations Intergovernmental Panel on Climate Change projects that unabated fossil fuel use could elevate atmospheric CO2 concentration to 800 ppmv by 2100, approaching the 870 ppmv mean concentration of the last 19 natural extinction events. Reversing this first global anthropogenic mass extinction requires reducing net anthropogenic CO2 emissions to zero, optimally by 2% per year starting immediately. Plain Language Summary The rising concentration of carbon dioxide (CO2) in Earth's atmosphere from burning fossil fuels poses a risk to biodiversity from ocean acidification, threatening marine algae that produce similar to 50% of planetary oxygen. This risk is estimated here based on the relationship between marine biodiversity loss and atmospheric CO2 concentration in the fossil record. Biodiversity loss varies cyclically with atmospheric CO2 concentration on million-year timescales, but is not correlated with long-term global temperature nor with radiative forcing (RF) of temperature by CO2. Atmospheric CO2 is therefore a plausible cause of past mass extinctions, while long-term temperature change and RF by CO2 are excluded. Biodiversity and atmospheric CO2 cycle at periods similar to each other and to geological and astrophysical cycles, consistent with causal linkages. The concentration of CO2 in today's atmosphere corresponds to a decline in fossil biodiversity of 6.39%, implying that current human-induced emissions of CO2 are killing ocean life now. The United Nations Intergovernmental Panel on Climate Change projects that continuation of the global fossil fuel economy could raise atmospheric CO2 to concentrations approaching the average of past mass extinctions by the year 2100. Arresting this first human-induced global mass extinction requires eliminating net human-induced emissions of CO2 starting immediately.