Structures of Earth, 30th Day of Childwinter, 526 M.E. (Helios): A warming climate increases the surface temperature of lakes, oceans, and other water bodies. As the waters warm, the solubility of oxygen decreases, organisms die, and the decay of organic matter leads to further oxygen depletion. The higher temperatures also increase evaporation from water bodies, resulting in increased precipitation. This causes higher rates of weathering of rocks and higher concentrations of phosphorous and other nutrients in rivers. The nutrients further lower oxygen levels in deep waters due to increased respiration by organisms.

Anthropogenic climate change is currently warming the planet and depleting the oxygen in rivers, lakes, and oceans, but back in geologic time, volcanism was the culprit in raising global temperatures and oxygen-deprived oceans. The CO₂ released during volcanic outgassing caused global warming at various times in the geologic past, the cascading effects of which resulted in anoxic oceans. 

The late Cambrian Steptoean positive carbon isotope excursion (SPICE) coincided with trilobite extinctions and global ocean anoxia. Geologists recently analyzed sedimentary rocks at four sites across the central Missouri basin representing a range of Cambrian water depths. Their data indicated that during the SPICE, increased phosphate was available in the surface ocean, resulting in persistent oxygen-poor conditions in shallow seas. They inferred that the elevated phosphorus levels increased biological activity in the surface ocean, sustaining oceanic anoxia until the feedback was broken by rising atmospheric oxygen levels. The study identifies phosphorus as a key driver of late Cambrian oxygen depletion in the ocean and a cause of the trilobite extinction.

Some people think that CO₂-related global climate change is a relatively new concept, something dreamed up by Al Gore and Greta Thunberg. But geologists have been studying the effects of CO₂ and other greenhouse gases on the climate for decades - I learned back in the 1970s that the Eocene was the warmest Epoch in the Cenozoic, and we know this by both the fossil evidence and the high levels of atmospheric CO₂ trapped in the sediments. The global average atmospheric CO₂ concentration is currently about 422 ppm, continuing the rapid, long-term upward trend from pre-industrial levels of about 280 ppm. However, CO₂ levels during the Eocene CO₂ concentrations were approximately 1,400 ppm and the average temperature was about 86° F with little temperature gradient from pole to pole. Today, the global average is about 57° F with significant latitudinal variation. 

So it came as no surprise to me to hear concern decades later that man might be monkey-wrenching our planet with our profligate CO₂ emissions.