text: pCO, pCO, or is the partial pressure of carbon dioxide (CO), often used in reference to blood but also used in meteorology, climate science, oceanography, and limnology to describe the fractional pressure of CO as a function of its concentration in gas or dissolved phases.   The units of pCO are mmHg, atm, torr, Pa, or any other standard unit of atmospheric pressure.  The pCO of Earth's atmosphere has risen from approximately 280 ppm (parts-per-million) to a mean 2019 value of 409.8 ppm as a result of anthropogenic release of carbon dioxide from fossil fuel burning.  This is the highest atmospheric concentration to have existed on Earth for at least the last 800,000 years.