For the first time, a large-scale population study has traced the rising carbon dioxide in Earth's atmosphere into the bloodstream itself, finding that Americans' serum bicarbonate levels have climbed seven percent since 1999 — almost in lockstep with CO₂ rising from 369 to over 420 parts per million. Conducted by researchers across three Australian universities and drawing on two decades of U.S. health data, the work raises a question that reframes the climate conversation: the atmosphere has not merely warmed around us, but may be quietly reshaping the chemistry within us. The human body, ev