Atmospheric CO₂ doesn't just warm the climate. It enters people's blood.
Larcombe and colleagues (2026, Air Quality, Atmosphere & Health) analyzed blood chemistry data from the U.S. National Health and Nutrition Examination Survey — approximately 7,000 people sampled every two years from 1999 to 2020, spanning 21 years. Serum bicarbonate, a direct marker of dissolved CO₂ in the blood, rose by approximately 7% over that period. The trajectory tracks atmospheric CO₂ with uncomfortable precision: from 369 ppm in 2000 to over 420 ppm today.
This is not a health crisis. Not yet. The current bicarbonate levels are well within the normal range. But the normal range exists because human physiology evolved in an atmosphere where CO₂ was between 180 and 280 ppm for the entirety of the genus Homo. We are now at 420 ppm and climbing. The 7% shift in blood chemistry is the body doing exactly what it should — compensating for changing input through acid-base buffering. The question is whether compensation has limits.
If current trends continue, the modeling suggests average serum bicarbonate could approach the upper boundary of the accepted healthy range within 50 years. “Approach the upper boundary” is measured clinical language for a trajectory that, extended far enough, takes the average person's blood chemistry outside the envelope where the body works well. Not everyone at once, not catastrophically, but as a population-wide drift toward a boundary that wasn't supposed to move.
Children born today face a lifetime of exposure to an atmosphere measurably different from what human respiratory and renal systems evolved to regulate. A child born in 2025 will reach middle age around 2070, by which point atmospheric CO₂ could exceed 500 ppm under moderate emissions scenarios. Their blood chemistry will have been drifting upward since birth, compensating continuously for an atmosphere their physiology wasn't designed for.
The finding is notable for what it doesn't require. It doesn't require believing any particular climate model. It doesn't require projecting sea level rise or temperature change. It takes observed atmospheric CO₂ — measured, not modeled — and observed blood chemistry — measured in real people, not simulated — and shows that one is tracking the other across two decades of data. The mechanism is high school chemistry: CO₂ dissolves in blood, forming carbonic acid, which the kidneys buffer by retaining bicarbonate. More atmospheric CO₂, more dissolved CO₂, more bicarbonate. The body is adapting in real time.
This makes climate change personal in a way that polar ice or coral reefs don't. Those are distant systems, visible in photographs and charts. Serum bicarbonate is the CO₂ already inside you. Every blood draw already contains the data. The 7% shift has been accumulating in clinical databases worldwide, unnoticed because each individual's value falls within the normal range. The signal is in the population trend, not any single measurement. No one's doctor has flagged it because no one's result is abnormal — yet.
The healthy range for serum bicarbonate is itself a product of the atmosphere we evolved in. As atmospheric CO₂ rises and blood chemistry drifts, the range will either be redefined (acknowledging the new normal) or will start catching people whose compensation is less effective. The elderly, those with chronic kidney disease, people with respiratory conditions that limit CO₂ exhalation — these populations have less buffering capacity. They will reach the boundary first. Not because they are sick in any new way, but because the atmosphere is making demands their physiology was never asked to meet.