Climate change is usually measured in degrees, centimeters of sea-level rise and extreme weather events. But new research suggests the changing atmosphere is also reaching into something far more personal: human blood.
In a study published in the journal Air Quality, Atmosphere and Health, scientists from The Kids Research Institute Australia, Curtin University and The Australian National University (ANU) examined more than 20 years of U.S. population health data. Using blood test results from roughly 7,000 people sampled at two-year intervals between 1999 and 2020 in the National Health and Nutrition Examination Survey (NHANES), they found persistent shifts in blood chemistry that closely tracked the upward trend in atmospheric CO2.
Since 1999, average serum bicarbonate levels — a blood marker closely associated with carbon dioxide in the body — have risen by about 7 percent. Over the same period, average calcium and phosphorus levels decreased. These biological trends unfolded while atmospheric CO2 climbed from about 369 ppm in 2000 to more than 420 ppm today.
The mechanism is straightforward: as CO2 increases, the body can retain additional bicarbonate to keep blood pH stable. That response preserves a delicate balance, but maintaining it over decades could have physiological consequences. "If current trends continue, modeling indicates average bicarbonate levels could approach the upper limit of today's accepted healthy range within 50 years," said study author Associate Professor Alexander Larcombe. "Calcium and phosphorus levels could also reach the lower end of their healthy ranges later this century."
Humans evolved when atmospheric CO2 was roughly 280 to 300 ppm. In the past decade, atmospheric levels have risen by an average of about 2.6 ppm per year — and 2024 alone saw an increase of 3.5 ppm. Co-author Dr Phil Bierwirth, a retired environmental geoscientist, cautioned that the study does not establish direct cause and effect, but the consistency across a large population deserves attention. "I actually think that what we are seeing is because our bodies are not adapting," he said. "It appears we are adapted to a range of CO2 in the air that may now have been surpassed... Maybe we can never adapt, such that it is vitally important to limit atmospheric levels of CO2."
The findings may matter most for children and teenagers, whose developing bodies face the greatest lifetime exposure to elevated CO2. The researchers argue that rising CO2 should be treated not only as an environmental concern but as a long-term public health factor, and they recommend monitoring atmospheric composition alongside biological markers as part of future climate policy.
