A study drawing on more than two decades of US population health data has found that blood bicarbonate CO2 rise trends are tracking atmospheric carbon dioxide concentrations with an unsettling consistency, prompting researchers to call for bicarbonate to be treated as a long-term public health indicator alongside established climate metrics.
The research, published in Air Quality, Atmosphere and Health, was conducted by scientists from The Kids Research Institute Australia, Curtin University and The Australian National University (ANU). They examined blood chemistry data from the US National Health and Nutrition Examination Survey (NHANES), covering roughly 7,000 people tested at two-year intervals between 1999 and 2020.
What the blood data actually shows
The headline finding is a roughly 7% increase in average serum bicarbonate levels since 1999. According to ScienceAlert, the average blood concentration of bicarbonate rose from 23.8 to 25.3 milliequivalents per litre, an increase of about 0.34% per year across the study period. Over the same two decades, average calcium and phosphorus levels declined. Atmospheric CO₂, meanwhile, climbed from about 369 parts per million (ppm) in 2000 to more than 420 ppm today.
Bicarbonate is a key marker for the body’s acid-base balance. As CO₂ rises, the body can retain additional bicarbonate to keep blood pH stable. The researchers argue that the parallelism between the atmospheric and biological trends is too consistent across a large population to dismiss.
Associate Professor Alexander Larcombe, the study’s lead author, said the results indicate that the body may already be adjusting to changes in atmospheric composition. ‘What we’re seeing is a gradual shift in blood chemistry that mirrors the rise in atmospheric carbon dioxide, which is driving climate change,’ he said.
His modelling suggests the consequences could accumulate over coming decades. ‘If current trends continue, modelling indicates average bicarbonate levels could approach the upper limit of today’s accepted healthy range within 50 years,’ A/Prof Larcombe said. Calcium and phosphorus levels, he added, could reach the lower end of their healthy ranges later this century.
A complication the study cannot resolve
The research does not establish a direct cause-and-effect relationship, and the NHANES dataset introduces a complication that the authors acknowledge. As The Conversation has reported, participants in the NHANES study likely spend most of their time indoors, where CO₂ concentrations often exceed 1,000 ppm in poorly ventilated spaces. That is far above current outdoor atmospheric levels of around 420 ppm. Whether it is the outdoor atmospheric trend, rising indoor concentrations, or both that are driving the blood chemistry shift remains an open question the study cannot settle.
Fellow author Dr Phil Bierwirth, a retired environmental geoscientist affiliated with the ANU Emeritus Faculty, said the correlation across such a large and diverse population is itself the reason for concern. ‘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.’
Dr Bierwirth elaborated: ‘The normal range maintains a delicate balance between how much CO₂ is in the air, our blood pH, our breathing rate and bicarbonate levels in the blood. As CO₂ in the air is now higher than humans have ever experienced, it appears to be building up in our bodies. Maybe we can never adapt such that it is vitally important to limit atmospheric levels of CO₂.’
Humans evolved when atmospheric CO₂ concentrations were approximately 280 to 300 ppm. During the past decade, atmospheric levels rose by an average of about 2.6 ppm each year, and 2024 alone saw an increase of 3.5 ppm.
Blood bicarbonate CO2 rise and what it means for climate policy
The researchers argue that the potential physiological effects of rising CO₂ should be considered in future climate policy discussions, alongside its established environmental consequences. A/Prof Larcombe was careful to avoid alarmism. ‘We’re not saying people are suddenly going to become unwell when we cross a certain threshold,’ he said. ‘But this suggests there may be gradual physiological changes occurring at a population level, and that’s something we should be monitoring as part of future climate change policy.’
The study recommends tracking atmospheric composition alongside biological markers across populations. The researchers also note that the findings may be particularly relevant for children and teenagers, whose bodies are still developing and who face the greatest lifetime exposure to elevated atmospheric CO₂.
A/Prof Larcombe is part of the Wal-yan Respiratory Research Centre, a partnership between The Kids Research Institute Australia, Perth Children’s Hospital and Perth Children’s Hospital Foundation. The study appears in Air Quality, Atmosphere and Health, volume 19, issue 3, 2026.
