An Approach for Assessing Human Respiration CO₂ Emissions Using Radiocarbon Measurements and Bottom-Up Data Sets

Carbon dioxide (CO₂) is a major greenhouse gas in the atmosphere and has large impacts on climate change. Its fossil fuel (CO_2ff) and biogenic (CO_2bio) sources are well investigated, while CO₂ emissions from human respiration (CO_2hr), a subset of CO_2bio, have received less attention. Especially as a source of carbon emissions in densely populated megacities, the role of CO_2hr emissions in the carbon cycles was largely neglected. Here we fully characterize the respiratory CO₂ emission rates (CERs) of Chinese people for the first time. Using the example of the megacity Beijing in China, we estimate the CO_2hr emissions and present a method for quantifying its fraction in the atmospheric CO₂ based on radiocarbon (¹⁴C) measurements and inventory data sets. The results show that males and females have similar age trends in CERs, but the gender difference is significant, especially between the ages of 20 and 60, the average CERs was 33% higher for males than for females (P < 0.05). The CO_2hr emissions were about 22.2 ± 0.6 kt CO₂ per day, which was equivalent to 7.5% of daily CO_2ff emissions in winter. The proportion is likely to be twice in summer due to the seasonal fluctuations of fossil fuel emissions. More importantly, the respiratory emissions could increase atmospheric CO₂ concentration by about 2 ppm, accounting for 14% ± 6% of average CO_2bio concentration in winter. This study highlights the importance of human respiration in carbon emissions in megacities and has implications for a better understanding of the regional carbon budget.