Declining oxygen level as an emerging concern to global cities

Rising CO₂ concentration and temperatures in urban areas are now well-known, but the potential of an emerging oxygen crisis in the world's large cities has so far attracted little attention from the science community. Here, we investigated the oxygen balance and its related risks in 391 global large cities (with a population of more than 1 million people) using the oxygen index (O_I), which is the ratio of oxygen consumption to oxygen production. Our results show that the global urban areas, occupying only 3.8% of the global land surface, accounted for 39% (14.3 ± 1.5 Gt/yr) of the global terrestrial oxygen consumption during 2001−2015. We estimated that 75% of cities with a population more than 5 million had an O_I of greater than 100. Also, cities with larger O_I values were correlated with more frequent heatwaves and severe water withdrawals. In addition, cities with excessively large O_I values would likely experience severe hypoxia in extremely calm weather. Thus, mitigation measures should be adopted to reduce the urban O_I in order to build healthier and more sustainable cities.