Stagnation tends to occur in areas where the temperature is fairly even, the atmosphere is stable and there is little rainfall. It is a serious issue for the climate because it allows ozone and particulate matter to accumulate near the Earth's surface.

Such pollution can cause respiratory infections, heart disease and lung cancer. According to the World Health Organization, urban air pollution leads to an estimated 1.3 million deaths worldwide each year. The problem is currently worst in eastern Europe and Russia, where more than one in every 2,500 deaths is attributable to air pollution.

Climate scientist Daniel Horton of Stanford University, US, together with colleagues from Stanford and Purdue University in Indiana, believes the problem will get worse over the next century. The researchers have quantified air stagnation in an ensemble of 16 climate simulations using the US National Climate Data Center air stagnation index (NCDC ASI), which rates the co-occurrence of light near-surface winds, light higher-altitude winds and no precipitation – key ingredients for air stagnation.

According to Horton and colleagues, climate simulations could be biased. To measure these biases, they compared the simulations of the late 20th century to observational data of wind speed and precipitation collected over the same period. They then removed the same biases from simulations of future climate, from which they deduced the NCDC ASI.

The researchers found that the highly populated and highly industrialized regions of the eastern US, the Mediterranean and eastern China could see between 12 and 25% more stagnant air days per year late this century compared with late last century. In general, mid-latitudes are most sensitive to stagnation, said Horton, because of a reduction in the pole-to-equator temperature gradient likely to be effected by climate change. This would lead to a weakened polar jet, and fewer storm systems.

Although an increase in air stagnation would likely lead to more deaths from air pollution, the researchers cannot give a quantitative estimate of the toll on human life. "There are many factors that will determine the impact of poor air quality on a population in the coming century, including that population's access to health care, their access to warnings of impending poor air quality, and their ability and willingness to regulate pollutants; in essence, their vulnerability," explained Horton. "The model simulations used in this study do not account for changes in pollutant emissions (outside of carbon dioxide), nor do we account for population dynamics."

The study is published in Environmental Research Letters (ERL).