Every year in the US, there are thousands of deaths thought to result from ozone and more than 100,000 deaths linked to fine particulate matter. The presence of these pollutants in the atmosphere is affected by factors like temperature, wind speeds, water vapour pressure and precipitation. Scientists predict that, with temperatures rising and air currents becoming more stagnant in the US, the amount of ozone and particulate matter will rise.

Until now, however, scientists had not studied in detail what effects climate change-driven pollution has already had. "We looked backwards to see what those impacts actually were," said Iny Jhun of the Harvard School of Public Health, US.

Jhun and colleagues looked back through US weather records and found that in the period 1994 to 2012, temperatures rose and wind speeds dropped in most regions, just as climate simulations predict will continue to happen in the future. They used US Environmental Protection Agency (EPA) measurements for the actual levels of ozone and particulate matter (PM2.5) over the same period, and employed statistical models to estimate what the pollution would have been if the weather had not changed. They called this difference in pollution the "weather penalty" – the amount of pollution specifically attributable to climate change.

The weather penalty includes direct effects of weather conditions (e.g. photochemical reactions), indirect effects such as more heating use on cold days, and effects of other meteorological phenomena with ground-level weather manifestations, like transport of cold, dry air mass, according to the scientists’ paper in ERL.

Finally, turning to previous studies that have linked pollution to mortality, the researchers estimated that this weather penalty must have led to 290 deaths from ozone and 770 deaths from particulate matter, each year. Over the 19 year study period, this amounted to 20,300 excess deaths – a small fraction of the total deaths attributable to pollution in the US, but still significant.

Jhun believes the study has a two-fold message. "Past changes in weather conditions have already had significant adverse impacts on air quality and health," she said, "and the weather penalty on air pollution and its associated health impacts will likely persist in the future climate."

Now the researchers would like to look at the weather penalties of other climate variables, such as cloud cover. "We also want to build a more fine-tuned model that can estimate the weather penalty and [its] uncertainties for each year, rather than multiple years combined," said Jhun. "But more sophisticated statistical methods are needed for that."

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

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