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Stagnant Days of Summer on the Rise

By Climate Central

Air pollution can be a problem any time of year, but summer often brings the conditions that trap pollutants in the lower atmosphere. Nearly stationary domes of hot air are frequently responsible for summer heat waves. This lack of air movement traps pollutants, such as particulates and ozone, raising health problems from respiratory distress to eye irritation. 

For the graphics above, we take a look at how the number of summer days with stagnant air has changed since 1973. To determine these days, we followed the NCEI Air Stagnation Index (ASI), which uses upper atmospheric winds, surface winds, and precipitation to calculate the level of stagnation. Over the past 4 decades, the most substantial increase in summer stagnation days has been in the Southeast and the Southwest.

A Mixed Future

Looking forward, climate change is expected to increase the frequency of heat waves. With more domes of hot air, more stagnation days could be expected — and two studies support this case. Following the ASI, researchers used ensemble simulations from different climate models to determine how the number of stagnation days may change by the end of the century. Both studies looked at the global picture, but we focus on the U.S.

The simulations in the original study assumed society will adopt a balance of fossil fuel and renewable energy sources. Under that scenario, researchers found an additional six to nine stagnation days per year across parts of the Southeast and the Southwest. However, different mechanisms were responsible for the increase. In the Southeast, it was the weak winds in the upper atmosphere and at the surface. While in the West, the additional stagnation days came from the lack of precipitation to scour out pollutants — in addition to more days with weak surface winds.

The second study assumed a business-as-usual emissions scenario, where fossil fuel use increases and greenhouse gases further accumulate. In this study, the researchers found similar patterns of stagnation change to their previous study. But in this case, there were even more stagnation days across the Southwest.

While this does not necessarily mean air quality will worsen in the coming decades, it is a reminder that weather and climate can have a direct impact on air quality and on society’s efforts to reduce harmful air pollution.

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