Summer U.S. Forecast: Hotter, More Polluted Than Usual

The odds favor a hotter than average summer for much of the Western U.S., and a closer to average one for the Eastern U.S., according to the May seasonal forecasts from The Weather Company’s WSI branch, and Columbia University’s International Research Institute for Climate and Society (IRI). Meteorological summer began on June 1, and the first week of summer has been on the cool side for much of the United States. However, a shift in the jet stream pattern is coming late this week and into early next week, which will bring hot conditions to much of the eastern half of the U.S. WSI anticipates the possibility of a weak El Nino late summer, perhaps with enough influence on the tropical atmosphere to limit the magnitude of the heat across the northern Plains, Great Lakes, and Eastern U.S. Similarly, in an outlook issued in mid-May, a model-based outlook from IRI and NOAA gave slightly-better-than-even odds of El Niño developing. However, the offiical NOAA/IRI forecast from early May has lower odds, between 40% and 50%. If El Niño does not develop, the odds of stronger and more widespread U.S. heat this summer will rise.

NOAA is predicting increased odds of a hot summer not only for the Western U.S., but also for the South and Northeast. WSI also predicts a hot summer for the Southeast during July and August. So, what does this mean for air pollution levels this summer?

Figure 1. The June-July-August forecast from the International Research Institute for Climate and Society, updated in May, predicts above-average chances for a hot summer over much of the Western U.S., Europe, and Russia. Image credit: Graphic based on outlook from IRI, part of The Earth Institute, Columbia University.

Seasonal forecasts of wind have little skill

Air pollution episodes are highly dependent on meteorology. If there are strong winds, air pollution levels are lower than when winds are light, since light winds allow pollutants to accumulate near their source. Since winds tend to be lighter in summer than winter, more air pollution episodes with stagnant air are usually observed in the summer. However, predictability of wind patterns is limited to about ten days in advance, and there is little we can say months in advance about how strong the winds might be this summer.

Seasonal forecasts of temperature do have skill

Meteorologists do have skill at predicting temperature patterns months in advance, though, since temperatures over land are connected to ocean temperature patterns, which typically change on a time scale of months. The skillfulness of these forecasts is especially strong when forecasters are most confident about the state of the El Niño-Southern Oscillation in the eastern Pacific. For example, NOAA forecasters anticipated the presence of a weak La Niña for the winter of 2016-17, and they successfully called for a cooler-than-average winter toward the northern U.S. and a warmer-than-average winter in the South (though the cool pocket was shifted further west than usual, and temperatures overall were warmer than expected). Such outlooks are most challenging to issue for summertime, largely because El Niño and La Niña are often in flux during the spring and summer—the so-called “spring predictability barrier.” 

Figure 2. A strong positive correlation between temperature (horizontal axis) and ground-level ozone pollution (vertical axis) in New York City. Data is for May to October (“smog season”) for the years 1988 to 1990. The higher the temperature, the higher the ozone level, regardless of a city’s size. Climate change is projected to bring higher average temperatures over this century, which will increase ground-level ozone if precursor emissions of the chemicals that create ozone (hydrocarbons and nitrogen oxides) are not reduced. Image credit: Union of Concerned Scientists.

Expect more ozone where temperatures are warm

Of most value to make forecasts for are the two pollutants known to be deadly for large numbers of people: ground-level ozone and particulate matter—especially the tiny particles known as PM2.5 (those that are less than 2.5 microns or 0.0001 inch in diameter). PM2.5 pollution is thought to kill about ten times more people than ozone. Ozone is created via chemical reactions that occur more readily at warmer temperatures. Thus, we can expect that the Western U.S. will have greater-than-average problems with ground-level ozone pollution this summer, due to the higher-than average temperatures predicted.

PM2.5 levels are not as strongly dependent on temperature, so it is difficult to make any general forecast for PM2.5 pollution this summer. There are some regions where high temperatures will increase the photochemical reactions that create small particles in the atmosphere, creating higher PM2.5 levels, though. Also, a hot summer will drive higher wildfire activity, and these fires will create smoke that has high levels of polluting PM2.5.

Heat waves are dangerous in more ways than one

Research into heat-wave disasters of the last 20 years, including the catastrophic heat wave that struck Europe in 2003 that killed 71,310 people, has shown that prolonged heat takes its toll in several ways. For years, researchers focused on the heat itself: if people are unable to find relief from high temperatures, especially over a string of hot nights, their body temperatures can rise to the level where deadly heat stroke becomes a real risk.

It’s now clear that high temperatures can team up with poor air quality to make heat waves even more dangerous. In the 2003 European heat wave, up to 40% of the “excess deaths” in some regions (the number of deaths beyond those one would normally get at a certain time of year) were attributed to ozone and/or particulate pollution. These pollutants not only affect our respiratory systems, but the tiniest particles can also work their way into our bloodstreams, raising the risk of heart attacks and strokes in the long term, and even during one-off heat waves for folks who are especially vulnerable. A study of several heat waves in nine European cities found that respiratory-related death rates increased by anywhere from 4% in Munich to 67% in Rome, while cardiovascular deaths rose by an estimated 8% in Munich and 38% in Rome.

There's a lot that each of us can do to protect our health from the impacts of air pollution. To cite just one example we discussed last month, running your car's ventilation system in "recirculation" mode can make a big difference when you are driving. The U.S. Environmental Protection Agency has many other tips on protecting indoor air quality, minimizing the impact of outdoor air pollution on your health, and reducing outdoor air pollution where you live and work.

WU is launching air quality products soon!
 

Bob Henson contributed to this post.