Exposure to high levels of airborne pollutants is an ongoing problem, as is exposure to extreme temperatures. If these two overlap, then it's possible that the health impacts will be greater. A recent paper published in PNAS uses 15 years of climate observations in the US and Canada to show that the two problems do indeed cluster together and occur in overlapping, large-scale episodes. The largest of these episodes has the hottest temperatures and the highest level of pollution.

The authors used both surface station and meteorological data from the years of 1999 to 2013. They divided this data into one-degree squares. Then they carefully gridded daily Earth surface values for ozone, particulate matter (as a proxy for air quality), and temperature, and they identified climatologically extreme events.

In this analysis, they saw that extreme weather and pollution events clustered into multi-day episodes that tended to be spatially connected. This means that the episodes typically affected grid squares that were adjacent to each other or contiguous. The researchers saw that the weather tended to drive several types of extreme events at once, with problems often coinciding or happening adjacent to each other in either space or time.

However, the timing of the extreme events wasn't always consistent, so there's not likely to be a simple mechanism connecting weather with different pollutants. Instead, the authors argue that these large-scale patterns make a strong argument that we should probably do some research to identify the mechanisms.

“Super episode”

The authors did see the occurrence of large, weeklong episodes of high temperatures and pollution that they referred to as "super episodes." For these events (even the smaller ones), the size of the heat waves is larger than the increase in the high pollution events. In other words, super episodes of extreme pollution and high temperature are primarily driven by the temperature, with pollution increasing as a secondary effect. The researchers hypothesize that this is due to a difference in emissions occurring over the course of several years, because pollution precursors (such as fossil fuels and factory byproducts) vary greatly in different regions of the US. Climate increases, on the other hand, are a more uniform and universal phenomenon.

In looking at these extreme episodes, the authors saw that the size of an episode was extremely valuable in terms of predicting its severity. The longest episodes of extreme temperature and pollution also had the highest pollution levels and hottest temperatures. So, rather than seeing the worst problems coming in short critical events, the issues tended to drag on for days.

There were also regional differences in frequency. The most common extreme heat and pollution events occurred in the US Southwest, and the minimum for these types of events occurred over the Great Lakes and the Northeast.

Some good news

Emissions reductions have been effective in reducing pollution, particularly in more recent years. Unfortunately, those reductions haven't targeted carbon dioxide, so temperature events are likely to increase in severity in the future.

The knowledge that the most extreme events are also the longest events should help inform preparedness plans. Cities will need to be ready to support citizens most vulnerable to these conditions for several days in a row and not just for a day here or there. Future preparedness plans will need to be more robust and anticipate the need for lengthy support programs.

PNAS, 2017. DOI: 10.1073/pnas.1614453114 (About DOIs)