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Twenty years after ‘95: What climate change means for heat waves, cities and forecasting

Posted October 2, 2015
Experts met to discuss the challenges of managing urban heat islands, using the deadly 1995 Chicago heat wave as a case study. Image by Dustin Phillips (Flickr) CC BY-NC-ND 2.0;

Experts met to discuss the challenges of managing urban heat islands, using the deadly 1995 Chicago heat wave as a case study. Image by Dustin Phillips (Flickr) CC BY-NC-ND 2.0;

The 1995 Chicago heat wave was a defining moment for the city. With four days of non-stop, sweltering heat and humidity, hospital emergency rooms were overwhelmed. There were thousands of heat-related illnesses and more than 700 deaths.

Two decades later, public health and government officials are acknowledging the threat of future heat waves. And they’re turning to climate scientists for information and advice.

Doug Sisterson, a meteorologist at the U.S. Department of Energy’s Argonne National Laboratory, discussed the widespread impacts of climate change at a September 16 forum hosted by the University of Illinois at Chicago. His talk provided the scientific backdrop for a discussion of past and future heat waves.

The event, organized by the university’s School of Public Health, marked the 20th anniversary of the famous Chicago heat wave.

One of the most common misconceptions about climate change is that locally cold temperatures are incompatible with global warming. On the contrary, climate disruption makes it difficult to predict local weather patterns with certainty, but guarantees more heat waves and other extreme weather events.

Cities face a unique challenge: Temperatures in urban areas can be as much as 10 degrees Fahrenheit hotter than in open air, thanks to the urban heat island effect.

Plants help cool their surroundings by providing shade and using the sun’s radiant energy to evaporate water off leaves. But cities replace this vegetation with concrete, a material that captures the sun’s energy as heat and holds it for longer. Tall, clustered buildings also contribute to the heat island by blocking wind, which would otherwise flush out some of the warm air.

Official recording stations for temperature are usually installed in open regions to collect data that will be broadly representative. Cities tend to boast a heterogeneous landscape, which means no single recording will be sufficient.

According to Sisterson, this means that the microclimate of an urban area is often overlooked.  “Urban areas worldwide have anegligible impact on global warming, but global warming will have a substantial impact on the urban microclimate,” he said.

The National Weather Service has traditionally taken measurements at the two Chicago airports, avoiding the downtown area entirely, he said.

“We’re starting to realize that we’ve been ignoring the microclimate of a city, where most people live,” Sisterson said. “In the last few years, there’s been a big push to get instruments into urban areas.


Climate disruption, according to Sisterson, will also manifest in different ways depending on the region of the world. It’s not necessarily going to be just extreme heat—it could be extreme cold, extreme drought, extreme flooding or a higher percentage of severe storms, he said.

Unfortunately, he can’t be much more specific than that, because with global warming comes a big forecasting problem.

Weather patterns are governed by jet streams, large currents that move air around the globe. As long as these jet streams patterns are predictable, meteorologists have reasonable foresight about local temperature and precipitation.

But climate change is throwing the jet streams off kilter.

These currents are powered by the temperature difference between the continents and the major oceans, which normally sets up semi-permanent regions of high and low pressure to direct jet streams.

When greenhouse gases trap more heat in the atmosphere, global air temperatures rise. But this increase affects land and water differently, which impacts the regions of high and low pressure, skews the jet streams and alters weather patterns worldwide.

“We know that extreme weather events are going to be more severe and more frequent on a global scale,” Sisterson said. “But what’s going to happen in our backyard? When exactly will it strike? What form of persistent weather patterns will take root? We don’t have all the pieces in place yet to make that kind of a detailed forecast.”

What is likely, he said, is that in the next few decades the Midwest will experience more heat waves like the one that struck Chicago in 1995.

And heat waves are only part of the problem.

The Chicago area offers a recent example of the far-reaching effects of climate change. The city may be notorious for its bitterly cold winters, with record low temperatures in the 2013-14 season, but the five years leading up to this were the warmest winters in recent history. The snowy season was starting later and ending earlier.

This gave plants an extended growing period, but also allowed certain species of bugs and viruses to flourish and spread disease.

Global warming is about more than temperature, Sisterson said, because of these feedback effects. It introduces new burdens on human health and can throw off the balance of plant and animal populations.

The forum was a valuable opportunity for diverse professionals and for the general public—it’s important that people tune in to the climate disruption conversation, grasp some of the complicated realities and discover how it all relates to their area of expertise, Sisterson said.

Other speakers at the event included Jan Semereza, an epidemiologist then with the Center for Disease Control who  led the 1995 investigation, Ed Donoghue, the Cook County Medical Examiner in 1995, John Wilhelm, former health commissioner of the Chicago Department of Public Health appointed by former Mayor Daley to head the task team investigation about the 1995 disaster, WGN-TV Chief Meteorologist Tom Skilling and representatives from the Centers for Disease Control from the European Union, as well as local,  state and federal  government agencies.

The morning session focused on lessons learned since 1995. Populations most vulnerable to heat stress include the elderly as well as low-income communities without air-conditioned homes. Chicago has become a leader in organizing neighborhood watches and cooling centers during stretches of extremely hot weather; panelists discussed whether these systems are effective and what other improvements are needed for future preparedness.

“I’m happy as a climate researcher to be able to participate,” Sisterson said, “to be able to say that climate disruption is real and we all need to understand what it means so that we can prepare for the consequences, especially in urban areas with high population density.”

Source: ANL

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