What is the most deadly natural disaster likely to strike Indianapolis?
Perhaps an EF5 tornado? In a few minutes on May 23, the nation’s deadliest tornado since 1947 cut a six-mile path of destruction through Joplin, Missouri, killing 151 and causing over $3 billion in damages.
Maybe the New Madrid fault will roar back to life resulting in a major Indy earthquake?
No, the most likely deadly disaster is an extreme heat event, according to climate researcher and IUPUI Professor Daniel Johnson, who spoke at the Indiana Environmental Health Summit earlier this week in Indianapolis. (Johnson's presentation available at the link above.)
Johnson defined an extreme heat event, commonly called a “heat wave,” as a period of prolonged high ambient air temperature coupled with high humidity. In a typical year, extreme heat is the number one cause of weather-related death in North America, outpacing all other causes combined.
Famous recent extreme heat events include the Chicago heat wave of 1995 that killed more than 600 residents and prompted that city to take action. A European heat event in 2003 claimed over 40,000 lives and a Russian heat event last summer killed 56,000.
A major factor in extreme heat events is the urban heat island effect, where heavily developed areas tend to be considerably hotter than the surrounding countryside, due primarily to absorption of heat by dark, heat-absorbing roofs and dark paved surfaces. In the country, vegetation creates shade and transpires moisture for natural cooling. Other factors have to do with slow-moving weather systems and domes of stagnant air with high ground-level ozone pollution. The Chicago extreme heat event of 1995 overtaxed the Chicago region’s electrical grid, causing brownouts and blackouts that shut down air conditioners and fans.
People living in dense urban environments may be more vulnerable to extreme heat if they also lack access to transportation or air conditioning or if they have illnesses or other risk factors. Risks increase for the very young and very old and for people who live alone. Part of Johnson’s research is to map these “socially vulnerable” populations in relation to urban hot spots to show areas of greatest risk of mortality. This type of mapping can be used to target areas for mitigation of the urban heat island effect and to inform emergency planning for socially vulnerable populations.
Johnson predicted an increase in the frequency of extreme heat events, due to climate change, the most direct effect of which is increased heat. Scientists at Stanford reinforced that prediction this week announcing research that suggests permanently hotter summers are on the way. A 2007 study (PDF) by the Pew Center on Global Climate Change found a similar connection between climate change and increasing frequency and severity of heat waves in the Midwest. The Union of Concerned Scientists published a report (PDF) in 2009 with implications for the Midwest with continued warming under various emissions scenarios, including an analysis for Indianapolis (see graph below).
Extreme heat events are particularly problematic in temperate climates, such as Indiana, where the population may not be acclimated to heat, especially earlier in the summer. As if to reinforce Johnson’s presentation, one of those early season events occurred this week when Minneapolis set a record of 103 degrees, causing I-95 to buckle in nine locations. Twenty-five other cities reported record highs. Fortunately, this week’s event was cut short by a cold front.
Johnson showed maps of Indianapolis with areas of increasing heat island effects as urban sprawl spread over the past few decades, but the maps also showed areas where the effect had been reversed in neighborhoods with aggressive tree planting programs.
Other mitigation strategies for the urban heat island effect include using heat reflective or vegetated roofs, shading paved areas, using light-colored paving and preserving or restoring open space. In addition to potentially saving lives, urban heat island mitigation can also conserve energy and water, clean the air, and make cities more beautiful and livable.
These urban heat island mitigation strategies will be like rearranging the deck chairs on the Titanic, however, unless we begin to reduce greenhouse gas emissions dramatically. In the future, it may no longer be accurate to refer to extreme heat events as "natural" disasters.