Climate change, heat waves, and mortality projections for Chicago

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Abstract

Over the coming century, climate change is projected to increase both mean and extreme temperatures as heat waves become more frequent, intense, and long-lived. The city of Chicago has already experienced a number of severe heat waves, with a 1995 event estimated to be responsible for nearly 800 deaths. Here, future projections under SRES higher (A1FI) and lower (B1) emission scenarios are used to estimate the frequency of 1995-like heat wave events in terms of both meteorological characteristics and impacts on heat-related mortality. Before end of century, 1995-like heat waves could occur every other year on average under lower emissions and as frequently as three times per year under higher. Annual average mortality rates are projected to equal those of 1995 under lower emissions and reach twice 1995 levels under higher. An “analog city” analysis, transposing the weather conditions from the European Heat Wave of 2003 (responsible for 70,000 deaths across Europe) to the city of Chicago, estimates that if a similar heat wave were to occur over Chicago, more than ten times the annual average number of heat-related deaths could occur in just a few weeks. Climate projections indicate that an EHW-type heat wave could occur in Chicago by mid-century. Between mid- and end-of-century, there could be as many as five such events under lower, and twenty-five under higher emissions. These results highlight the importance of both preventive mitigation and responsive adaptation strategies in reducing the vulnerability of Chicago's population to climate change-induced increases in extreme heat.

Introduction

Extreme heat and oppressive heat events are known to produce elevated rates of both illness and death due to heat stress (Martens, 1998, McGeehin and Mirabelli, 2001, Schär et al., 2004). In addition to their direct impacts, sustained extreme heat events exacerbate preexisting cardiovascular, respiratory, and other conditions (Ellis and Nelson, 1978, Kalkstein and Valimont, 1987).

A direct effect of rising temperatures is an increase in the frequency and severity of extreme heat and heat wave events. As climate changes, very hot days and heat wave events are projected to become more frequent and severe (Tebaldi et al., 2006). At the same time, the risk of severe, prolonged heat wave events, such as those that occurred over Chicago in 1995 and Europe in 2003, is also increasing (Meehl and Tebaldi, 2004, Stott et al., 2004).

Excessive heat is currently the leading cause of weather-related deaths across the United States (NCDC, 2004). During the summer of 1980, as many as 10,000 deaths in the United States may have been associated with oppressive heat (NCDC, 2004). Although some research has suggested an overall decrease in heat vulnerability in recent decades (Davis et al., 2002), especially as air conditioning has become more commonplace (Smoyer, 1998), there is still a clear vulnerability to heat, and dramatic mortality episodes have occurred in the United States within the last 10 years (Klinenberg, 2002). One recent study indicates that, following a decline from the 1970s to the early 1990s, heat vulnerability has remained relatively constant since and may even be increasing in some cities (Sheridan et al., 2008).

In recent decades, two well-documented heat wave events have centered over major cities: Chicago in 1995 and Paris in 2003. Much can be learned regarding the potential impact of climate change on extreme heat and heat-related health concerns for the city of Chicago by examining the meteorological conditions leading to these events and their impact on urban mortality and morbidity rates.

In July of 1995, the city of Chicago experienced a heat wave unprecedented in its 123-year-old weather records (Livezey and Tinker, 1996). Maximum daily temperatures were equal to or greater than 32 °C (90 °F) for seven consecutive days and greater than 38 °C (100 °F) for 2 days at the peak of the heat wave (Fig. 1a). Even more importantly, there was no relief at night, as nighttime minimum temperatures were over 27 °C (80 °F) during the hottest days.

During the 1995 heat wave, 739 “excess” deaths were recorded (Semenza et al., 1999). Initially, 514 of these were classified as heat-related (Whitman et al., 1997), but a recent reanalysis estimates a greater total of 697 heat-related deaths during the 1995 heat wave (Kaiser et al., 2007). Although some deaths may have merely been anticipated by a few days to weeks, Semenza et al. (1999) estimated that only 26% of deaths were due to this type of displacement or “harvesting,” leaving over 500 deaths directly attributable to the heat wave event. Even this number may be an underestimate, as Shen et al. (1998) found that excess mortality rates during the Chicago heat wave were higher than the estimated heat-related mortality (24–26 per 100,000 as opposed to 19 per 100,000), likely due to an overly narrow classification of heat-related death.

During the 1995 heat wave, there were also more than 3000 excess emergency department visits (Dematte et al., 1998), and more than 1000 hospital admissions as compared to what would normally be expected at that time of the year (Semenza et al., 1999). Most hospital admissions were due to dehydration, heat stroke, and heat exhaustion among people with underlying medical conditions. Of those admitted with heat stroke, 21% died in hospital and 28% during the following year (Dematte et al., 1998).

The effects of the heat wave were likely enhanced by micrometeorological effects such as the urban heat island (leading to higher temperatures at weather observing stations closer to the center of Chicago, and lower temperatures at suburban sites), and the fact that the moderating effect of the lake was minimized by the southerly winds prevailing during the heat wave, which virtually eliminated the cooling effect of lake breezes. At the same time, socio-economic factors also enhanced the risk of heat-related health impacts. Changnon et al. (1996) highlight several of these, including an inadequate heat wave warning system, power failures, inadequate ambulance service and hospital facilities, an aging population, and improper ventilation due to lack of resources (i.e., residents who were unable to afford air conditioning). For some neighborhoods, risk factors even included the fact that people were afraid to open their windows due to crime.

Further analyses focus on statistical correlations of risk factors with mortality, ranking the different risk factors. The results emphasize the importance of access to air conditioning (O'Neill et al., 2005, Naughton et al., 2002) and vulnerability due to existing medical conditions and/or social isolation (Semenza et al., 1996, Naughton et al., 2002). Age, race, and social class were also contributing factors. For people ages 65 years and up, hospital admissions were up by 35% during the heat wave, as opposed to an increase of 11% for the general population; mortality rates for that age group were also higher (Whitman et al., 1997, Semenza et al., 1999). In terms of race, heat-related deaths were disproportionately larger in the black community and smaller in the Hispanic community, as compared to the Chicago-wide average (Whitman et al., 1997, Semenza et al., 1999). Independent of race, the relative affluence levels of neighborhoods were also a mitigating factor, with wealthier and more commercially successful areas showing lower mortality rates, likely because more of their inhabitants were better able to afford central air conditioning (Browning et al., 2006, O'Neill et al., 2005).

Many of the lessons learned during the 1995 heat wave have already been acted on. A second heat wave in 1999, just slightly less severe than the 1995 event (Fig. 1b), resulted in only 114 excess deaths attributed to heat (Palecki et al., 2001). Furthermore, more than half the deaths were for people less than 65 years old, suggesting that adaptation strategies focused on the elderly population were succeeding (Naughton et al., 2002). Despite these successes, however, future climate changes may challenge even currently successful adaptation strategies. Whether this is likely to be the case is the first question we investigate here, by assessing the projected frequency of 1995-like heat wave events for the 21st century: first, in terms of their meteorological characteristics, and second, in terms of their mortality characteristics.

During the summer of 2003, western Europe was impacted by a heat wave of historic proportions (Trigo et al., 2005). For most of that summer, temperatures were well above average across a broad region extending from the British Isles to the Iberian Peninsula and eastward to Germany and Italy. The most extreme conditions centered in France where in Paris, maximum temperatures equaled or exceeded 38 °C (100 °F) for 6 days, and the heat broke long-standing maximum and minimum temperature records during August 3–13 (Planton et al., 2004). The temporal extent of this heat wave event was also unprecedented. For June 1 through August 31, 2003, maximum temperatures were above average for all but 8 days in Paris and, for at least half of those days, average maximum temperatures were exceeded by 6 °C (10 °F) or more (Fig. 2a). Minimum daily temperatures were also abnormally high. Electricity demand rose by more than 4%, exacerbating concerns regarding the ability to cool both nuclear and fossil fuel power plants (Salagnac, 2007).

Initial estimates of the death toll incurred by the European Heat Wave (EHW) of 2003 centered around 30,000 (UNEP, 2004). This estimate was first revised upward by Valleron and Boumendil (2004) to over 40,000 deaths. More recently, an analysis by Robine et al. (2008) identified a total of 70,000 excess deaths in 16 European countries during the summer of 2003. This latest estimate was based on excess mortality rates rather than recorded heat-related deaths, again highlighting the potential for heat-related mortality to be underestimated through individual death counts.

Estimates vary regarding the magnitude of the harvesting effect of the EHW, whether some of the estimated 70,000 deaths were merely anticipated by a period of days to months. However, most agree that this effect was relatively small for this event. Robine et al. (2008) find no harvesting effects at all in their estimates of 70,000 excess deaths in 16 countries during the summer of 2003. A modest harvesting effect, amounting to about one-third of total deaths in France, is estimated by Toulemon and Barbieri (2008).

Analysis of the 2003 EHW indicates that its duration and magnitude is beyond that of any similar event that has occurred in the United States or Europe over the last 150 years. During the EHW, for example, the city of Paris reported 2600 excess emergency room visits, 1900 excess hospital admissions, and 475 excess deaths (Dhainaut et al., 2004).

As with Chicago, a number of factors have been identified that rendered certain segments of the European population more at risk or vulnerable to extreme heat both during and after the heat wave event. For example, nearly two-thirds of all excess deaths were female; also, a disproportionately large amount of deaths were experienced by age groups over 65 years, with excess deaths increasing by as much as 27% for people over the age of 85 years (Robine et al., 2008) and for those at home or in retirement institutions, as compared to hospitals (Fouillet et al., 2006). Case studies in Germany and France quantified the contribution of elevated ozone levels to increased risk and identified respiratory disease as being most strongly correlated with extreme heat (Hoffmann et al., 2008, Filleul et al., 2006). Not unrelated, populations in urban areas showed the greatest sensitivity to the extreme heat (Salagnac, 2007).

Following the EHW, Lagadec and Laroche (2005) highlighted a number of contributing risk factors, including the absence of alert systems to inform the population of the heat risk, and other preventative measures that could have been taken by the public heath sector. As with Chicago, again, the response of the public health authorities – at least in France – appears to have borne fruit. Actual mortality for a heat wave event in 2006 was two-thirds lower than predictions based on historical mortality rates that included the EHW (Fouillet et al., 2008). This suggests that, overall, increased awareness and adaptation measures may have decreased the vulnerability of the French population to extreme heat.

It is clear that adaptation capacities differ significantly between European and North American cities – both in the behavior of their inhabitants and as in the built environment. The importance of air conditioning (particularly central air as opposed to window units) in determining the vulnerability of Chicago's population to extreme heat has already been highlighted by O'Neill et al., 2005, Naughton et al., 2002 for Chicago, as well as by Salagnac (2007) for France. In fact, recent work has shown that the death rates in five major US cities would have been considerably lower than the number encountered in Europe had a heat wave of the same magnitude as the 2003 EHW occurred on this side of the Atlantic. Both increased air conditioning availability and differing urban structures (less green space and more concrete per unit area in many European cities) can account for most of this difference (Kalkstein et al., 2008).

Nonetheless, a heat wave comparable to the EHW in duration and intensity has yet to occur over North America. Moreover, there is a well-documented pattern of increased mortality in US cities as a result of extreme heat waves (e.g., St. Louis, 1966, 1980, 2006; New York, 1975, 1984, 2006; Philadelphia, 1991, 1993; Chicago, 1995, 1999). This raises the question of what the health impacts of a similar event would be for Chicago. This is the second question we investigate here – first, projecting the meteorological conditions of the EHW over Chicago to estimate projected mortality rates, and then calculating the future probability of such an event in terms of its impact on heat-related mortality.

This analysis is part of a larger study examining the potential impacts of climate change on the US Great Lakes and Midwest regions in general, and the city of Chicago in particular. Given Chicago's history of high-impact heat waves and large urban population, this analysis focuses on that city. At the same time, these results are likely to be qualitatively relevant to other large urban centers within the Great Lakes watershed whose populations have demonstrated sensitivity to extreme heat, including Toronto, Minneapolis, and Detroit (Dolney and Sheridan, 2005; O'Neill et al., 2005; Schuman et al., 1964). Specifically, we draw on a large existing body of literature that documents the characteristics and effects of extreme heat in general on urban populations, and two individual heat wave events in particular – the Chicago heat wave of 1995 and the European heat wave of 2003 – to assess the projected impacts of climate change on public health in Chicago. We do so using four complementary analysis methods.

First, we calculate the projected frequency of 1995-like heat wave events in the future, in terms of their meteorological characteristics. This heat wave is the most extreme event in recent memory, and thus serves as an easily recognizable indicator to Chicago's population of a future such event.

At the same time, however, Chicago has already demonstrated the ability to dramatically increase its resilience. Comparing mortality rates for similar heat waves in 1995 and 1999 showed a reduction of nearly four-fifths over that time (Palecki et al., 2001). For that reason, we also estimate projected changes in the frequency and intensity of offensive air mass events associated with elevated heat-related mortality, in order to generate projections of increases in year-to-year mortality that include adaptation. In this way, we attempt to estimate the frequency of future summers with mortality rates, rather than merely meteorological conditions, similar to those of 1995. We hypothesize that, given the already demonstrated potential of city inhabitants to adapt to extreme heat conditions, the number of summers with 1995-like mortality is likely to be significantly less than the number of summers with 1995-like extreme heat.

Third, we use a novel “analog city” approach developed by Kalkstein et al. (2008) to superimpose the meteorological conditions in Paris during the EHW event on the city of Chicago. All other conditions in Chicago remain the same as observed for that city, including its demographics, vulnerability, and infrastructure. We estimate the likely impacts of an EHW-like heat wave on the city today, given its current risk factors. We hypothesize that mortality rates are likely to be lower than those experienced by Paris as the population of Chicago is likely to be less vulnerable to extreme heat than that of Paris; at the same time, however, Chicago's mortality rates are likely to be significantly higher than average due to the unprecedented intensity and duration of the heat wave event.

Finally, we use the meteorological characteristics of the EHW to estimate the likely timing of such an event occurring over Chicago, and the frequency of an EHW event by mid- and end-of-century under higher and lower scenarios of future climate change. Throughout the analysis, whenever possible, we deliberately distinguish between the impacts expected under a higher vs. a lower future emissions scenario. Our intention is to highlight the importance of mitigation in limiting future change, as well as the need for adaptation even under a lower emissions scenario.

Section snippets

Climate projections for estimating future heat wave conditions and mortality rates

Hourly and daily weather observations for the Chicago Midway Airport weather station were used to derive the historical characteristics of extreme heat and offensive air mass events in Chicago. The Midway station was selected from eight long-term weather stations in and around Chicago as it is the closest NWS weather station to the majority of Chicago's urban population with both daily and hourly records of temperature and humidity, and daily records of wind direction and speed, sea level

Projected changes in the frequency of heat wave events for Chicago

Over the coming century, climate change is expected to increase not only average summer temperatures but also the frequency of extreme heat associated with heat wave events. Furthermore, climate model simulations indicate that the atmospheric circulation patterns associated with both the severe 1995 heat waves in Chicago as well as the Paris heat wave in 2003 are expected to become more intense, more frequent, and longer-lasting in the second half of the 21st century (Meehl and Tebaldi, 2004).

Discussion and conclusions

Significant increases in extreme heat, and prolonged heat wave events, are projected to continue in coming decades, consistent with observed global trends in past decades (Meehl and Tebaldi, 2004). Model uncertainties notwithstanding, extreme heat and associated human health risks under the higher emissions scenario are generally twice those projected to occur under lower emissions by end-of-century. These results suggest that more heat stress ailments can be expected in the future.

It is clear

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