As the reality and significance of climate change begin to permeate discussions across many policies and decision-making spheres – from the community-‐level to international negotiations – many are recognizing the importance of heat stress, extreme heat, and its impact on labor productivity. These are not the only health risks that are being altered by a changing climate; shifts in the transmission and range of zoonotic and vector-‐borne diseases, direct threats of mortality and morbidity related to hazards like flooding or typhoons, malnutrition due to food insecurity, and other types of health impacts should be anticipated. Some of these health threats, such as those related to food insecurity or malaria have traditionally garnered more research and policy attention than heat stress. Recent efforts are now calling greater attention toward the importance of heat stress, but much work is still needed in many locations, where monitoring and communication of weather conditions (early warning) as well as general awareness of weather-‐health impacts and precautions, remain low. Still further work needs to be done in finding context-‐appropriate responses for dealing with heat and in exploring the inter-‐linkages between various health impacts and knock-‐on effects, such as how reduced labor productivity could lead to lower crop yields (and thus food insecurity) as farmers are physically unable to tend to their crops or to loss of household incomes and inability to seek medical care. (https://www.rockefellerfoundation.org/wp-content/uploads/Climate-Change-Impacts-on-Heat-Stress-by-2050.pdf)
Climate change is causing temperatures to increase around the globe, and leading to an increase in the number of hot days and nights. Hot, humid days and nights contribute to heat stress, heat-related deaths, reduced labor productivity, and can exacerbate poverty. While everyone can be negatively impacted by extreme heat, certain people such as workers, especially those working outdoors in the sun or engaging in physical labor, the elderly and those with chronic illnesses, children, and pregnant women are particularly at risk of suffering harm during such hot spells.
Extreme heat events are a leading cause of weather-related human mortality in many countries world-wide (CDC 2006). High-risk groups such as the elderly, the very young, individuals with health problems, and the urban poor are particularly susceptible to extreme heat events (Uejio et al. 2011). As climate continues to warm, extreme heat events are expected to worsen in severity (hotter days and nights), frequency (number of hot days and nights), and duration (heat waves) (Delworth et al. 1999; Meehl and Tebaldi 2004; IPCC 2007). In the absence of acclimatization, and adaptation and mitigation measures, human mortality due to extreme heat events can be expected to increase in a warming climate (IPCC 2007). Even acclimatization may not protect humans from unprecedented or intolerable heat stress under severe warming scenarios due to human physiological limits
In addition to global-scale warming, extreme heat can be exacerbated in urban areas because of the urban heat island (UHI) effect (Stone 2012). Multiple local factors resulting from the unique morphological, radiative, and thermal properties of urban areas are responsible for the UHI (Oke et al. 1991). First, the urban canyon can decrease surface longwave radiation loss and increase absorption of solar radiation (lower albedo). Second, urban areas have reduced evapotranspiration due to replacement of vegetation with impervious surfaces. Third, urban surfaces generally store more sensible heat during the day that is then released at night. Fourth, anthropogenic heat emissions from vehicles, commercial and residential buildings, industry, and power plants also contribute directly to the heat island effect (Sailor 2010).
Analysis of extreme heat events has indicated a role of urban heat island effects in enhancing premature mortality and morbidity (e.g., Conti et al. 2005). Enhanced urban temperature, combined with urban effects on humidity, radiation, winds, and air pollution all should be considered (Fischer et al. 2012). Furthermore, urban areas have a distinct asymmetry in their diurnal temperature cycle compared to rural areas, with the heat island being more pronounced at night than during the day. This characteristic of urban areas may exacerbate threats to human health due to heat stress because it deprives urban dwellers of the opportunity to cool off (Rogot et al. 1992).
Numerical modeling provides an opportunity to examine extreme heat events and heat stress in the context of climate change. Heat stress can be assessed in terms of temperature alone as is commonly done using climate models (Meehl and Tebaldi 2004), but it is recognized that humidity can aggravate the physiological effects of high temperature (Epstein and Moran 2006). In general, most indices of heat stress combine temperature and humidity but make assumptions about other factors that affect heat stress, such as radiation and wind speed, as well as human physiology and clothing (Epstein and Moran 2006). Several previous numerical modeling studies have examined changes in heat stress associated with climate change, however, few have considered the combined effects of temperature and humidity on urban areas explicitly (Delworth et al. 1999; Sherwood and Huber 2010; Diffenbaugh and Ashfaq 2010). Fischer et al. (2012) contrast the urban and rural heat stress response to a doubling of CO2 using a land surface model including an urban canyon model [the Community Land Model (CLM) with CLM Urban (CLMU)] coupled to a global climate model (the Community Climate System Model (CCSM4); Gent et al. 2011). Heat stress was quantified with the Simplified Wet-Bulb Globe Temperature (SWBGT; Willett and Sherwood 2012). They found substantially higher heat stress in urban areas compared to rural areas, the contrast is most pronounced at night and over mid-latitudes and subtropics. Under doubled CO2, the occurrence of nights with extremely high heat stress increased more in urban than rural areas.
Hot weather has long been recognized as detrimental to human health and labor productivity by the scientific, health, and labor research communities. Despite this recognition, implementation, and updating of existing health policy mechanisms, public and business awareness of the health and labor risks associated with extreme heat remain low. Capacities for accessing, perceiving, and understanding heat risks, and taking appropriate actions are diverse and strongly linked with socioeconomic status, culture, and ability to participate politically – similar to capacities related to dealing with other natural hazards and building resilience to climate change.
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