Key Message: Wide-ranging Health Impacts
Climate change threatens human health and well-being in many ways, including impacts from increased extreme weather events, wildfire, decreased air quality, threats to mental health, and illnesses transmitted by food, water, and disease-carriers such as mosquitoes and ticks. Some of these health impacts are already underway in the United States.
Process for Developing Key Messages:
The key messages were developed during technical discussions and expert deliberation at a two-day meeting of the eight chapter Lead Authors, plus Susan Hassol and Daniel Glick, held in Boulder, Colorado May 8-9, 2012; through multiple technical discussions via six teleconferences from January through June 2012, and an author team call to finalize the Traceable Account draft language on Oct 12, 2012; and through other various communications on points of detail and issues of expert judgment in the interim. The author team also engaged in targeted consultations during multiple exchanges with Contributing Authors, who provided additional expertise on subsets of the key message. These discussions were held after a review of the technical inputs and associated literature pertaining to human health, including a literature review,1 workshop reports for the Northwest and Southeast United States, and additional technical inputs on a variety of topics.
Description of evidence base
The key message and supporting text summarizes extensive evidence documented in several foundational technical inputs prepared for this chapter, including a literature review1 and workshop reports for the Northwest and Southeast United States. Nearly 60 additional technical inputs related to human health were received and reviewed as part of the Federal Register Notice solicitation for public input.
The effects of decreased ozone air quality on human health have been well documented concerning projected increases in ozone,2,3,4,5,6,7,8,9,10 even with uncertainties in projections owing to the complex formation chemistry of ozone and climate change, precursor chemical inventories, wildfire emission, stagnation episodes, methane emissions, regulatory controls, and population characteristics.11 Ozone exposure leads to a number of health impacts.12,13,14
The effects of increased temperatures and atmospheric CO2 concentration have been documented concerning shifts in flowering time and pollen initiation from allergenic plants, elevated production of plant-based allergens, and health effects of increased pollen concentrations and longer pollen seasons.15,16,17,18,19,20,21,22,23,24,25,26 Additional studies have shown extreme rainfall and higher temperatures can lead to increased indoor air quality issues such as fungi and mold health concerns.27,28,29,30
The effects of wildfire on human health have been well documented with increase in wildfire frequency18,31,32,33,34,35,10,36 leading to decreased air quality37,38,39,40,41,42 and negative health impacts.38,43,44,45
The effects of temperature extremes on human health have been well documented for increased heat waves,46,47,48,49 which cause more deaths,50,51 hospital admissions52,53,54 and population vulnerability.55,56
Precipitation Extremes - Heavy Rainfall, Flooding, and Droughts:
The effects of weather extremes on human health have been well documented, particularly for increased heavy precipitation, which has contributed to increases in severe flooding events in certain regions. Floods are the second deadliest of all weather-related hazards in the United States.57,58 Elevated waterborne disease outbreaks have been reported in the weeks following heavy rainfall,59 although other variables may affect these associations.60 Populations living in damp indoor environments experience increased prevalence of asthma and other upper respiratory tract symptoms.61
Disease Carried by Vectors:
Climate is one of the factors that influence the range of disease vectors; 62,63,64 a shift in the current range may increase interactions with people and affect human health.65,66,67,68 North Americans are currently at risk from a number of vector-borne diseases.69,70,71,72,73,74,75,76,77,78,79,80,81There are some ambiguities on the relative role and contribution of climate change among the range of factors that affect disease transmission dynamics.65,82,62,63,69,66,67,68,64 However, observational studies are already underway and confidence is high based on scientific literature that climate change has contributed to the expanded range of certain disease vectors, including Ixodes ticks which are vectors for Lyme disease in the United States.83,84,85
Food- and Waterborne Diarrheal Disease:
There has been extensive research concerning the effects of climate change on water- and food-borne disease transmission.86,87,88,89,90,91,92,93,94,95,96 The current evidence base strongly supports waterborne diarrheal disease being both seasonal and sensitive to climate variability. There are also multiple studies associating extreme precipitation events with waterborne disease outbreaks.59 This evidence of responsiveness of waterborne disease to weather and climate, combined with evidence strongly suggesting that temperatures will increase and extreme precipitation events will increase in frequency and severity (Ch. 2: Our Changing Climate), provides a strong argument for climate change impacts on waterborne disease by analogy. There are multiple studies associating extreme precipitation events with waterborne disease outbreaks and strong climatological evidence for increasing frequency and intensity of extreme precipitation events in the future. The scientific literature modeling the projected impacts of climate change on waterborne disease is somewhat limited, however. Combined, we therefore have overall medium confidence in the impact of climate change on waterborne and food-borne disease.
Harmful Algal Blooms:
Because algal blooms are closely related to climate factors, projected changes in climate could affect algal blooms and lead to increases in food- and waterborne exposures and subsequent cases of illness.95,96,97,98,99,100,101 Harmful algal blooms have multiple exposure routes.102,103,104
Climate change is expected to have global impacts on both food production and certain aspects of food quality. The impact of temperature extremes, changes in precipitation and elevated atmospheric CO2, and increasing competition from weeds and pests on crop plants are areas of active research (Ch. 6: Agriculture, Key Message 6).105,106,107,108,109,110,111,112,26 The U.S. as a whole will be less affected than some other countries. However, the most vulnerable, including those dependent on subsistence lifestyles, especially Alaska Natives and low-income populations, will confront shortages of key foods.
Mental Health and Stress-Related Disorders:
The effects of extreme weather on mental health have been extensively studied.113,114,115,116 Studies have shown the impacts of mental health problems after disasters,117,118,119 with extreme events like Hurricane Katrina,120,121 floods,122,123 heat waves,124 and wildfires125 having led to mental health problems. Further work has shown that some people with mental illnesses are especially vulnerable to heat. Suicide rates vary with weather,126,127,128 dementia is a risk factor for hospitalization and death during heat waves,124,129 and medications for schizophrenia may interfere with temperature regulation or even directly cause hyperthermia.130,131 Additional potential mental health impacts include distress associated with environmental degradation, displacement, and the knowledge of climate change.114,115,132,133
New information and remaining uncertainties
Important new evidence on heat-health effects134,135,136 confirmed many of the findings from a prior literature review. Uncertainties in the magnitude of projections of future climate-related morbidity and mortality can result from differences in climate model projections of the frequency and intensity of extreme weather events such as heat waves and other climate parameters such as precipitation.
Efforts to improve the information base should address the coordinated monitoring of climate and improved surveillance of health effects.
Assessment of confidence based on evidence
Overall: Very High confidence. There is considerable consensus and a high quality of evidence in the published peer-reviewed literature that a wide range of health effects will be exacerbated by climate change in the United States. There is less agreement on the magnitude of these effects because of the exposures in question and the multi-factorial nature of climate-health vulnerability, with regional and local differences in underlying health susceptibilities and adaptive capacity. Other uncertainties include how much effort and resources will be put into improving the adaptive capacity of public health systems to prepare in advance for the health effects of climate change, prevent harm to individual and community health, and limit associated health burdens and societal costs.
Increased Ozone Exposure: Very High confidence.
Allergens: High confidence.
Wildfires: Very High confidence.
Thermal Extremes: Very High confidence.
Extreme Weather Events: Very High confidence.
Vector-borne Infectious Diseases: High or Very High confidence for shift in range of disease-carrying vectors. Medium confidence for whether human disease transmission will follow.
Food- and Waterborne disease: Medium confidence.
Harmful Algal Blooms: Medium confidence.
Food Security: Medium confidence for food quality; High confidence for food security.
Threats to Mental Health: Very High confidence for post-disaster impacts; Medium confidence for climate-induced stress.
Strong evidence (established theory, multiple sources, consistent results, well documented and accepted methods, etc.), high consensus
Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.), medium consensus
Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought
Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts