“Any climate impact on health is mediated by other physical and environmental factors. There is no such thing as a direct climate impact.”—Ben Zaitchik
“It will help if we are clearer in our communications within and outside the health sector and if we talk about what we are modeling for. What are our goals? What are we trying to achieve?”—Kris Ebi
Throughout the workshop, speakers and panelists described a wide array of potential health risks of climate change and discussed why models of health risks are needed for public health and policy. Speakers focused in particular on health risks, like waterborne and vector-borne diseases, resulting from climate-induced changes in air quality, water quality, and temperature.
John Balbus, of the US National Institute of Environmental Health Sciences, described the main indicators of climate change and their connections to human health outcomes. The primary manifestations of a changing climate in the physical environment are “higher CO2 concentrations, increasing high temperatures, rising sea levels, and increasing severity of some kinds of extreme weather phenomena, such as hurricanes”, Balbus said. The pathways by which those climate-related changes affect human health are a complex mix of natural and human systems (Figure 2-1). For example, Balbus explained, higher temperatures may lead to the spread of the ranges of deer, mice, and ticks—“the ecologic causal chain that brings Lyme disease to humans”. Severe storms, such as Superstorm Sandy and Hurricane Katrina, can overload the functionality of health-care systems, which in turn may have negative consequences in particular for the elderly and those who have chronic medical conditions.
Jan Semenza of the European Centre for Disease Prevention and Control, described data published in the 2013 Intergovernmental Panel on Climate Change4 “Summary for Policymakers” that demonstrate increases in surface temperature and changing patterns of precipitation around the world. The reports observe that precipitation is increasing in northern latitudes and decreasing in southern latitudes. “Climate change is happening now,” said Semenza, and the warming and precipitation trends are expected to continue. Semenza explained that the differences between wet and dry regions and between wet and dry seasons
Figure 2-1 Illustration of the impact of climate change on human health
The inner circle represents the physical environment. The first ring represents the four primary manifestations of climate change in the physical environment (climate drivers): increasing carbon dioxide concentrations in the atmosphere (yellow), rising temperatures (red), rising sea levels (blue), and more extreme weather (green). The four climate drivers can act through natural and human systems to cause conditions listed in the second ring, such as changes in vector ecology, extreme heat, and changes in water and food supply. Surrounding the second ring are the types of health effects that may result from the conditions listed in the ring. Source: George Luber, workshop presentation, slide 21.
are expected to become greater later in this century. Among the health risks associated with a warmer and wetter climate include increased enterococcus and vibrio species5 in coastal and marine waters, respectively; and greater geographic distribution and abundance of the mosquito species that caries West Nile Virus, noted Semenza.
George Luber of the US Centers for Disease Control and Prevention Climate and Health Program noted that the effects of global climate change will vary substantially by region. Some places are warming faster than others, and the percent of extreme temperature events
5Enterococcus is a genus of bacteria that can cause gastroenteritis (“stomach flu”). Vibrio is a genus of bacteria that can cause gastroenteritis, severe wound infections, and septicemia (blood poisoning).
(locations that experience above normal temperatures) has increased. “From a public-health standpoint, when it comes to weather what we really care about is the extremes,” Luber said.
The increasing frequency of extreme weather events increases the likelihood of complex emergencies – “disasters within a disaster”. Luber emphasized that complex emergencies occur at magnitudes greater than the norm, and may exceed the coping capacity of critical systems that are in place to protect health. They can involve multiple system failures, such as loss of telecommunication, egress and transportation, and electric power. Luber named Hurricane Katrina as an example of a complex emergency – 800,000 people were displaced, and thus the challenges accompanying lost livelihood and community impacted both New Orleans and the cities where citizens took up new residence. The combination of large-scale ecologic perturbations caused by climate change and by global trade and travel can lead to novel health threats, Luber said. The increasing spread of Lyme disease in association with the expanding range of suitable habitat for the deer and birds that serve as reservoirs for the black-legged ticks that are the vectors of the disease-causing bacteria is one example. The rising rate of ciguatera6 poisoning from Gulf of Mexico fish that results from increases in sea temperature decreases in reef habitat related to oil-drilling platforms is another. A third example is the surge in West Nile fever in eastern Europe in 2010, Semenza said.
Effects of climate change on health and economics are already being documented throughout the world, said Anthony Janetos, director of Boston University’s Frederick S. Pardee Center for the Study of the Longer-Range Future. “There hasn’t been an overall assessment related to climate-change effects that I’m aware of that concludes that the risks of health effects in human populations will decrease,” Janetos said. Rather, the climate-assessment literature concludes that risks to health from multiple pathways—including disease, heat stress, waterborne illnesses, recovery from events, and malnutrition—will increase.
The increases in temperatures throughout the world linked to climate change can affect both the abundance and the activity of animal vectors that can spread disease to humans, said Nick Ogden, senior research scientist in the Public Health Agency of Canada. Those vectors include mosquitoes, flies, ticks, fleas, and lice that reside mainly in wildlife. The potential spread of mosquitoes due to climate change is particularly a concern because they are the vectors for some of the most important global vectorborne diseases in humans such as malaria, dengue fever, and chikungunya.
Among tropical diseases, dengue has been spreading in the Americas over the last 50 years, and both the US and Canadian governments are monitoring recent cases of chikungunya. “We’re now getting a lot of people coming back to Canada from the Caribbean with chikungunya,” Ogden said. Many of them have the virus in their blood, and it is possible for one of them to be bitten by an Asian tiger mosquito, Aedes albopictus, which could then transmit both chikungunya and dengue fever within North America.
Canada is also monitoring the spread of Lyme disease across its borders. Migrating birds can easily transport the ticks that serve as vectors for the disease into Canada, Ogden commented. Over the last decade, the number of reported cases of Lyme disease in Canada has risen dramatically. The primary ways that climate change is affecting water systems are through sea-level rise and changes in the ocean’s pH, changes in the water cycle itself, and
6A food-borne illness caused by eating fish whose flesh contains toxins originally produced by dinoflagellates (protists).
increases in extreme precipitation events. Juli Trtanj, of the US National Oceanic and Atmospheric Administration, spoke about research that is helping to capture the data needed to model how climate change may affect waterborne infectious diseases and water-related illnesses. It is a complicated issue that includes how runoff from urban and rural environments and infrastructure affects surface and groundwater sources of drinking water and coastal waters, she explained. That phenomenon is affected by temperature, nutrients in the water, changes in pH, salinity, and other biologic and chemical factors.
“Climate change will amplify existing health threats,” emphasized Michelle Bell, of Yale University. For example, she explained, human exposure to tropospheric ozone is expected to increase substantially as a result of climate change. Many areas around the world already have a growing ozone problem that is due to the expansion of transportation and industry; and many people in the United States live in areas where tropospheric ozone concentrations exceed the US Environmental Protection Agency’s health-based standard, she said. Increasing ozone concentrations in southern New York state are projected to raise the number of emergency-department visits due to asthma, Luber noted.
There will also be a marked increase in mortality due to multiple stressors and other effects on vulnerable populations, particularly in vulnerable locations. “The cumulative effect might be very great,” Luber explained.
As climate change affects rates of famine and increases the likelihood of conflicts over scarce resources, it is also likely to undermine human health in other ways. Stéphane Hallegatte of the World Bank pointed out that there could be a huge effect on the dynamics of health worldwide. When people who have escaped from poverty fall back into it, the reason is related to health issues in 50–80% of the cases, he said. Famine and conflicts are also likely to decrease the capacity of developing countries to control the transmission of vectorborne diseases, Ogden said. Another route by which exotic vectorborne diseases can be transmitted throughout the world is migration of infected refugees who are fleeing the developing world because of war, famine, or pestilence, he said.
Many efforts are under way to model the health risks of climate change. But why? A key argument for modeling is that health is likely to be important in inspiring people to take action on climate change, said Ben Zaitchik, of Johns Hopkins University. Nonetheless, What are our goals? What are we trying to achieve? asked Kristie Ebi, of ClimAdapt LLC. Ebi pointed out that there is broad confusion in the health sector about the goals of modeling the health risks posed by climate change. “The health sector needs a wide variety of models to achieve a wide diversity of purposes,” Ebi stressed (Box 2-1). Scientists develop different models with different sets of assumptions because “we don’t know what the future is going to look like,” she said. Rather than wait until thresholds are crossed, models help scientists to “understand the thresholds before we have lots of cases of impact.”
One use of models that many researchers are exploring is to inform climate services that enable people and institutions to make decisions about their actions, said Zaitchik. For example, satellite data on local soil-surface temperatures and moisture levels can be used to develop models of malaria risks. Zaitchik and colleagues have developed an application that funnels such information to local malaria clinics.
Goals of Modeling Health Risks Posed by Climate Change
Ebi noted that models of health risks posed by climate change are needed to fulfill diverse purposes. She emphasized that modeling discussions should begin with communication about why a model is being developed or used. Ebi listed some of the purposes for which scientists are developing models:
- To understand exposure–response relationships, such as the health effects of heavy precipitation.
- To look into the future on the basis of what is known about current exposure–response relationships.
- To develop better early warning systems, which are not yet designed to address what may happen in a changing climate.
- To assess costs and benefits of health risks climate change or climate adaptation policies.
Modelers are grappling with improving the use of models for policy decisions by producing “deeply coupled” models that incorporate information about the climate, the physical environment, and human actions, Zaitchik said. Such models could be used to investigate whether, for example, the 2003 heat wave in Europe had an effect on European Union climate policy or whether Superstorm Sandy had an effect on US policy. A paradigm shift may be required to get to that point, he observed.
“We are talking about a lot of things when we talk about modeling the health effects of climate change,” Balbus said. “We need to think in an integrated and holistic fashion about the total burden, but assessing it quantitatively will require considering some of the effects separately,” he explained. For example, before modelers are able to develop complex, integrated, quantitative models, the effects of air pollution may need to be assessed separately from the effects of nutritional stresses related to effects on the agricultural system. Ultimately “many approaches need to be brought together and figured out in one place,” he said.
Ebi cautioned against overinterpreting the numerical outputs of models. Rather, she said, “we are modeling for insights”—trying to understand how systems work and what may happen in a particular set of conditions. “We need to be more explicit that these are Bayesian experiments. We are gaining insights from models to inform what we do, how we go about things, and what needs to be done in terms of policies and budgets,” she concluded.
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