Navigating complexity to address how climate change affects population health

Jennifer McPartland, PhD’08, no longer works in advocacy, but spent a decade working for the Environmental Defense Fund (EDF) as the Senior Scientist for Healthy Communities. There, she spent years working to reduce the population’s exposure to harmful chemicals by pushing for effective federal policies and voluntary market-based leadership. Her policy efforts primarily focused on the “universe of chemicals” regulated under the Toxic Substances Control Act (TSCA). Over time, she said, “ancillary issues like climate change became an explicit part of that conversation, as we worked out how to manage and reduce exposure to harmful chemicals.”

When it came to her work, the most obvious intersection was between toxic substances and extreme weather events. “Increased severity of weather events can lead to increased emissions of toxic substances,” she said. “For example, instances where flooding causes previously contained compounds in the environment to disperse, or pollutant emission spikes that occur as part of refinery and petrochemical facility shutdown procedures when hurricanes strike.”

As a science-based organization, EDF is engaged in academic research collaborations to better understand and predict how different weather events may increase harmful emissions from various types of facilities.

“The aim of these efforts is to help strengthen the Environmental Protection Agency’s efforts to meaningfully and effectively protect local communities surrounding such facilities who bear the brunt of these emissions,” McPartland said.

Now, serving on a National Academies committee on environmental health, McPartland spends a lot of time thinking not just about environmental exposures and general population health, but specifically about more vulnerable segments of the population.

“Climate change only exacerbates existing issues,” said McPartland, who has a doctorate in microbiology and molecular biology. “When we’re contending with the complexities of addressing how pollution disproportionately affects vulnerable communities, climate change tends only to make situations worse. For example, air pollution that is related to or induced by climate change will disproportionately impact communities with higher rates of asthma or cardiovascular disease. Communities with poor infrastructure are more susceptible to flooding and the consequences of flooding.

“These burdens, these harms, the future they don’t affect all people equally,” she said. “It’s an ethical imperative that we don’t treat the globe or the United States like a monolith.”

McPartland points to the challenges of navigating complexity as a central difficulty when enacting policies to meaningfully address the ways climate change is affecting population health. “With climate change, as with many things, policy practices often lag behind current scientific understanding. As an example, up to this point, regulators typically assess potential chemical risk in isolation one chemical at a time. Evaluation of potential risks from co-exposures to multiple substances i.e., the real world is rare. Fortunately, practices are starting to improve, but this needs to accelerate.”

She says that while there is growing awareness that climate change is affecting our health, it’s not always clear to the average citizen. “People fairly easily understand climate change, extreme weather events and destruction,” she said. “But that climate change can directly affect our health and well-being may be less intuitive for some. And even when the recognition is there, trying to get your arms around it and identify solutions is really, really tough.”

Infectious disease and climate change

A growing body of research indicates that a warming planet will exacerbate transmission of pathogenic diseases. Temperature and precipitation changes are associated with expanded ranges for disease vectors such as mosquitoes and ticks; warmer winters may allow pathogens to more readily survive and promote disease outbreaks. The disruption of animal habitats by drought, wildfires and other extreme weather events brings an increased risk of pathogen spillover as animals and humans share space, while flooding can cause wastewater overflow and contamination of water and food sources.

Zoonotic diseases, human/animal dynamics and climate change

Interviewing Cara Brook, PhD, can be a bit of a challenge. An assistant professor in the Department of Ecology and Evolution, she spends much of her time halfway around the world in Madagascar, where she studies viral dynamics in Old World fruit bats. In a post-COVID world, the public is more aware than ever that bats can host zoonotic diseases that can make the jump to humans. Brook says this is due to the uniqueness of the bat immune system.

“Bats are the only flying mammals, and flight is extremely metabolically costly. To evolve the ability to fly and mitigate this metabolic damage, bats had to evolve numerous unique anti-inflammatory properties that appear to also allow them to better tolerate viral infections without experiencing extreme disease,” she said. “As a result, we think that bat viruses have evolved traits, such as high replication rates, that are not harmful to the bats but cause massive pathology when they spill over to humans and other animals.”

Brook doesn’t necessarily consider herself a climate change researcher, but she does see the relationship between climate change and her work on understanding how viruses persist in bat populations and the dynamics of bat-to-bat transmission, particularly because of how climate change places stress on the animals. “We see spikes in virus transmission during the winter season, which is when bats are experiencing a nutritional deficit,” she said. “These spikes might lead to increased risk of exposure for humans and, as a result, increased risk of a spillover. Climate can drive nutritional stress through a number of different factors, such as changing the flowering or fruiting times for food resources and extreme weather events that can destroy trees and eliminate fruit crops.”

The increased risk of bat-to-bat transmission occurs when more bats congregate at fewer food resources, putting more animals in the same location at the same time. Additionally, when bats are stressed, a cascade of responses, including down-regulated immunity and heightened viral shedding, means that the bats are more infectious than at other times of the year. External factors that increase the stress placed on bats drive transmission rates even higher. Deforestation due to human activity and extreme weather events increase the risk of zoonotic spillover as humans and bats come into closer contact.

Part of the challenge in understanding how climate change can affect the spillover of zoonotic diseases is that the relationship can be very heterogeneous. Things like humidity, precipitation and temperature can all affect disease transmission, while extreme weather events can create such unexpected environmental factors as standing water.

“Disease responses are nonlinear,” said Brook. “Different pathogens might respond to something like an elevated temperature in different ways. For example, the white-nose fungus that has decimated North American bat populations tends to