Indirect impacts of climate on human health are ones in which the human body is not immediately affected by the climate system but instead feels the delayed impacts through some intermediary. Indirect impacts affect many more people in many more ways than the direct impacts. The list is long and includes, among others:
Several food-borne pathogens are spreading due to climate change. Depending on the crop type, drought and excess moisture encourage crop pests and spread molds. Climate-driven rusts, blights, and rots are devastating already stressed crops and thereby indirectly affecting human health through decreased food security.
But it's not just our crops falling ill from increased pathogens in a changing climate. Instances of some types of pathogens that make their way to people are also on the rise. The US Global Change Research Program's Climate and Health Assessment from 2016 describes some of these pathways and consequences (and in their bulleted list [2], they include ample citations to support each bullet point - I'm linking out to that directly in hopes the links there are updated as necessary):
Water-borne diseases are closely related to weather and climate. Cholera, cryptosporidiosis, and several other significant diseases are spread by fecal contamination of water supplies and are often closely associated with floods and heavy downpours. In other cases, drought can concentrate disease pathogens in pools and low flows. Climate change is causing increased intense rainfall events in many parts of the world and drought in others; it is not surprising that water-borne illness is a growing problem in those areas with the most extreme changes. The key to understanding the spread of water-borne illness in a changing climate is exposure. As this image shows, various factors (all from human activity) are increasing our exposure to these illnesses.
Disease vectors, such as mosquitos and ticks, are moving into new areas as warmer, wetter climates push poleward and upslope. The spread of malaria and dengue fever, for instance, is believed to be at least partly due to mosquitos expanding their range with the changing climate.
Click through this series of maps illustrating the projected expansion of mosquitos carrying Aedes-borne viruses (dengue fever* and Zika among them). These projections are based on an RCP (representative concentration pathway [5]) of 8.5 which is really a worst-case scenario in which we continue on with our business as usual practices and fail to mitigate climate change. So, with any luck, this won't happen! But, you can also see a set of projections based on a more plausible RCP with the original paper (Ryan et al., 2019 [6]). These projections demonstrate the rapid increase in exposure for highly populated areas, some expansion into new areas - meaning newly vulnerable populations.
*In 2019, countries across tropical Asia saw higher than usual instances of dengue fever.
Let's take a look at another vector, ticks. Look at the massive growth in reported cases of Lyme Disease [10] between 1996 and 2014. Many of you live in Pennsylvania or surrounding states, so you're likely all too familiar with ticks and Lyme Disease. Most of us have a story or two of pulling a deer tick off ourselves, our children, or our pet. I've spared you a close-up image of a deer tick on this page, you're welcome.
Rodents and their fleas are also a concerning disease vector with changing distributions in a changing climate. For example, Hantavirus borne by fleas on deer mice is closely linked to the ENSO cycle in the US Southwest (Hjelle and Glass, 2000 [12]), and evidence points to climate-related distribution changes in Europe of Hantavirus borne by other rodents (European Center for Disease Prevention and Control, n.d. [13]). In Brazil, warmer temperatures mean an increased expansion of sugarcane production. Rodents (particularly deer mice) find sugarcane quite tasty and estimates suggest that this combination of increased sugarcane production and warming temperatures could expose an additional 20% of Brazilians [14] to this potentially fatal virus.
Sometimes, when I'm having a conversation about climate change (as one does!), I hear people talk about our greenhouse gas emissions as pollutants. I've personally never really thought of them that way (even though, perhaps I should because they do overlap), because I tend to think of air pollution as particulate matter - soot, big puffs of black exhaust from the back of a truck, things like that. Greenhouse gas emissions seem categorically different to me - like secret pollutants we can't see. However, it's important to understand that regardless of how you conceive of GHGs and pollutants, our GHG emissions are affecting the more traditional air pollutants in ways it's important for us to understand.
Air quality and its effect on human health is in large part a function of the weather, which creates conditions that promote or depress the formation, concentration, deposition, dispersion, and transport of pollutants. Changing climate means changes in location, timing, and intensity of air pollution events. Ground-level ozone, the most noxious constituent of photochemical smog, is associated with a host of health problems and is strongly and positively correlated with temperature and solar radiation; increases in temperature and longer summer seasons correspond to increases in general ozone concentrations and the number of days violating air quality standards. Here's a short EPA podcast from 2016 called The Future of Breathing: Connecting Air Quality and Climate Change [15] I'd like you to spend 2 minutes on (or read its transcript).
Particulate matter (PM) has even greater health effects than ozone; this pollutant is also linked to temperature and humidity in its formation. Forest fires also release many particulate air pollutants and toxic gases and are known to affect health. Forest fires are on the increase worldwide and forecast to escalate dramatically in the future.
Let's circle back to thinking about vulnerability and how people might experience this type of impact of a changing climate differently than those around them. The American Lung Association [16] estimates 26 million people in the US are living with asthma. Poor air quality is just one potential indirect impact on human health that affects people with asthma more acutely.
One of the biggest climate impacts on human health is through seasonal allergens. Over the past few decades, spring is coming earlier and fall is ending later in the Northern Hemisphere, increasing the length of the allergy season. (Do you hear the collective groan of the roughly 30% of the population who suffer from seasonal allergies?) Changing distributions of plants and molds (thanks to extreme precipitation events and changing temperature patterns) are causing the spread of allergens into areas where they did not exist earlier. There is also some evidence that the rising atmospheric CO2 concentration is fertilizing some allergen-rich species, like ragweed (Albertine et al., 2014 [18]).
What do you notice about this map? The northern latitudes of the US and the western part of the country are seeing rapidly expanding numbers of frost-free days relative to the southern US. It's just another reminder that while impacts are global in nature, they materialize very differently across smaller geographic scales.
For this week's lesson, let's focus specifically on the effects climate change impacts can have on one's mental health. This is separate from thinking about the mental health implications associated with people worrying about climate change in general (which is a rapidly growing issue). We'll probably have a chance to tackle that in Unit 3: Solutions.
This area seems a bit understudied compared to the more obvious impacts and isn't discussed as much publicly. Flooding, droughts, Lyme Disease, asthma - we've talked about a lot of potential issues connected to climate change that might seem more connected. But, the mental health implications of living through many of these impacts is of serious consequence and worth our time to better understand.
Mental health is inextricably linked to physical and community health (see those bidirectional arrows between the physical, mental, and community symbols - it's all connected!).
Links
[1] http://www.fao.org/tempref/docrep/fao/011/i0378e/i0378e.pdf
[2] https://health2016.globalchange.gov/food-safety-nutrition-and-distribution#table-125
[3] http://climateandhealthalliance.org/uncategorized/climate-change-and-food-safety/
[4] https://health2016.globalchange.gov/water-related-illness
[5] https://www.ipcc-data.org/guidelines/pages/glossary/glossary_r.html
[6] https://journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0007213
[7] https://www.npr.org/2019/07/31/747012629/bangladesh-reports-more-than-13-000-dengue-cases-in-july?fbclid=IwAR0NQWLtp3v2EGO_8zHF2ySpBF1-5MHk6DUqZ0XU7WWWN7tXAhoSRXSwp7c
[8] https://www.straitstimes.com/asia/se-asia/philippines-declares-national-alert-after-456-dengue-deaths
[9] http://outbreaknewstoday.com/peak-dengue-season-in-singapore-8000-cases-5x-last-years-number-26932/
[10] https://www.cdc.gov/lyme/index.html
[11] https://www.cdc.gov/lyme/datasurveillance/index.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Flyme%2Fstats%2Findex.html
[12] https://academic.oup.com/jid/article/181/5/1569/2191142/
[13] https://www.ecdc.europa.eu/en/climate-change/climate-change-europe/rodent-borne-diseases
[14] https://www.reuters.com/article/us-brazil-climatechange-health/brazil-risks-rodent-borne-hantavirus-rise-due-to-sugarcane-climate-change-scientists-idUSKBN1A52IV
[15] https://19january2017snapshot.epa.gov/research/science-bite-podcasts_.html#future
[16] https://www.lung.org/about-us/blog/2019/05/asthma-and-climate-change.html
[17] https://www.epa.nsw.gov.au/about-us/publications-and-reports/state-of-the-environment
[18] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4221106/
[19] https://nca2014.globalchange.gov/report/our-changing-climate/frost-free-season
[20] https://s3.amazonaws.com/climatehealth2016/low/ClimateHealth2016_08_Mental_Health_small.pdf