Colin Carlson, a young disease ecologist and faculty member at Georgetown University, has become increasingly alarmed about the impact of climate change on infectious diseases. In a radio interview, he was asked whether the resurgence of locally transmitted malaria in Florida and Texas was linked to climate change, and his concern about the planet’s trajectory had boiled over.
Carlson expressed his fear based on a paper he had read, which suggested that if atmospheric carbon dioxide levels exceeded 1200 parts per million, Earth’s clouds could start to disappear. He emphasized that this would be a significant, potentially catastrophic change in the planet’s functioning, rather than a small, incremental shift.
His passion for addressing climate change and its consequences reflects the growing urgency among scientists and experts to address the escalating threats posed by global warming. Climate change is impacting various aspects of our planet, from infectious diseases to extreme weather events, and researchers like Carlson are committed to raising awareness and seeking solutions to mitigate its effects.
Colin Carlson emphasizes the complex relationship between infectious diseases and climate change. While there is a plausible connection between the warming climate and the spread of diseases carried by mosquitoes and ticks, attributing specific cases to climate change can be challenging.
For instance, he explains that cases of malaria in Florida and Texas may not be directly attributable to climate change because the mosquitoes capable of carrying the disease have always been present in southern states. Instead, the introduction of the parasite by a traveler returning from abroad can lead to local transmission. Carlson notes that such cases are “weird edge cases” that resemble the types of situations climate change might bring about.
Attributing infectious disease outbreaks to climate change is a complex endeavor, similar to attributing droughts or hurricanes to climate change. Researchers like Erin Mordecai at Stanford University are working on developing methods to quantify the impact of temperature changes on disease transmission, aiming to determine how much climate change contributes to the rise in cases for specific diseases.
The link between climate change and infectious diseases is incredibly complex due to the multitude of factors at play. Climate change influences various environmental variables, including temperature, humidity, rainfall patterns, animal behavior, and human behavior. Each of these factors can affect different diseases in unique ways, depending on the type of pathogen (virus, bacterium, parasite, or fungus), the transmission vector (mosquitoes, ticks, water, or air), and the geographical region.
Researchers are faced with several questions:
- How will existing diseases be affected as their current habitats and climates change?
- Will diseases expand into new regions due to changing climatic conditions?
- Will climate change promote the emergence of new diseases?
Answering these questions is vital for public health preparedness, but it’s also a challenging scientific problem coupled with communication difficulties.
Researchers like Colin Carlson and Erin Mordecai have made progress in understanding the connection between climate change and diseases like malaria. Carlson, for example, has presented strong evidence that climate change has contributed to increased malaria transmission in certain parts of Africa. However, the overall picture is far from straightforward, and researchers are working diligently to unravel the intricate relationship between climate and infectious diseases.
The Intersection of Malaria, Mosquitoes, and Temperature: Unraveling the Complex Relationship
It was Mordecai’s fervent study of grasses that initially piqued her curiosity, but it was the urgency of a different issue that would alter the course of her research. While pursuing her graduate studies at the University of California, Santa Barbara, her passion for mathematics and biology led her to investigate how plant pathogens transferred from invasive grass species to native ones. However, a paper by her advisor, ecologist Kevin Lafferty, shed light on a more pressing concern – the non-linear relationship between human diseases and climate. This revelation intrigued Mordecai, prompting her to shift her focus.
Her new pursuit involved a deep dive into the impact of temperature on mosquitoes and the pathogens they carry. As cold-blooded creatures, mosquitoes are profoundly influenced by temperature, shaping virtually every aspect of their lives and their potential to spread diseases. Temperature affects their lifespan, the number of eggs a female mosquito lays daily, and their likelihood of biting. It also plays a pivotal role in determining how mosquito-borne pathogens establish themselves within the insect after it feeds on the blood of an infected host and how long this process takes.
In laboratory settings, researchers can meticulously study how temperature influences each of these traits. The general relationship appears hump-shaped, featuring an optimal temperature at which these traits thrive, with exponential declines in performance as temperatures move away from this ideal. Courtney Murdock, an infectious disease ecologist at Cornell University, compares this pattern to human preferences, saying, “There are some temperatures that you experience that are too cold, and some temperatures that are too bloody hot. And there’s the temperature that you’d like to hang out at, usually around 75°F.
ALSO READ: Shocking Revelation – Dark Secrets Behind Elon Musk’s Neuralink Monkeys Experiment Exposed
For years, experts believed the temperature optimum for malaria transmission was around 31°C. However, Mordecai and her colleagues challenged this notion in a 2012 paper published in Ecology Letters. Their research revealed a significantly lower temperature optimum of 25°C, with a rapid decline in transmission rates above 28°C. Mordecai notes that climate warming may actually decrease malaria transmission in many regions of sub-Saharan Africa. Conversely, areas like the highlands of South America and southern Africa are experiencing conditions increasingly favorable to malaria transmission. Meanwhile, previously malaria-controlled regions in Europe and North America may face heightened challenges in maintaining the disease’s containment.
However, it’s essential to acknowledge that focusing solely on the relationship between mosquitoes, diseases, and temperature overlooks numerous other climate-related factors. Changing rainfall patterns, more frequent extreme weather events like droughts and floods, and human migration in response to these global shifts all leave their mark on disease patterns.
Unraveling the Climate-Disease Link Through Historical Data
To gain insights into the intricate relationship between climate change and disease, some scientists are delving into historical data to detect signs of climate-induced shifts in disease incidence. This approach involves examining past records to ascertain whether climate change has already impacted real-world disease patterns. One particularly contentious dataset, concerning malaria cases in the Kericho tea estates of eastern Kenya, has been at the center of a decades-long scientific dispute.
The Kericho tea estates, managed by Brooke Bond Kenya Ltd., offered healthcare services to employees and their families, maintaining meticulous health records. In the late 1990s, Dennis Shanks, a scientist at the U.S. Army Medical Research Unit in Nairobi, made a significant discovery. He uncovered boxes of malaria admission and case records dating back to 1965, hidden in a loft at the Kericho tea estate. This find opened the door to a wealth of historical data.
Shanks, along with Robert Snow, a malaria researcher at the Kenya Medical Research Institute, and other collaborators, digitized and analyzed these records. In 2000, they published their findings, which revealed a substantial increase in malaria cases during the 1990s. Intriguingly, they noted that the mean monthly temperature in the region had not experienced significant changes during the same period. This discrepancy prompted them to suggest that factors other than climate change, such as lapses in mosquito control programs and an outbreak of drug-resistant malaria, may have been responsible for the surge in cases.
This publication ignited a spirited debate within the scientific community. In 2006, Mercedes Pascual, a computational ecologist at New York University, revisited the temperature data from the Kericho estates and asserted that a warming trend did indeed exist when examining a longer time frame. She argued that the highlands, where the estates were located, should have been among the places where the most significant impacts of climate change on malaria transmission were observed.
The debate persisted, with multiple reanalyses of the medical data and attempts to correlate it with climate data. Robert Snow, one of the original paper’s authors, expressed concerns that the growing emphasis on climate change could divert attention away from more immediate challenges related to malaria control.
On the contrary, Mercedes Pascual disagreed with Snow’s perspective. She argued that research efforts, funding, and attention dedicated to malaria have historically outweighed those directed toward climate change. Only recently has research on climate-disease interactions begun to gain more substantial traction within the scientific community.
A Child Prodigy’s Research Unveils Climate-Malaria Relationship
Ethan Carlson, a child prodigy who entered the University of Connecticut at the age of 12, is gaining attention for his research that seeks to elucidate the influence of climate change on malaria transmission. Carlson’s scientific journey began by examining the likelihood of parasites facing extinction due to climate change, eventually leading him to investigate the broader impact of climate change on diseases caused by these parasites. Personally invested in climate change due to his age, Carlson acknowledges that he will experience more significant effects of climate change than many of his colleagues.
To investigate the climate-malaria relationship, Carlson turned to climate econometrics, a field previously employed to estimate how human-induced climate change affects factors like food production and conflict. This methodology involves constructing elaborate “what if” scenarios. Carlson and his team utilized a comprehensive dataset published by Robert Snow in 2017, encompassing 115 years of malaria data in sub-Saharan Africa. They developed 1000 plausible models to assess how temperature shifts, floods, and droughts could have contributed to changes in malaria incidence. By simulating thousands of alternative worlds with no climate change, they compared malaria incidence in these hypothetical scenarios with the actual world. This approach allowed them to estimate the likelihood that climate change exacerbates the disease.
In a preprint published earlier this year, Carlson’s team reported a 66% probability that human-induced climate change has already increased malaria cases in sub-Saharan Africa as a whole. They estimated that, for every 100,000 children between the ages of 2 and 10 in the region, 84 malaria cases can be attributed to climate change at any given time. Although this may seem modest, given the approximately 300 million children within that age range in sub-Saharan Africa, Carlson emphasized that over 200,000 children currently suffer from malaria solely due to climate change. The highest impact of climate change on malaria incidence is observed in eastern and southern Africa, while west and central Africa have likely experienced a reduction in malaria transmission due to climate change.
Carlson underscored the importance of public health interventions, such as bed nets, vector control measures, and treatments, which have achieved substantial reductions in malaria cases. He emphasized that climate change matters but added, “Public health interventions have been 200 times more important.”
Despite these findings, Carlson’s study offers a glimmer of hope. It suggests that if global warming is limited to 2°C, the trend could eventually lead to a net reduction in malaria cases across the continent by the middle of the century.
Impacts on Other Mosquito-Borne Diseases and Challenges Ahead
While some findings may provide reassurance for malaria, the situation varies for other mosquito-borne diseases. Mordecai’s transmission curves reveal that some diseases have significantly higher temperature optimums than the most lethal form of malaria, caused by the Plasmodium falciparum parasite transmitted by Anopheles gambiae mosquitoes. Diseases like Rift Valley fever, Ross River virus, dengue, and Zika, all positioned to the right of falciparum malaria on the temperature diagram, are expected to experience and have already experienced more substantial increases due to climate change.
Dengue is particularly concerning, as it already causes hundreds of millions of infections and an estimated 20,000 deaths annually. The threat is likely to intensify as global temperatures rise. Mordecai explains that, while climate change affects malaria in various ways depending on the region, it consistently drives dengue in the same direction everywhere. Urbanization and insufficient mosquito control measures have contributed to the resurgence of the dengue-carrying mosquito Aedes aegypti in many areas. Dengue is predicted to worsen and is currently doing so.
For malaria, any reprieve may be short-lived. Mosquitoes and parasites could adapt to higher temperatures. Additionally, another mosquito species carrying the same parasite, Anopheles stephensi, could become more significant. Stephensi, responsible for urban malaria, is expanding across Africa and tolerates much higher temperatures.
These complexities are also applicable to diseases that do not rely on insects for transmission. Influenza, for instance, exhibits a seasonal pattern, surging during winter. This pattern is not solely due to people gathering indoors but also because cold, dry air favors virus transmission. Changes in air temperature and humidity will likely influence the virus’s spread and potentially its evolution. However, the precise mechanisms remain unclear, with disease ecologist Jessica Metcalf from Princeton University describing the situation as “an unbelievably tortured tangled thing.”
Conveying Complexity and Urgency
Effectively communicating the complexity and uncertainties surrounding the relationship between climate change and diseases like malaria can be challenging. While simple statements like “mosquito-borne pathogens will thrive in a warming world” may be appealing, they can also be misleading. On the other hand, conveying nuanced information, such as the fact that climate change may result in more malaria in some areas and less in others, risks obscuring the urgent message that climate change poses a significant threat to public health.
ALSO READ: Boosting NASA’s UFO Investigations: The Case for Increased Budget Allocation
Climate researcher Aaron Carlson acknowledges the difficulty of striking the right balance in communication. He believes that the science is not being “walked back” but rather that there is a high rhetorical bar set for scientific findings to be considered a dramatic health impact.
Soon after Carlson’s radio interview, a new communication challenge emerged as another locally transmitted malaria case was reported in the United States, this time in Maryland, where such cases had not been seen for 40 years. While this development is indeed unusual, climate change is not the likely cause of this malarial threat—at least not yet.
