Abstract
Despite recent advances in climate change attribution, many health impacts remain unmeasured1. Here we leverage over a century of clinical data2 to investigate whether human-caused climate change has increased the burden of childhood malaria across sub-Saharan Africa. We find a robust effect of temperature and extreme precipitation on prevalence, consistent with previous findings at local scales and in laboratory experiments. We estimate that rising temperatures have probably increased malaria in East and southern Africa, but averted a comparable number of cases in West Africa, with a net impact of 1 excess case per 1,000 children (95% confidence interval (CI) −4 to 6) across the continent. Over the coming century, we project that climate change could marginally accelerate the elimination of malaria in West and central Africa, where the present-day burden is highest; across the continent, this could avert 1 (low greenhouse gas emissions: 95% CI −2 to 6) to 20 (high greenhouse gas emissions: 95% CI 0–52) cases per 1,000 children by the end of the century. However, reducing future global warming from 3 °C to under 2 °C could prevent an average of 5 excess cases per 1,000 in high-elevation (more than 1 km) East Africa and in southern Africa (95% CIs −3 to 13 and −4 to 14, respectively) by 2100. Our study resolves a decades-old debate about one of the first suspected health impacts of climate change, providing a template for future work measuring its true global burden.
Research Highlights
What role has climate change played—and what role will it play—in the spread of childhood malaria?
Context
Malaria is the single deadliest climate sensitive infectious disease. It hits children the hardest, with 95 percent of global malaria cases being children ages 2-10 and 80 percent of global malaria deaths being children under 5. Though malaria is present in countries around the world, the vast majority of cases are in Africa.
Malaria is spread by a parasite carried by mosquitoes. Both the parasite and the mosquitoes that carry it are highly impacted by temperature. Lab-based studies have found that mosquito-based transmission of malaria peaks at around 25°C (77°F), and becomes far less likely when the temperatures are either below 16°C (61°F) or above 34°C (93°F). Because of this, some regions of southern Africa and high-elevation East Africa have historically been cold enough that malaria has not been a significant problem. Instead, the risk has concentrated in West Africa and Central Africa.
Researchers and public health officials have long argued over the role that human-caused climate change has played in shaping past and present malaria burdens. This study resolves this debate, quantifying climate change’s historical fingerprint and its future impact on childhood malaria across sub-Saharan Africa.
Research Design
While many studies have tested the links between climate and disease occurrence, often in controlled laboratory settings, very few have been able to causally trace how human-caused climate change has contributed to observed population-scale changes in disease prevalence. This study provides that direct causation for one of the world’s most deadly infectious diseases.
The authors use a comprehensive dataset of over 50,000 geolocated blood sample surveys across Sub-Saharan Africa spanning more than a century that captures a snapshot of the amount of malaria in the population at any moment in time. From that, they build a model linking state/province level malaria prevalence averages for children between 2 and 10 years old to temperature and precipitation, separating out other factors such as economic development, heath care, and population changes.
Having quantified in observational data how the climate influences the prevalence of malaria, they combine this statistical model with climate model outputs to compare a world with human-caused climate change to a simulated counterfactual world without it. This allows them to estimate how much climate change impacted malaria prevalence rates between 1901 and 2014 at subnational level. Finally, they project how future climate change could further alter malaria cases between 2015 and 2100, based on low, intermediate and high future greenhouse gas scenarios.
Findings
Climate change has caused 1 excess case of malaria for every 1,000 children ages 2 to 10 (a 0.5 percent increase), with southern and east Africa experiencing far greater increases.
The study finds that climate change has likely increased childhood malaria in east and southern Africa since 1901, but averted a similar number of cases in west Africa. The net effect of this shifting burden is approximately 1 excess case for every 1,000 children ages 2 to 10. This spatially shifting burden falls strongly along latitudinal and elevational lines. For example, the Ethiopian highlands are cool, and climate change has made them warmer and more suitable for mosquitoes to spread malaria. The spread of malaria increased by over 8 cases per 1,000 in this region over time (a 5 percent increase), with similarly large increases in colder southern latitudes. In contrast, in west Africa, temperatures exceed optimal transmission levels and climate change has already begun to suppress the spread of the disease, avoiding 4 cases per 1,000 per year by 2014, reducing prevalence by 1-2 percent. In the central African basin, the current extremely high rates of malaria prevalence have been largely unaffected by climate change, as baseline temperatures sit very close to the optimal transmission climatology.
Climate change could reduce malaria by about 2 percent (5 cases per 1,000 children) as temperatures get too hot for the disease by the end of the century, helping eradicate the disease where it is currently most rampant.
Across sub-Saharan Africa, past increases in malaria are expected to be offset by mid-century as it becomes too hot for mosquitos to transmit the disease in many regions. By the end of the century, climate change is expected to decrease prevalence by 5 cases for every 1,000 children. Of course, these estimates depend on how future emissions unfold: in a world where there is more stringent emissions reductions (low emissions), about 1 case per 1,000 children is averted, while under the worst climate change scenario (high emissions), transmission reduces substantially and average prevalence falls by 20 cases per 1,000 children, a 9 percent reduction.
As with climate changes’ historical footprint, the future outlook tells a story of shifting burdens. Future climate change will accelerate efforts to eradicate malaria in the places where temperatures become the hottest: west and central Africa. Under an intermediate future climate change scenario, cases decrease by about 3 fewer cases per 1,000 children in central Africa (a 1 percent reduction), and 16 fewer cases per 1,000 children in west Africa (a 5-6 percent reduction). In areas like Nigeria and the Democratic Republic of the Congo—today’s malaria hotbeds—climate change could decrease malaria cases by just as much as eradication efforts such as bed nets and antimalaria drugs have in these countries today.
While average prevalence declines as temperatures rise across sub-Saharan Africa in coming decades, transmission continues to shift along latitudinal and elevational lines in temperature. Localized increases in prevalence will be felt in high-elevation east Africa and in cooler parts of southern Africa, potentially reaching increases of up to 30 cases per 1,000 children (about a 20 percent increase) in an intermediate emissions scenario by end-of-century in regions like the Rift Valley and coastal southern Africa.
Limiting future global warming to under 2°C could prevent 5 excess cases per 1,000 children in southern Africa and in high-elevation regions by 2100.
Broadly, the study shows that mitigating climate change keeps average temperatures in sub-Saharan Africa closer to the optimum range for malaria transmission. However, for many cooler areas, such as high-elevation east Africa and much of cooler southern Africa, reducing greenhouse gas emissions may prevent substantial climate change-driven increases in malaria. These are regions where malaria burdens are low today, and thus where health systems must adapt to an evolving threat.
For example, by mid-century, limiting global warming to below the 2°C limit in the Paris Agreement is projected to prevent an estimated 2 cases of malaria per 1,000 children in southern Africa compared to an intermediate emissions scenario that reaches 3°C. By the end of the century, these benefits could be even greater, with 5 cases averted per 1,000 children in southern Africa and more in the east African highlands where warming substantially elevates transmission.
Closing Take-Away
The study is the most comprehensive look to date at the impact of climate change on any infectious disease and brings new clarity to a decades-long debate over climate change’s impact on malaria. It finds that programs to fight malaria should take climate and future temperature change into consideration, especially in areas where malaria could worsen in the future. Decision-makers in these regions should be prepared to implement more aggressive programs. Disease surveillance and control, improved healthcare, and economic development can easily counter-balance climate change impacts in most places. At the same time, more work is needed to make the near-term climate predictions—what to expect over the next year to decade—that decision-makers need to control malaria today.
