Motivation

Malaria is a vector-borne disease caused by Plasmodium parasites, which are transmitted to human hosts through bites of infected female mosquitoes belonging to the Anopheles genus. The most prevalent form of the parasite in Africa, P. falciparum, kills about 400,000 people annually¹. Both rainfall and temperature impact the presence and the spread of Anopheles mosquitoes. Rainfall provides breeding sites while temperature affects mosquito’s biting, development and mortality rates.

A mosquito becomes infected when biting an infected human host. Then, an incubation period is necessary for the parasite to develop in the mosquito’s body before it becomes infectious and can transmit the disease to another human host. This incubation period, commonly referred as to the sporogonic cycle, is temperature-dependent: the higher the temperature, the shorter the development period².

Current climate change already affects rainfall and temperature³. Future climate change, in conjunction with other critical socio-economic factors, might impact vector-borne diseases such as malaria further.

First, simulations of future climate and instabilities

To predict changes in vector-borne diseases risk, it is preliminarily necessary to simulate how climate will evolve in future. Climate models driven by greenhouse gases emission scenarios, such as standard IPCC RCPs scenarios, are utilized to estimate future climate trajectories⁴. The RCP scenarios depict a more or less optimistic future based on the quantity of greenhouse gases that could be emitted in the upcoming decades and centuries. The most pessimistic scenario, RCP8.5, corresponds to a significant increase in CO2 emissions, and does not consider the necessity of reducing fossil energy consumption in future. Sadly, in view of recent emissions trends, the observed trajectory is close to this extreme scenario. Several studies have used RCP scenarios to run malaria mathematical models to estimate the risk of malaria transmission in future. Such studies estimate an increase in malaria risk over altitude regions in Africa at the end of the 21^st century, while risk could decrease over the warmest plains due to extreme temperatures limiting mosquito survival⁵.

These 'standard' climate change scenarios do not consider the impact of non-linear melting of the polar ice sheet on climate. The study of paleoclimates has revealed that Greenland ice sheet melting is not a continuous process. Indeed, specific past periods such as ‘Heinrich events’ correspond to periods of iceberg debacle⁶. These rapid melting processes induce climatic instabilities that obviously have a very important impact on global climate change.

We based our study on the RCP8.5 scenario to which we added an idealized and rapid Greenland ice sheet melting. We simulated the ice sheet melting by adding freshwater inputs into the North Atlantic Ocean for 2020-70 using the IPSL (Institut Pierre Simon Laplace) climate model⁷ (Fig. 1). As a matter of fact, the amount of freshwater released over this period corresponds to an additional sea level rise (SLR) of 1 m, and we also tested a more extreme scenario (corresponding to +3m SLR). Such rapid ice melting induces changes in the Atlantic meridional overturning circulation (AMOC) that impacts the atmospheric circulation at large scale, thus leading to significant changes in temperature and precipitation over the African continent.

Second, simulations of malaria transmission risk in future

We used calibrated climate model outputs for the standard RCP8.5 scenario and for the ice melting experiments to drive an ensemble of mathematical malaria models of varying complexities (Fig. 2).

Based on the standard RCP8.5 simulation, malaria models suggest a decrease in malaria transmission over the Sahel in future due to high temperatures that negatively impact mosquito survival. Conversely, a temperature increase allows temperatures to become more suitable for malaria transmission over the East African plateau (Fig. 3).

If we focus on the ice melting scenario, temperature increases more moderately leading to a more moderate increase of future malaria risk over East Africa. Importantly, additional sea-ice melting in Greenland causes a southward shift of the African rain-belt. This shift amplifies drought conditions over the Sahel, leading to a more pronounced decrease in malaria transmission in future. Rainfall significantly increases over South Africa, leading to a significant increase in malaria transmission over Southern Africa (Fig. 3).

We argue that rapid climate destabilization scenarios, such as an accelerated ice melting at high latitudes and permafrost melting, should be investigated further in future climate change risk assessments.

References

1. World malaria report 2020: 20 years of global progress and challenges. Geneva: World Health Organization; 2020. Licence: CC BY-NC-SA 3.0 IGO.
2. Shapiro, Lillian LM, Shelley A. Whitehead, and Matthew B. Thomas. "Quantifying the effects of temperature on mosquito and parasite traits that determine the transmission potential of human malaria." PLoS biology 15.10 (2017): e2003489.
3. Pachauri, Rajendra K., et al. Climate change 2014: synthesis report. Contribution of Working Groups I, II and III to the fifth assessment report of the Intergovernmental Panel on Climate Change. IPCC (2014).
4. Moss, R. H. et al. The next generation of scenarios for climate change research and assessment. Nature 463, 747–756 (2010).
5. Caminade, C. et al. Impact of climate change on global malaria distribution. Proc. Natl Acad. Sci. 111, 3286–3291 (2014).
6. Hemming, S. R. Heinrich events: massive late pleistocene detritus layers of the north atlantic and their global climate imprint. Rev. Geophys. 42, RG1005 (2004).
7. Defrance, D. et al. Consequences of rapid ice sheet melting on the Sahelian population vulnerability. Proc. Natl Acad. Sci. USA 114, 6533–6538 (2017).