A rapid melting of the Greenland ice cap is one of the known risks of climate change, in particular for the rise in sea level that it would cause.
Such a phenomenon would however have other more unexpected consequences, thousands of kilometers away. In Africa, for example, this melting could affect the transmission of malaria by mosquitoes, as we show in a recent study published in Nature Communications and bringing together researchers from different laboratories in France (LSCE), Italy (ICTP) and the United Kingdom (University of Liverpool).
As a reminder, malaria is a disease caused by the parasite plasmodium, which in 2020 caused the death of 627 people According to the latest WHO report, of which 96% in Africa. The continent also concentrates 228 of the 241 million cases reported in the world during the same year, or 95%. 77% of deaths also concern children under five.
Temperature and transmission
This transmission is not possible directly between human beings: the parasite needs a vector, in this case a female mosquito of the species Anopheles. When the latter bites a sick person, it absorbs the parasites present in the latter's blood, which develop in the body of the insect before being retransmitted to a new host during a subsequent bite.
The development time of the parasite in the mosquito, between its ingestion and its transmission, depends on the temperature: the higher it is, the shorter this time will be. The mosquito being an ectothermic insect, its body temperature depends directly on that of the environment. In the event of too slow development, linked to a low temperature, the mosquito will be dead before being able to retransmit the parasite.
Mosquitoes Anopheles are also sensitive to weather conditions. For them to live and grow, temperatures must be between 16 and 40°C. They also need water for the laying of eggs and the development of larvae, which takes place in an aquatic environment (puddles, ponds, etc.). On the other hand, too intense precipitation can destroy the eggs and the larvae.
The survival of mosquitoes threatened?
With global warming, temperatures in Africa are rising. Regions could become too hot during the XNUMXste century for mosquitoes to survive, this is the case in the Sahel region of West Africa.
Other areas, which were previously too cold to make sustained transmission of malaria possible, will on the contrary reach temperatures high enough to allow the survival of the mosquito, the development of the parasite and therefore theoretically the transmission of the disease; this is the case of the East African highlands.
This has been demonstrated through numerical simulations of the future climate. To achieve them, it is necessary to choose a scenario describing greenhouse gas emissions over the course of the century according to certain assumptions. From these greenhouse gas emissions, the model, which includes physical equations describing the climate system, simulates the climatic conditions of the coming century.
The precipitation and temperature values thus generated are fed into other numerical models allowing the study, this time, of the risk of transmission of malaria linked to the changes.
Ice cap melting and ocean circulation
However, climate models do not represent the full complexity of the Earth system. They do not take into account, for example, the impact of a possible rapid melting of the Greenland ice sheet.
However, scientists know thanks to the study of past climates, that a large quantity of ice is likely to be released in a sudden and difficult to predict manner. As it melts, it would supply fresh water to the North Atlantic Ocean, which is a key region of the global climate.
In this zone, the warm surface waters coming from the equator cool down and become salinized, because the seawater which forms the pack ice releases the salt it contains there. This densifies these waters which plunge towards the seabed by towing the ocean currents, following the principle of a treadmill. This movement is the engine of ocean circulation, known as “thermohaline circulation”.
The possibility of an influx of fresh water in the North Atlantic would therefore reduce the density of the waters and slow down this plunge in cold waters. If the engine of the thermohaline circulation is slowed down, the entire ocean circulation changes. The transport of heat by the ocean would then be slowed down.
Reduced temperature rise
The ocean and the atmosphere being in perpetual interaction, a transformation of the marine currents will also impact the atmospheric circulation, which would result in climatic changes (atmospheric pressures, winds, temperatures, precipitations…) which can reach Africa.
The rise in temperatures linked to the increase in greenhouse gases is mitigated by the melting of the Greenland ice caps. The modification of the atmospheric circulation also causes a displacement of the tropical rains towards the South.
These changes in temperature and precipitation differ from standard simulations that only take into account the increase in greenhouse gases. But they also affect the life cycle of the mosquito, the development of the parasite and therefore the transmission of malaria in Africa, which then shifts south in these simulations.
Malaria moved to southern Africa?
The purpose of our article is precisely to compare the effect of global warming with and without simulation of a sudden melting of the Greenland ice on malaria in Africa. When the simulations take into account the additional impact of this rapid melting of the ice, three important results are noteworthy.
In the Sahel region, the phenomenon of reduced risk of malaria transmission is on the one hand amplified. In addition to the increase in temperatures linked to global warming, this region also receives less precipitation, the rain band having shifted significantly towards the south.
The increase in the risk of transmission of malaria in East Africa is also mitigated, temperatures do not increase as much when the melting ice is taken into account. On the other hand, a risk of malaria transmission appears in southern Africa, due to higher rainfall.
If the climate is able to hinder the transmission of malaria, let's not count on it to eradicate the disease: public health policies and economic and social development are today the main key capable of preventing this scourge. Several countries, such as China, have moreover succeeded in eliminating it from their territory despite a climate still favorable to transmission.
Image credit: Shutterstock/punghi
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