气候变化对疟疾有何影响？

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Climate change could make a number of infectious diseases worse. I expected malaria would be one of them.

That’s concerning, given the huge global burden that it already carries: it kills almost half a million children every year.

But a more complex story emerges from the scientific literature. One where rising temperatures could actually reduce the burden of malaria in some areas, while making it worse in others.

What impact has climate change already had on malaria? Will it make the global fight against it harder or easier in the future?

These are the questions that I want to answer here.

The global burden of malaria today

Before we look at how climate change could change the dynamics and distribution of malaria in the future, we need a quick primer on the global burden of the disease today.

This is important because climate change could cause a decline in malaria in some regions, while increasing the prevalence in others. The impact on the overall global burden of malaria will mostly depend on what happens in the worst-affected regions today.

Malaria is an infectious disease present across much of the tropics and subtropics. In the map below, you can see the band of new infections across South America, South and Southeast Asia, and Africa.

The difference between sub-Saharan Africa and other parts of the tropics, though, is huge. Infection rates in sub-Saharan Africa are around 100 times higher than in South America and 50 times higher than in South Asia.

The fact that severe malaria is mostly an African story today is even clearer when we look at where people die from it. More than 95% of the 600,000 people who die from malaria every year live in sub-Saharan Africa. You can see this regional breakdown below.1

But this chart also makes clear that within sub-Saharan Africa, there are substantial differences in the burden.

> Infection rates in sub-Saharan Africa are around 100 times higher than in South America and 50 times higher than in South Asia.

Around half of global deaths occur in West Africa — countries like Nigeria, Sierra Leone, Cameroon, Côte d’Ivoire, and Liberia. The rest are split between the central and eastern parts of the continent. Southern Africa has extremely low rates of malaria.

This dominance in West Africa is not just about population (we might expect more deaths in denser areas). As you can see in this map of death rates, those living in West Africa are at a much higher risk of dying from malaria.

These are the important background facts to make sense of this article’s focus. Most deaths from malaria occur in sub-Saharan Africa. Within the continent, West Africa is hit the worst. And children are most at risk: tragically, around 70% of malaria deaths are among children. That’s almost half a million children dying of the disease every year.

Malaria is a very temperature-dependent disease

To understand the link between malaria and climate change, we need to know the basics of the disease’s “perfect conditions”.

Several mosquito species in Africa carry and spread malaria. A dominant one is Anopheles gambiae. It carries and spreads the parasite Plasmodium falciparum, which is what infects humans.

Over many decades, scientists have studied the ideal conditions for these mosquitoes to thrive. The spread of malaria peaks in these species at around 25°C.2 Transmission drops close to zero below 16°C and above 34°C.

That means the suitability of malaria forms a bell-shaped curve like the one below. It spreads best at 25°C; this suitability drops on either side of it, and eventually becomes negligible.

Temperature is not the only thing that matters. How established malaria already is in a given country or region does, too. As do rainfall patterns: female mosquitoes lay their eggs in standing water, and larvae need water to develop. So, rainfall creates favorable conditions for malaria to spread. There is a limit, though: heavy, persistent rain often washes larvae away, preventing them from developing. An area with moderate rainfall is therefore a good breeding ground for malaria to spread.

Humidity can also have an effect. Adult Anopheles dry out in low-humidity air, so they often live longer in wetter, more humid seasons (and when they live longer, they do a better job of spreading the parasite).

What effect has climate change already had on malaria?

The world has already warmed by around 1.4°C since pre-industrial times.3

What impact has this already had on the prevalence of malaria?

A recent paper by Colin Carlson and colleagues, published in Nature, estimated the impact of historical climate change on the burden of malaria so far, and projected how this might affect it in the future under different warming scenarios.4 The paper focused on the burden on children in Africa — as we established earlier, this is the most affected demographic, both in terms of cases and deaths.

They estimate that over the last century — from 1901 to 2014 — climate change has led to a small increase in the prevalence of childhood malaria across sub-Saharan Africa. By “increase”, they mean that rates were slightly higher than they would have been compared to a hypothetical scenario without climate change. The prevalence of malaria has actually declined in recent decades, but for reasons unrelated to climate.

Across the region, this increase was estimated at roughly 1 case per 1,000 children aged 2 to 10. To put this into context, around 24% of children (or 240 in every 1,000) in this region were estimated to carry the malaria parasite at the end of the study period. So, the overall impact has been relatively small and is quite uncertain. 59% of the study’s simulations showed an increase in malaria prevalence, with uncertainty ranging from a reduction of 4 cases per 1,000 to an increase of 6 cases per 1,000.

The bigger impact has been in changing the prevalence of malaria across different parts of the continent. Hotter temperatures might not have led to a large increase, on average, but they have affected its distribution.

Climate change has probably led to a decline in malaria in West Africa, but an increase in the southern and eastern parts of the continent. The following map summarizes this change: you see blue in the West but red, marking an increase, in the East and South.

Remember that West Africa is where the burden of malaria has been, and still is, the highest. A decline there due to climate change has been almost entirely offset by an increase elsewhere on the continent, which is why the effect, on aggregate, is very small.

The chart below shows malaria prevalence in two scenarios: the realistic one the world has lived through, and an alternate reality without human-driven climate change. Malaria has declined in West Africa due to climate change, but has increased in the other three subregions.

To understand why climate change has made things better in West Africa, but worse elsewhere, let’s return to our bell-shaped curve of where malaria thrives.

> Climate change has probably led to a decline in malaria in West Africa, but an increase in the southern and eastern parts of the continent.

The study by Carlson and colleagues found results very similar to previous studies on the optimal conditions for malaria-carrying mosquitoes: transmission peaks at around 25°C.

In 1901, the average temperature in West Africa was around 26°C — very close to, but just past, the ideal temperature.5 Climate change has increased temperatures, pushing them away from the optimum and down the slope.

What has happened elsewhere? East and Central Africa started below the optimum, at around 22°C, and Southern Africa far below, at around 17°C. Higher temperatures in all of these regions have pushed them closer to the optimum. East and Central Africa now sit at around 24°C, and Southern Africa at around 18°C.

It’s this temperature effect that has driven these diverging patterns across sub-Saharan Africa. The authors of the st