Malaria and climate change

Key Questions

Q: Not all scientists agree that climate change affects malaria. How can we be sure?

A: Well it's not quite as simple as that. Actually, all scientists agree that climate change has the potential to affect malaria, because it is well known that transmission is highly sensitive to temperature and rainfall. Most scientists would therefore expect some changes in malaria transmission, if predicted future climate change scenarios turn out to be correct.

However, the real controversy relates to whether changes in climate over the last 30 years or so have already had an impact on malaria transmission. Some reports suggest that they have – others not. The problem is that, whichever way you look at it, the evidence base is rather small. There are very few sites where we have good quality malaria and climate data going back several decades, and arguably none where we also have good data on other factors that might complicate the malaria/climate relationship, such as drug resistance, insecticide resistance and changes in the effectiveness of health services. So we can only be sure about how climate changes are affecting or will affect malaria by finding more places where we can study the interactions between these various risk factors.

Q: Does climate change complicate the design of early-warning systems for malaria, which factor in climate variables?

A: It is worth pointing out that there are only very few examples of operational early-warning systems for malaria and that these have not been evaluated with much rigour. Yes, climate change might make climate-based early warning more difficult, because the “typical” past patterns that underlie the early warning systems might not cope well with changes short-term climate patterns between years or seasons. But it should be noted that, even in the absence of climate change, progress in developing early-warning systems has been very slow, reflecting the fact that many factors other than climate change hinder their development (e.g. the complexity of the malaria disease system, data shortages, resource shortages). 

Q: Have malaria early-warning systems not been evaluated with rigour because it is technically hard to do so?

A: There certainly are technical difficulties associated with this type of evaluation; if the system works well, then any predicted epidemic should be prevented, but how do you prove that the epidemic would have happened in the absence of an early-warning system? But more generally these systems have only been implemented very recently and there has not been sufficient time to carry out proper evaluations.

Q: What role does technology like remote sensing have in predicting disasters related to climate change?

A: Earth-observing satellites have a potentially very important role in predicting disasters. There are now a large range of satellite sensors providing information on climate variables themselves (rainfall, temperature, etc) and on longer term drivers of climate variability (e.g. sea surface temperature and other elements of El Niño). Satellite data therefore give us the potential to develop forecasts of climate conditions several months in advance, and to track actual weather conditions in real-time. The challenge is for disease control programmes to harness this information effectively to inform their activities.

Q: Have any vector-borne diseases re-emerged largely as a consequence of climate change?

A: As noted previously, some researchers believe that the emergence of malaria in highland areas of Africa reflects changes in climate, although others do not subscribe to this view. There is no clear evidence that climate change has yet led to the re-emergence of any other vector-borne diseases. For important diseases like dengue, schistosomiasis, lymphatic filariasis and Chagas disease, there have been very substantial changes in the scale and global distribution of cases in the last few decades, but these appear to reflect societal and disease control factors more than changes in climate or environment.

Q: Could you elaborate on why substantial changes in the scale and global distribution of cases of dengue and other diseases appear to reflect societal and disease control factors more than changes in climate or environment?

A: To clarify, year-to-year variations in climate have been implicated in year-to-year variations in malaria and dengue (although not chronic diseases like schistosomiasis, lymphatic filariasis, etc) but there is no strong evidence to directly relate long-term changes in disease patterns to climate trends. For some diseases, non-climatic drivers have clearly had a major impact on disease transmission. For example, globally, improvements in water supply and sanitation have led to major reductions in schistosomiasis outside Africa. Also, some diseases that have historically been associated with very high public health burdens are now targeted for elimination by global programmes. African trypanosomiasis and Chagas disease are good examples: for these diseases any effects of climate change are likely to be of secondary importance.

To reiterate – nobody is saying that climate change is not a potentially important driver of vector-borne distributions in time and space; however, it is important that climate effects are put in the wider context of other potentially important drivers.