Climate Change and Mortality

In much of this blog I have explored the threat of rising sea levels and the impact that this is likely to have on a number of societies. However, in this post I now want to change my focus somewhat and look at the how human health may be directly linked to rising temperatures.

Non-infectious health effects

Heat related mortality occurs mainly when there is a large difference between temperature extremes and the mean climate, for example people are most at risk at the start of summer when they have not acclimatised to higher temperatures. For example across Europe during the first two weeks in august 2003 as many as 45000 heat related deaths were reported (Patz et al,2005).

Non-infectious heat related mortality is not likely to increase due to gradual increases in mean temperature but due to increases in extremes. It is likely that global warming will result in more frequent and more severe heat waves and thus the number of heat related deaths is likely to increase. It is also important to note that the effect of heat waves is amplified in large cities due to the heat island effect and therefore people living in large cities are more vulnerable to heat-related mortality.

I think that the issue of non-infectious health effects is something that is very important to take note of. This is as, living in an affluent western society it is very easy to separate yourself from the issues associated with climate change and see it simply as a problem for those living in less developed countries. However, this issue highlights that global warming truly is a global threat.

Infectious Disease

Infectious agents (bacteria, viruses) and their vector organisms (mosquitoes, ticks etc.) are devoid of thermostatic mechanisms and therefore their reproduction and survival is largely dependent on fluctuations in temperature. Some studies have shown that Malaria transmission is associated with anomalies in maximum temperature while others have suggested that variables such as drug resistance, human migration, land use change or presence of vector control programmes are much more important determinants of the spread of Malaria.  However, Patz et al   point to a study which shows a strong correlation between a warming trend in central Ethiopia between 1968 and 1993 and the increasing prevalence of Malaria. This study took confounding factors such as those mentioned previously into account but found that they were unable to account for the observed trend in the prevalence of Malaria.  Such studies are somewhat controversial due to the varying quality of data that it is possible to obtain, however, there is a strong suggestion that with increasing temperatures Malaria is likely to become increasingly prevalent.

Another extremely serious disease caused by a mosquito borne virus is Dengue haemorrhagic fever. It is well known that the species of Mosquito which is the principle carrier of the dengue virus is largely affected by ecological drivers such as levels of temperature, moisture and solar radiation. As such models have been produced which predict the size of the mosquito population based on a number of climate variables. The results of this model have then been compared to the number of DHF cases and it has been shown that these two variables show a high level of positive correlation. With climate change it can therefore be expected that the number of cases of DHF will rise.

While the previous examples I have given are of tropical diseases these are not the only diseases on which climate has an important effect. For example Patz et al also explain that in continental Europe higher than average temperatures have resulted in a 30% increase in the number of cases of Salmonellosis.

Impacts

The World Health Organization has carried out a study which investigates the global burden of disease which in the year 2000 was attributable to anthropogenic climate change. Their assessment, which Patz et al describe as using extremely conservative assumptions about the link between climate and health, shows that climatic changes which have occurred since the mid-70s are already responsible for around 150,000 deaths per year. This figure is based on estimates of the increase in climate sensitive health outcomes such as cardiovascular disease, diarrhoea, malaria, flooding and malnutrition, all outcomes which quantitative studies of the climate-health relationship have been carried out. The global spatial distribution of mortality due to climate change can be seen in figure 1, from this it is obvious that vulnerability is not globally uniform and therefore any response must be well targeted based on in depth regional assessment if it is to be successful.

Figure 1: WHO estimated mortality per million people by the 2000 due to anthropogenic climate change (Patz et al, 2005)

It has also been estimated that by the year 2030 the risk of various threats to human health will have doubled. Extremely large increases in the risk of flooding are expected with more modest increases in diseases such as malaria and diarrhoea. However, it is important to note that the smaller increase in risk associated with disease is likely to pose the greatest threat to a large number of communities. For example in sub-Saharan Africa flooding kills around 1 person per million per year, compared to malaria which kills 1600 per million and diarrhoea which kills 1000 per million. It is therefore easy to see that even a modest increase in the prevalence of malaria or diarrhoea is likely to have a devastating impact in such areas.

Overall Patz et al conclude that sadly population vulnerability is still very much dependent on economic factors which dictate the ability of a society to adapt. In addition regions where the risk of climate sensitive disease is greatest are also those with the most limited ability to adapt. Again we see an ethical dimension to this crisis, 90% of malaria occurs in Africa a continent with some of the lowest levels of greenhouse gas emissions which have created this problem. It is therefore clear that the emissions of some of the world’s wealthiest countries are putting at increasing risk some of the world’s poorest and most vulnerable people who unlike those wealthier nations have little ability to adapt to this growing threat.