Take malaria, for example. Some very simple links have been established that relate climate to the life cycle of mosquitoes and the dynamics of malaria transmission (Rogers and Randolph 2000, Tanser et al. 2003). A quantitative understanding of this relationship can then be used to calculate a marginal change in the number of malaria deaths when the average temperature rises by a given amount – when all other potentially relevant factors remain fixed.Here is how Ryan describes the image above:
But of course, we know that all things will not be equal. Many social, political and cultural factors will come into play. Malaria epidemiology may be related in part to climate conditions, but the amount of suffering and death caused by malaria ultimately should have little to do with climate or climate change. For example, the absence of malaria in the southeastern United States, where environmental conditions are conducive to the disease, is due to a massive Centers for Disease Control (CDC) eradication program begun in 1947, which rendered the problem insignificant within four years (CDC 2004). Defeating the disease may be more difficult in some areas than others, but it is nonetheless treatable and controllable through means entirely unrelated to climate change (Sachs 2002).With that in mind, consider this crude figure showing malaria deaths over time in a hypothetical country where the disease has been a burden historically . . .
The red wedge represents the marginal increase in deaths that a climate impacts model might tell us to expect, all other things being equal. But the baseline projection is actually quite unlikely, especially in the context of an unstable government, a fragile and decaying agro-economic system or, conversely, a transitioning economy with the capacity to eradicate the disease. Whether the problem is largely solved by effective intervention, or greatly exacerbated by non-climate-related disasters like a civil war, overpopulation or some other collapse, the marginal change due to climate is rendered less important. Even if the baseline proves relatively accurate, the impact due to climate change pales in comparison to the massive failure of efforts to intervene in an eminently solvable problem that causes 8 millions deaths a year.If anything, Ryan has over-stated the climate-impacts of malaria as compared to the analysis in the Nature paper which concluded:
. . . the proposed future effects of rising temperatures on endemicity are at least one order of magnitude smaller than changes observed since about 1900 and up to two orders of magnitude smaller than those that can be achieved by the effective scale-up of key control measures.Ryan's essay can be found here.