Attributing observed change in disease occurrence to climate change

There are well-documented observations of various non-human systems, both physical and biotic, that have undergone changes that are reasonably attributable to associated regional warming over the past few decades. This includes melting of glaciers, shrinkage of sea-ice, changes in seasonal timing of bird nesting and plant flowering, and changes in timing and paths of insect migration. Attribution of climatic influence is much easier for these relatively simple systems, mostly lacking compelling alternative explanations. This, however, is not the case for patterns of infectious diseases in human populations - there are always several, sometimes many, plausible explanations for any observed change in pattern of occurrence. Hence, caution is needed in the interpretation of climate-associated changes in human infectious diseases. In general, no single report is conclusive.

Within a particular "climate envelope," many other social, economic, behavioral, and environmental factors also influence disease transmission. Against this complex and "noisy" background, it is unavoidably difficult to make a quantitative attribution to climate change of any observed change in the occurrence of some specified infectious disease. Vector-borne infectious diseases vary greatly in the complexity of their transmission modes, and hence some are much easier to study via modeling than are others. To date, the formal modeling of how climate change would affect vector-borne diseases has focused on malaria and dengue fever. Modeling future impacts is conceptually simpler for dengue than for malaria. The two main pathogen variants of malaria (falciparum and vivax) and its transmission relies on several dozen regionally dominant mosquito species, whereas dengue fever is transmitted principally by one mosquito vector, Aedes aegypti.

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