It seems to have taken ecologists a little while to notice what the health professionals were running around saying about climate change invariably increasing infectious diseases and how vector borne diseases are already on the rampage. Possibly because, as Kevin Lafferty, the author of the paper that sparked the recent debate notes:
"For many ecologists, interest stops at the surface of the organism they study."
Now that they have noticed some of the highly questionable things that have been said, ecologists have weighed in on all sides of the debate in an Ecology forum. Even though there was some disagreement in the ranks about the extent that climate will drive infectious diseases, they all seemed to be in agreement that its not as simple as everyone has been making out, and it is often very hard to pinpoint where the effects of climate on disease begins and ends when taken into consideration with all the other inter-related variables. Also interesting to note was the fact that in most of the articles, the inference was that climate change was being discussed from the perspective of climatic variabilities, and not so much from a purely anthropogenic perspective.
Kevin Lafferty, from the U.S. Geological Survey's Western Ecological Research Center, notes in his article The ecology of climate change and infectious diseases, that:
although the globe is significantly warmer than it was a century ago, there is little evidence that climate change has already favored infectious diseases. While initial projections suggested dramatic future increases in the geographic range of infectious diseases, recent models predict range shifts in disease distributions, with little net increase in area. Many factors can affect infectious disease, and some may overshadow the effects of climate.Richard Ostfield, from the Cary Institute of Ecosystem studies then weighs in with an article entitled Climate change and the distribution and intensity of infectious disease, in which he notes:
...unequivocal demonstrations of a causal link between climate change and human infectious diseases are rare (albeit increasing). Some diseases are likely to decrease in incidence and range with climate warming (Harvell et al. 2002), and others are likely to respond to precipitation or humidity more than to temperature, leading to poor predictive power under warming scenarios. Many diseases are strongly influenced by other ecological, sociological, economic, and evolutionary factors besides climate change.
In his concluding thoughts, he summises that "clear effects of climate change have now been established for several human infectious diseases" but then follows it up with the caveat:
Hay et al. (2002) close their critique of climate drivers of highland malaria by asserting, ‘‘The more certain climatologists become that humans are affecting global climates, the more critical epidemiologists should be of the evidence indicating that these changes affect malaria’’ (Hay et al. 2002:909). Good science demands that skepticism be applied equally to evidence for and against climatic effects on disease.
Mercedes Pascual and Menno Bouma then disagree on the basis that disease range shifts due to temperature are important (Do rising temperatures matter?) and that even if you think temperatures don't matter, then human activities such as changes in land use may increase infectious diseases anyway.
Harvell et al then use diseases in coral (these are ecologists remember) to point out that they think there is evidence of climate as a disease driver (Climate change and wildlife diseases: When does the host matter the most?), but then acknowledges that its complicated and host reactions and immune response may be important, too.
Then Andy Dobson from Princeton does a whole bunch of complicated things with numbers in
Climate variability, global change, immunity,and the dynamics of infectious diseases but seems to work out, among other things that:
Ultimately, the observed increase in malaria cases represent a complex interaction between climate change, human population expansion, the evolution of drug resistance, and the rapid expansion of the AIDS epidemic.Then reassures the reader that if you can give him lots of money he'll be happy to work it out for you.
Teasing out the relative importance of each requires careful analyses of nonstationary time-series data where long term trends in global change, and annual seasonal variation in external forcing, interact with the intrinsic tendency of epidemic systems to cycle; this can produce very complex dynamics that defy simple statistical analysis.
My absolute favourite response comes from Sarah Randolph, a zoologist from Oxford, who points out in Perspectives on climate change impacts on infectious diseases, that over the last 450 million years so many species have become extinct that we really shouldn't get so worked up about a few climate related shifts in species distribution. She points out that the only reason we are getting so hysterical is that those species shifts might be pathogens that directly or indirectly affect homo sapiens. In summary, she says:
The complexity within each disease system emphasizes that any expectation of a simple consistent response to climate change, i.e., a universal worsening of the situation, is ill founded.
Hitherto, there is no single infectious disease whose increased incidence over recent decades can be reliably attributed to climate change. The often-repeated statistic, that climate change is currently causing approximately 150 000 extra deaths per annum, may be stamped with the authority of the World Health Organization (Campbell-Lendrum et al. 2003), but is, in the opinion of many practicing disease ecologists, inestimable. Furthermore, large as this number is in terms of bereavements, it represents only ;0.15% of all-cause deaths (as a first approximation, assuming a global population of 6.7 3109 and mean life expectancy of 67 years). Other, more avoidable, causes of premature deaths from infectious diseases deserve more attention than climate change.
So, in general, even though they seemed to be quibbling about which bit goes where (remind me never to get a bunch of ecologists to put together an Ikea cot), they all seem to be in agreement that:
1. Its complicated.
2. We don't know much and need to learn more.
3. Nobody should have left the public health mob, the sociologists and the climate scientists unsupervised for so long.
4. Scientific debate and skepticism is a really good thing.
If you made it to the end of this post with me, congratulations! Now as a reward, you can go to my last post and watch the models strip off in the name of carbon reduction.