Nice day for a touch of cocolitzli, eh?:
At a recent meeting about drilling to look at long-term records of climate change, one of the participants brought up Bill Ruddiman's theory about the onset of recent climate change, and that made me think of some interesting connections.
Bill observed that CO2 and methane in the atmosphere regularly follow cycles that are driven by orbital changes, but that in the recent past, they deviated, rising in concentration when they should have fallen. The CO2 begins to rise about 8,000 years ago, and methane begins to rise about 5,000 years ago (New Scientist article, graphs). That is a lot sooner than can be attributed to industrialization (post 1700's), and Bill proposed that this was due to a major change in the land cover - the rise of human agriculture and the removal of forests.
A piece of this article also deals with the "Little Ice Age," a period during the Middle Ages when temperatures were lower than normal, and the winters were particularly hard in Europe. Rather than having the Little Ice Age exacerbate the effects of the Black Plague as is usually taught, Bill turned this on its head, and wondered whether in fact the arrival of the plague that killed off about 50% of the human population meant that all that land that went fallow in fact began absorbing CO2 as the forests regrew, and that this drop in CO2 caused the low temperatures.
So I immediately began to wonder whether in fact this was linked to the arrival of Europeans in the Americas, and the great changes in population that occurred here with the spread of disease. In Mexico, probably the most populous and highly organized society, the outbreaks of cocolitzli in 1545 wiped out about 90% of the population, and the 1576 episode then wiped out 50% of those remaining. This had to have had a large effect on the amount of land under agriculture, and thereby on the land cover.
I then went forward in time, and wondered if the cure for all this global warming that is going on would be a pandemic of H5N1 avian flu. If the published survival rate of 45% for this strain is combined with a global outbreak, it would have an enormous impact on humans, and therefore on land cover, and from there, on climate. (Avian flu timeline)
Mind you, there was a fair bit of criticism of the idea of the plague causing the LIA in the paleoclimate community. For one, when we look at marine records preserving many hundreds of thousands of years of sediments, there are fairly regular millenial cycles in the sediments that are probably caused by variations in temperature. Bill's theory does not explain this cyclicity - no evidence has been found for a parallel cyclicity for land cover, much less disease.
Could it happen this time? Of course. Is disease among humans what caused the Little Ice Age? Not clear. Is disease among humans and animals what usually causes these changes in climate? Probably not.