The UnAustralian

Sunday, January 04, 2004
Plague and Carbon Dioxide: Unexpected Interactions

While CO2 levels have slowly risen over the last 8000 years, there were several times in the last 2000 years, when CO2 have suddenly dropped (by amounts ranging from 4 - 10 ppm) before returning to their expected position.

One good question is, why?

Ruddiman has looked into this and come up with an interesting explanation. If the rise in CO2 is due to deforestation, could re-growth of forests account for the drop in CO2 levels? The reason for this re-growth is simple, during times of large scale disease, the human population drops significantly, leading to abandonment of farms, which in turn leads to forests starting to reclaim them. As the human population starts to recover, the farms are recovered.

Interestingly, the timings of the CO2 drops, and large scale plagues appears to correlate quite well. For example, from about 164 AD to 540 AD, the Roman Empire were hit by successive waves of epidemics, culminating in a bubonic plague pandemic which devastated Europe in 540 to 542 AD (smaller bouts of plague also came in waves for the next 50 years). This time period matches a drop in CO2 levels.

Likewise for the series of bubonic plague and smallpox which tore through Europe, Asia and the Americas from 1332 to the 1680s AD. The effects of these diseases was massive. For example, the Black Death is estimated to have killed off between a third and a quarter of Western Eurasia's population.

An important question to ask is, can the effects of these plagues account for the size of the CO2 drops? To account for a drop of 4 - 10 ppm in CO2, we must remove between 11 and 27 GtC from the atmosphere (astute readers may note that this doesn't scale with the numbers in my previous post, this is due to the differing time scales of the various natural processes involved).

With regards to the example of the epidemic's which hit Europe and North Africa between 164 and 590 AD, it would require ~ 22% of the farm land to be reforested. This number is conceivable (and is probably an overestimation, as an epidemic in China would have also contributed to CO2 removal). So disease could well be the main cause of the this 4 ppm drop.

By the time of the Black Death pandemic, considerably more deforestation had taken place, leading to much more land being available for the trees to 'take back'. The drop in CO2 levels is estimated to be between 5 and 10 ppm (thus needing between 14 and 27 GtC to be removed). This requires reforestation to occur at a level of between 25 and 45%. This is quite close to the estimated mortality levels.

The last drop (from 1550 to 1800 AD), is a bit harder to explain. A 10 ppm drop requires 27 GtC to be removed. Ruddiman estimates that the diseases which decimated the Native Americans could have removed between 5 and 10 GtC. While Europe was being hit with reoccurring waves of plague until the 1700's, it is unlikely that this could cause the loss of 33% of farmland required to explain the drop. Diseases in India and China may help to explain it, however, there may well be other factors at play.


William F. Ruddiman. The Anthropogenic Greenhouse Era Began Thousands of Years Ago Climatic Change 61: 261-293, 2003
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