Also, did the earth ever get colder as a result of something similar, such as the decimation of Native American tribes by disease?
Some studies seem to confirm this theory, but they aren't conclusive.
The argument essentially goes that so many people were killed by the migrating Mongols that farms and villages were left barren and untilled. This led to an increase in plant life, which in turn acted as a "carbon sink" for the carbon dioxide being produced. As a result of less carbon dioxide coming into the atmosphere, the planet cooled.
I saw something similar regarding the conquest of the new world as well. Native Americans pretty much used the land like Europeans at the time and removed a lot of forest to create farm land. When Old World diseases affected their populations, reforestation occurred and again took carbon dioxide out of the atmosphere. There is some evidence that this led to the little ice age in Europe.
Source: http://www.livescience.com/11739-wars-plagues-carbon-climate.html