Well, that explains it.

Bubonic Plague, or the black death, may have been responsible for Europe's "little ice age," the BBC reports today. Europe's chill linked to disease

Europe's "Little Ice Age" may have been triggered by the 14th Century Black Death plague, according to a new study.

Pollen and leaf data support the idea that millions of trees sprang up on abandoned farmland, soaking up carbon dioxide from the atmosphere.

This would have had the effect of cooling the climate, a team from Utrecht University, Netherlands, says.

The Little Ice Age was a period of some 300 years when Europe experienced a dip in average temperatures.

So there you have it. All the peasants died and trees sprang up on their untended farms, soaking up all the carbon-dioxide, cooling the earth.

So all we have to do is kill a gazillion peasants and plant a bunch of trees- global warming solved! Huzzah!