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.
Dr Thomas van Hoof and his colleagues studied pollen grains and leaf remains collected from lake-bed sediments in the southeast Netherlands.
Monitoring the ups and downs in abundance of cereal pollen (like buckwheat) and tree pollen (like birch and oak) enabled them to estimate changes in land-use between AD 1000 and 1500.
Pore clues
The team found an increase in cereal pollen from 1200 onwards (reflecting agricultural expansion), followed by a sudden dive around 1347, linked to the agricultural crisis caused by the arrival of the Black Death, most probably a bacterial disease spread by rat fleas.
This bubonic plague is said to have wiped out over a third of Europe's population.
Counting stomata (pores) on ancient oak leaves provided van Hoof's team with a measure of the fluctuations in atmospheric carbon dioxide for the same period.
"Between AD 1200 to 1300, we see a decrease in stomata and a sharp rise in atmospheric carbon dioxide, due to deforestation we think," says Dr van Hoof, whose findings are published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
This is because leaves absorb carbon dioxide through their stomata, and their density varies as carbon dioxide goes up and down.
But after AD 1350, the team found the pattern reversed, suggesting that atmospheric carbon dioxide fell, perhaps due to reforestation following the plague.
The researchers think that this drop in carbon dioxide levels could help to explain a cooling in the climate over the following centuries.
Ocean damper
From around 1500, Europe appears to have been gripped by a chill lasting some 300 years. There are many theories as to what caused these bitter years, but popular ideas include a decrease in solar activity, an increase in volcanic activity or a change in ocean circulation. The new data adds weight to the theory that the Black Death could have played a pivotal role.
Not everyone is convinced, however. Dr Tim Lenton, an environmental scientist from the University of East Anglia, UK, said: "It is a nice study and the carbon dioxide changes could certainly be a contributory factor, but I think they are too modest to explain all the climate change seen."
And Professor Richard Houghton, a climate expert from Woods Hole Research Center in Massachusetts, US, believes that the oceans would have compensated for the change.
"The atmosphere is in equilibrium with the ocean and this tends to dampen or offset small changes in terrestrial carbon uptake," he explained.
Nonetheless, the new findings are likely to cause a stir.
"It appears that the human impact on the environment started much earlier than the industrial revolution," said Dr van Hoof.