Europe's "Little Ice Age" may have been triggered by the
fourteenth 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, said.
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 south-east
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.
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, understood to be a bacterial disease spread by
This bubonic plague is said to have wiped out over a third of
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. This is because leaves absorb carbon dioxide through their
stomata, and their density varies as carbon dioxide goes up and
"Between AD 1200 to 1300, we see a decrease in stomata and a
sharp rise in atmospheric carbon dioxide, due to deforestation we
think," said Dr van Hoof, whose findings were published in the
journal Palaeogeography, Palaeoclimatology, Palaeoecology. 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
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
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.
A later, much greater cause probably adds fuel to
the fire of the Little Ice Age (or lack of it given the freezing
conditions) - that of the massive depopulation of North America
during the early European colonial period between about 1500-1700.
European disease cuts a swathe through the native peoples, so much
so that vast areas of managed forest are quickly reclaimed by
fast-growing foliage. The extra growth pulls down enough carbon
dioxide to contribute to the Earth's cooling.
The "Great Dying" of the indigenous peoples of the
Americas led to the abandonment of enough cleared land that the
resulting terrestrial carbon uptake had a detectable impact on both
atmospheric CO₂ and global surface air temperatures, which is what
Alexander Koch and colleagues wrote in their paper which was
published in Quaternary Science Reviews at the start of 2019.