The testimony of trees: How volcanic eruptions shaped 2000 years of
world history

Date:
September 28, 2020
Source:
University of Cambridge
Summary:
Researchers have shown that over the past two thousand years,
volcanoes have played a larger role in natural temperature
variability than previously thought, and their climatic effects
may have contributed to past societal and economic change.



FULL STORY ========================================================================== Researchers have shown that over the past two thousand years, volcanoes
have played a larger role in natural temperature variability than
previously thought, and their climatic effects may have contributed to
past societal and economic change.


==========================================================================
The researchers, led by the University of Cambridge, used samples from
more than 9000 living and dead trees to obtain a precise yearly record
of summer temperatures in North America and Eurasia, dating back to
the year 1 CE. This revealed colder and warmer periods that they then
compared with records for very large volcanic eruptions as well as major historical events.

Crucial to the accuracy of the dataset was the use of the same number
of data points across the entire 2000 years. Previous reconstructions of climate over this extended period have been biased by over-representation
of trees from more recent times.

The results, reported in the journal Dendrochronologia, show that the
effect of volcanoes on global temperature changes is even greater than
had been recognised, although the researchers stress that their work in
no way diminishes the significance of human-caused climate change.

Instead, the researchers say, the study contributes to our understanding
of the natural causes and societal consequences of summer temperature
changes over the past two thousand years.

"There is so much we can determine about past climate conditions from the information in tree rings, but we have far more information from newer
trees than we do for trees which lived a thousand years or more ago,"
said Professor Ulf Bu"ntgen from Cambridge's Department of Geography,
the study's lead author.

"Removing some of the data from the more recent past levels the playing
field for the whole 2000-year period we're looking at, so in the end,
we gain a more accurate understanding of natural versus anthropogenic
climate change." Comparing the data from tree rings against evidence
from ice cores, the researchers were able to identify the effect of past volcanic eruptions on summer temperatures.



========================================================================== Large volcanic eruptions can lower global average temperatures by
fractions of a degree Celsius, with strongest effects in parts of North
America and Eurasia.

The main factor is the amount of sulphur emitted during the eruption that reaches the stratosphere, where it forms minute particles that block
some sunlight from reaching the surface. This can result in shorter
growing seasons and cooler temperatures, that lead in turn to reduced
harvests. Conversely, in periods when fewer large eruptions occurred,
the Earth is able to absorb more heat from the Sun and temperatures rise.

"Some climate models assume that the effect of volcanoes is punctuated
and short," said Bu"ntgen. "However, if you look at the cumulative effect
over a whole century, this effect can be much longer. In part, we can
explain warm conditions during the 3rd, 10th and 11th centuries through
a comparative lack of eruptions." Reconstructed summer temperatures in
the 280s, 990s and 1020s, when volcanic forcing was low, were comparable
to modern conditions until 2010.

Compared with existing large-scale temperature reconstructions of the
past 1200-2000 years, the study reveals a greater pre-industrial summer temperature variability, including strong evidence for the Late Antique
Little Ice Age (LALIA) in the 6th and 7th centuries.

Then, working with historians, the scientists found that relatively
constant warmth during Roman and medieval periods, when large volcanic eruptions were less frequent, often coincided with societal prosperity
and political stability in Europe and China. However, the periods
characterised by more prolific volcanism often coincided with times of
conflict and economic decline.



========================================================================== "Interpreting history is always challenging," said Dr Clive Oppenheimer,
the lead volcanologist of the study. "So many factors come into play -- politics, economics, culture. But a big eruption that leads to widespread declines in grain production can hurt millions of people. Hunger can
lead to famine, disease, conflict and migration. We see much evidence
of this in the historical record.

"We knew that large eruptions could have these effects, especially when societies were already stressed, but I was surprised to see the opposite
effect so clearly in our data -- that centuries with rather few eruptions
had warmer summers than the long-term average." The new temperature reconstructions provide deeper insights into historical periods in which climactic changes, and their associated environmental responses, have
had an outsized impact on human history. This has clear implications for
our present and future. As climate change accelerates, extreme events,
such as floods, drought, storms and wildfires, will become more frequent.

"Humans have no effect on whether or not a volcano erupts, but the warming trend we are seeing right now is certainly related to human activity,"
said Bu"ntgen. "While nothing about the future is certain, we would
do well to learn how climate change has affected human civilisation in
the past."

========================================================================== Story Source: Materials provided by University_of_Cambridge. The original
story is licensed under a Creative_Commons_License. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Ulf Bu"ntgen, Dominique Arseneault, E'tienne Boucher, Olga
V. Churakova,
(Sidorova), Fabio Gennaretti, Alan Crivellaro, Malcolm K. Hughes,
Alexander V. Kirdyanov, Lara Klippel, Paul J. Krusic, Hans
W. Linderholm, Fredrik C. Ljungqvist, Josef Ludescher, Michael
McCormick, Vladimir S.

Myglan, Kurt Nicolussi, Alma Piermattei, Clive Oppenheimer,
Frederick Reinig, Michael Sigl, Eugene A. Vaganov, Jan
Esper. Prominent role of volcanism in Common Era climate variability
and human history.

Dendrochronologia, 2020; 125757 DOI: 10.1016/j.dendro.2020.125757 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200928090500.htm

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