Posts Climate, Environment, Forestry

There has been climate variation before, and the growth rings of trees, which have been there for a long time, reveal some bleak periods in the history of mankind.

It was exceptionally cold during the summers between 1627 BCE and AD 536 in Lapland. The cold weather damaged growing trees in extensive areas in the current area of Finland and probably also more extensively in the northern hemisphere. (photo)

Growth rings in pine trunks lifted from the bottom sediment of small lakes in Northern and Eastern Lapland reveal the climate conditions that prevailed thousands of years ago.

In the early summer, trees form large, thin-walled springwood cells that are light in colour, while the summerwood cells that are created in late summer are smaller and darker, and have thicker walls. This is how the growth rings, or annual rings that can be differentiated based on the above-mentioned colours, are formed in trees. The thickness of a growth ring is an indication of the weather during the specific year.

“Thick growth rings are formed in the trunks of trees when the conditions are favourable for growth, while the rings formed during poor conditions are very thin,” explains Samuli Helama from the Natural Resources Institute Finland (Luke), who studies dendrochronology or tree growth rings and their discernible variation.

Surely other plants and also people suffer when it is very cold in the summertime. The exceptional climate conditions were caused by particles or aerosols that were released into the air by a volcanic eruption. The aerosols darkened the Sun and hampered the photosynthesis of plants.

4. (Photo: Hannu Herva)
1. (Photo: Mauri Timonen)
3. (Photo: Hannu Herva)
A map of the research area and research sites 5–8 in Finnish Lapland. The points indicate the sites from which the samples were taken that were studied for frost damage between 1627 BCE and AD 536. The figure also shows the northern tree line of pine and the Arctic Circle. (Photos: 1 Mauri Timonen and 2,3, 4 Hannu Herva )
2. (Photo: Hannu Herva)













In the 1600s BCE, the current area of Finland was living in prehistoric times.

The growth rings of northern pines provide very specific information on the climate during this time.

“The growth rings depict the climate conditions with an accuracy of up to one calendar year. It is the only data that provides such a specific chronological link so far into the past. The accuracy is a major benefit, as it can be used to reliably reconstruct climate variation in the past,” Helama says.

Archaeological or other studies of the climate of the past could not get even near to the same accuracy of timing without dendrochronology.

Luke has created a growth ring calendar that covers more than 7,600 years based on the growth rings of Lapland pines. It can be compared to a variety of historical events. The calendar is among the longest in the world.

The growth ring calendar is considered an especially good means of interpreting climate variation, as humans did not influence the forests in Lapland much in the past centuries.

Why do we need the data on past climate conditions?

Helama says that we should consider how modern humans could survive if similar summer frosts or other abnormal climate conditions occurred now. Would our food stocks be sufficient if the yields in the northern hemisphere experienced a major collapse and the supply of imported grain was highly limited?

Would we know how to prepare for such a changing of the climate?

Slow, continuous warming of the climate is no longer a surprise, which is why a sudden cold spell would be so unexpected. The surprise element could throw society into disarray.

“Nothing is more likely than that something similar will happen. However, nothing is more unlikely than when it will happen.”

Volcanic eruptions also caused an exceptionally bleak and cold period in the mid-500s in the northern hemisphere.

The aerosols that the eruptions spread into the atmosphere darkened the Sun for a long time. These sudden changes have also been studied from the growth rings of Lapland pines.

The amount of incoming radiation can be reconstructed based on the variation of the carbon isotope ratios analysed from the growth rings. The isotope determination was completed in cooperation with the University of Helsinki Laboratory of Chronology.

The exceptionally poor conditions hampered agriculture. Presumably, the deteriorated solar radiation decreased the amount of vitamin D generated by people’s bodies. Hunger deteriorated people’s general state of health, and the misery was compounded by compromised disease resistance.

The harsh years coincided with a bubonic plague epidemic that rocked the Roman Empire. The epidemic, which started in 542, killed approximately half or even more of the population in the area that was then the eastern part of the Roman Empire. The plague spread through Europe to the Mediterranean and possibly also up to Finland. By the 700s, it had killed tens of millions of people.

The 1600s were also a total catastrophe for Finns.

The year 1601 was the “Great Straw-Year”, when crops died where they stood because of frost. There were several consecutive years of crop failure, as the result of which almost one-fifth of the population died of famine or infectious diseases. The cold weather was, yet again, caused by aerosols from a volcanic eruption.

The growth rings were especially narrow in Lapland in 1601, and the walls of the summerwood cells were exceptionally thin.  These are indicators of the coldness of the summer period.

The changing of the climate during the Great Straw-Year was probably caused by an eruption of the volcano Huaynaputina in Peru the previous year.

“This period has had a major impact on the Finnish genotype and the Finnish identity. It makes you wonder why us, you and me, are here, while others are not.”

According to Helama, volcanic eruptions cause natural climate variation that humans can influence very little. The eruptions are caused by huge geological forces. We can still try to prepare for such phenomena, though.

The current warming of the climate is mostly caused by human activity, and we should try to influence it.

Text: Riitta Salo-Kauppinen

Published in Finnish in Maaseudun Tulevaisuus newspaper on 12 of June 2019.

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