Finnish trees tell the story of 2,000 years of climate history

22 August 2012

Over the past 2,000 years, the climate in northern Europe has cooled more than previously assumed. This is the conclusion drawn by an international group of researchers following the examination of the growth rings of fossilized pines from Finnish Lapland. Scientists from Johannes Gutenberg University Mainz (JGU) also took part in this major project, most notably the climate geographer Professor Dr. Jan Esper.
 

A section of a pine tree lies on Professor Dr. Jan Esper's desk. "It fell into a lake in 1350," explains the professor of Climate Geography at JGU's Institute of Geography. "As a result, it has been remarkably well preserved." A relatively large number of trees in Finnish Lapland suffered the same fate and these circumstances have made it possible for an international research team to calculate the climatic cooling trend over the past two millennia more precisely than ever before.

The findings surprised the scientists – the rate of cooling is greater than had been assumed. Previously, the Intergovernmental Panel on Climate Change (IPCC) had assumed that the rate of cooling is 0.2 degree C per millennium; Esper and his colleagues in Finland, Scotland, and Switzerland have now concluded that the actual figure is more like 0.3 degree C. "Although this is a conservative estimate," explains Esper. So, while it is unlikely that this figure will need to be adjusted downwards, the upper limit is still unknown. "Who knows what will come out in the end?"

Lively discussion about new research findings

The publication of the findings in the magazine Nature Climate Change immediately set off a discussion. "Nonetheless, our reconstruction of the climate history of the region is not being seriously contested," says Esper. On the other hand, their hypothesis that the width of the growth rings in the trees examined is not as important as the density of the wood has attracted a lot more skepticism.

Not only that, but on the basis of their findings, the scientists also extrapolated climate data for the whole of the northern hemisphere over the past 2,000 years. "This aspect is the one generating the most controversy," says Esper smiling. "However, such a reaction is completely normal and is to be welcomed."

Dendrochronology makes it possible to look deep into the past, reconstruct the course of climate change, and date findings very precisely. Wherever wood is to be found, researchers can read the growth rings. Over the course of about 100 years, collation of countless findings has created a kind of reference library. The focus up to date has been on measuring the width of growth rings. But Esper and his colleagues chose to use another technique that has only been around since the early nineties.

Complex method determines cell-wall thickness

"We measure the cell-wall thickness of wood. This is a more accurate indicator." At the center of attention is the late growth wood towards the end of a growth ring. The thickness of the cell walls in this wood provides information about the temperatures in June, July, and August.

"Unfortunately, this technique is much more complex than previous methods," states Esper. A density measuring device and an engineer specialized in such work are needed. However, the basic principle seems pretty straightforward. The thicker a cell membrane, the denser the material. So the idea is to take a sample of each growth ring and measure its translucence with the aid of x-rays.

"We initiated our project in 2006." The scientists took samples that looked pretty much like the slice of wood on Esper's desk from well-preserved pines which had fallen into Finnish lakes over the past 2,000 years.

The luxury of a long-term project

"It's a luxury to have a long-term assignment to take on in addition to our routine projects," claims Esper. "We don't get any third-party funding." Only short-term projects tend to attract that type of sponsorship. "People generally want to see concrete results within a year." But Esper thinks that long-term projects are particularly important in this field. "If we had the time and money, we could extend the period we are looking at to 7,000 years."

Nonetheless, the current results cover the time all the way back to 138 BC and they show that the Roman era was far warmer than previously thought. As there are interactions between climate and culture, the new findings provide new food for thought.

Better understanding of climate development

In addition, these findings enable researchers to better understand climate change and create forecasts for the future. Why does the climate vary? Which factors play a role? We are able to answer these questions with ever-increasing precision.

About 8,000 years ago, the Earth was at its perihelion, its closest point to the sun in its elliptical path, during summer in the northern hemisphere. However, the perihelion shifts constantly over time. "Now it's in January." This is the central factor that determines the cooling trend.

A rate of 0.3 degree C over 1,000 years may not at first seem particularly significant. "But it is not so negligible when we compare it with the rate of global warming attributable to greenhouse gases, which is less than 1 degree C to date." And this information provides important evidence when it comes to forecasting future climate change.

Volcanic eruption darkened the world

Esper has been working on follow-up projects for quite some time. One involves analyzing the influence of volcanic eruptions on climate change. Growth-ring density measurements also play an important role in this work.

Mount Tambora on the island of Sumbawa (in what is now Indonesia) erupted in 1815. 1816 was the "Year Without A Summer". Measurements of the width of growth rings suggest that the following years also had very cold summers.

"However, it is the case that tree-ring growth has a kind of 'memory'," says Esper. The events of 1816 continued to have an effect in trees and influenced the width of subsequent growth rings. "But they had no effect on the density of the wood." The cell-wall thickness of late growth wood is a more direct indicator of climate and indicates that summers were actually warmer. So it is possible that the Tambora eruption did not darken the world for as long as has been assumed.

New measuring equipment for JGU

The density measurements provide new insight into climate change, making it necessary to review older conclusions based on measurements of growth-ring widths. "Unfortunately, we cannot yet carry out such measurements here at the University," says Esper. Up to now, his Swiss colleagues have been responsible for this aspect. The precision instruments are also manufactured in their country. There is only one single company that supplies them.

"However, the DFG has now approved the required funding so we can order our own density-measuring device," states Esper happily. As the graduate engineer Markus Kochbeck is working in the dendrochronology lab, this means there is already someone there who will be able to use it.

"However, we still have to wait as the suppliers have to meet two orders that were placed ahead of ours. And can you guess who placed them? That's right: The Chinese." However, JGU should get its instrument by late 2013. The device, that costs EUR 270,000, will then also be available in Mainz where it will help tell the story of climate change.