Climate in northern Europe reconstructed for the past 2,000 years

Jul 09, 2012
The reconstruction provides a high-resolution representation of temperature patterns in the Roman and Medieval warm periods, but also shows the cold phases that occurred during the Migration Period and the later Little Ice Age.

An international team that includes scientists from Johannes Gutenberg University Mainz (JGU) has published a reconstruction of the climate in northern Europe over the last 2,000 years based on the information provided by tree-rings.

Professor Dr. Jan Esper's group at the Institute of Geography at JGU used tree-ring density measurements from sub-fossil pine trees originating from Finnish Lapland to produce a reconstruction reaching back to 138 BC. In so doing, the researchers have been able for the first time to precisely demonstrate that the long-term trend over the past two millennia has been towards climatic cooling. "We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low," says Esper. "Such findings are also significant with regard to , as they will influence the way today's changes are seen in context of historical ." The new study has been published in the journal Nature Climate Change.

Was the climate during Roman and Medieval times warmer than today? And why are these earlier warm periods important when assessing the global climate changes we are experiencing today? The discipline of paleoclimatology attempts to answer such questions. Scientists analyze indirect evidence of , such as ice cores and , and so reconstruct the climate of the past. The annual in trees are the most important witnesses over the past 1,000 to 2,000 years as they indicate how warm and cool past were.

Researchers from Germany, Finland, Scotland, and Switzerland examined tree-ring density profiles in trees from Finnish Lapland. In this cold environment, trees often collapse into one of the numerous lakes, where they remain well preserved for thousands of years.

The international research team used these density measurements from sub-fossil in northern Scandinavia to create a sequence reaching back to 138 BC. The density measurements correlate closely with the summer temperatures in this area on the edge of the Nordic taiga; the researchers were thus able to create a temperature reconstruction of unprecedented quality. The reconstruction provides a high-resolution representation of temperature patterns in the Roman and Medieval Warm periods, but also shows the cold phases that occurred during the Migration Period and the later Little Ice Age.

In addition to the cold and warm phases, the new climate curve also exhibits a phenomenon that was not expected in this form. For the first time, researchers have now been able to use the data derived from tree-rings to precisely calculate a much longer-term cooling trend that has been playing out over the past 2,000 years. Their findings demonstrate that this trend involves a cooling of -0.3°C per millennium due to gradual changes to the position of the sun and an increase in the distance between the Earth and the sun.

"This figure we calculated may not seem particularly significant," says Esper, "however, it is also not negligible when compared to global warming, which up to now has been less than 1°C. Our results suggest that the large-scale climate reconstruction shown by the Intergovernmental Panel on (IPCC) likely underestimate this long-term cooling trend over the past few millennia."

Explore further: Asian monsoon much older than previously thought

More information: J. Esper et al., Orbital forcing of tree-ring data, Nature Climate Change, 8 July 2012. doi:10.1038/NCLIMATE1589 . www.nature.com/nclimate/journa… ll/nclimate1589.html

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gwrede
3.3 / 5 (8) Jul 09, 2012
I used to have a lot to say about AGW. But today, I have no idea.

Sure, burning a lot of coal and oil will heat the atmosphere, and CO2 levels and aerosols contribute, too. But is this significant, is this something we should avoid at _any_ cost, I don't know anymore.

Maybe we should prepare for change? If we did that, we'd be more prepared for some other things, too, like a major Icelandic volcano, water level changes, major draughts, extreme weather, etc. Even a smallish meteorite hitting a remote area will change the climate for at least some years. Shouldn't we be prepared, instead of throwing billions at trying to keep everything unchanged?
Torbjorn_Larsson_OM
2 / 5 (4) Jul 09, 2012
It is a background, not an AGW effect. Making the latter even slightly worse since the net GW is observed.

If we miss the 2 degC AGW increase and gets to 4-6 degC as seems likely, it would then take 13 000 - 15 000 years to get back due to short term orbital variety. (Planets exchange ellipticity (and so distance) for eccentricity, which seems the likeliest explanation for any change in the former. And Earth has not much of either.)

So I wouldn't see this as a life saver against near-term social problems.

@ qwrede: Um, what? They have already been able to estimate that current climate extremes costs more than trying to prevent it, since flooding et cetera is so costly, never mind the human cost. (Um, large article this spring, IIRC. Should be googeable.) This cost will likely sky-rocket. It makes moral and economical sense to try to mitigate AGW.
Howhot
3.7 / 5 (3) Jul 09, 2012
The thing about AGW is it is very one issue; CO2 level rise equated to global climate rise which to the lay person is equated to temperature rise. But season after season, that average background temperature continues to rise and rise. This being contra-indicated from the solar input (it should be cooling). 2000 years ago, climate tracked pretty well to solar and global inputs; 280ppmv CO2 was pretty consistent until 1970. Then it's like "BAMB! Hold on to your socks" climate change! Only now are we recognizing that it isn't cause by solar input, and it's not just a freak pattern of weather causing global climate change. It is us and our success at mastering our environment that has made climate change and it correlates to CO2 rise. Thus the AGW theory (actually fact) that the added greenhouse gas emissions from mankind's industrial productivity is causing a very long term temperature rise.

The social issue is just how does the human race deal with it?





rubberman
not rated yet Jul 10, 2012
"Thus the AGW theory (actually fact) that the added greenhouse gas emissions from mankind's industrial productivity is causing a very long term temperature rise.

The social issue is just how does the human race deal with it?"

Correct on both counts.
Claudius
1 / 5 (1) Jul 13, 2012
http://www.nature...589.html

"N-scan shows a succession of warm and cold episodes including peak warmth during Roman and Medieval times alternating with severe cool conditions centred in the fourth and fourteenth centuries (Fig. 2). AD 2150 (+1.05°C, with respect to the 19511980 mean) was the warmest reconstructed 30-year period, ~2°C warmer than the coldest AD 14511480 period (1.19°C) and still ~0.5°C warmer than maximum twentieth-century warmth recorded AD19211950 (+0.52°C)."
Claudius
1 / 5 (1) Jul 13, 2012
In the previous post, AD 2150 should be read AD 21-50. 19511980 should be read 1951-1980. AD14511480 should read AD1451-1480. AD19211950 should be read AD1921-1950.