Researchers track climate change in northwest Africa back to the Middle Ages

An international research team has figured out northwest Africa's climate history by using the information recorded in tree rings. The trees sampled contain climate data from the medieval period, including one from Morocco that dates back to the year 883.

The climate of a region that includes Morocco, Algeria and Tunisia has now been analyzed back to the year 1179 and shows that frequent and severe droughts occurred during the 13th and 16th centuries.

"Water issues in this part of the world are vital," said lead researcher Ramzi Touchan of the University of Arizona. "This is the first regional climate reconstruction that can be used by water resource managers."

In most of North Africa, instruments have been recording weather information for 50 years or less, too short a time to provide the long-term understanding of regional climate needed for resource planning, he said.

"One of the most important ways to understand the climate variability is to use the proxy record, and one of the most reliable proxy records is tree rings," said Touchan, an associate research professor at UA's Laboratory of Tree-Ring Research.

The team has developed the first systematically sampled network of tree-ring chronologies across Northwest Africa, said co-author David Meko, also of UA's Laboratory of Tree-Ring Research.

The network allowed the researchers to analyze the patterns of past droughts over the whole region, said Meko, a UA associate research professor. The width of the annual growth rings on trees in semi-arid environments is highly correlated with the amount of precipitation.

The team found the region's 20th-century drying trend matches what climate models predict will occur as the climate warms. The research is the first to compare projections from climate models with tree-ring-based reconstructions of the region's past climate.

The region's trees and dead wood needed to do such research are disappearing rapidly from a combination of a massive die-off of trees, logging and population pressures, Touchan said.

"We have a chance to do what we call salvage dendrochronology," Touchan said. These are areas where we need to get this information now or it's going to disappear."

Pointing to a cross-section of an old tree from Morocco, he said, "This is from 883–and this is from a stump. If we don't take them, they're gone. So this is a real treasure."

The team's paper, "Spatiotemporal drought variability in northwestern Africa over the last nine centuries," is now available online and will be published in a future issue of the journal Climate Dynamics. The National Science Foundation funded the research.

The team sampled several different species of conifer and oak trees, because research indicates that testing several different species from the same region provides a better indicator of regional climate.

The current tree-ring chronology builds on previous work in Northwest Africa by this team and by other researchers. The chronology incorporates samples from at least 20 trees from each of 39 different sites.

Persistent drought was more widespread across Northwest Africa before the year 1500 than for the four centuries following, the researchers found. However, the pattern of widespread regional drought then seems to re-emerge in the late 20th century.

The spatial extent of the new regional tree-ring chronology revealed that drought in Morocco is not driven by the same kinds of oceanic and atmospheric conditions as drought in Algeria and Tunisia.

Drought in Morocco is strongly related to the north/south seesaw of air-pressure anomalies in the North Atlantic Ocean called the North Atlantic Oscillation. However, drought in Morocco is only weakly related to El Nino. By contrast, drought in Algeria and Tunisia appears more linked to a warm tropical Atlantic Ocean.

Touchan hopes to expand the new network's geographic reach to across North Africa, including Libya and additional parts of Algeria. In addition, he wants to extend the chronology back in time to bridge the gap to archaeological material.

Tree-ring chronologies exist for centuries deep in the past, but they are "floating," meaning that there is no continuous record linking those chronologies to ones that reach back from the present, he said. "If we can bridge this gap, it will be a discovery for the world," Touchan added.

Touchan and Meko's co-authors are Kevin J. Anchukaitis of Columbia University's Lamont Doherty Earth Observatory in Palisades, N.Y.; Mohamed Sabir of the National School of Forest Engineering in Sale, Morocco; Said Attalah of the University of Ourgla in Algeria; and Ali Aloui of the Institute of Sylvo-Pastoral of Tabarka in Tunisia.

Click here to access the article from SpringerLink

See also The development of an Early Historic tree-ring chronology for Scotland

Source: University of Arizona

'Mummified' trees found in Norway date back to the 13th century

Norwegian scientists have found “mummified” pine trees, dead for nearly 500 years yet without decomposition. The discovery could lead to a wealth of information for scholars examining the medieval environment.

Dated to the early 1200s, the 40 dead Scotch pines were found scattered among living trees in what was once a dense forest that supplied wood for medieval boats and churches.

Norway’s wet climate seems perfect for encouraging organic matter to rot – particularly in Sogndal, located on Norway’s southwestern coastline, in one of the most humid, mild areas of the country.

“We were gathering samples of dead trees to reconstruct summer temperatures in western Norway, when our dendrochronological dating showed the wood to be much older than expected”, says Terje Thun, an associate professor at the Norwegian University of Science and Technology’s (NTNU) Museum of Natural History and Archaeology. Thun conducted the work with his colleague Helene Løvstrand Svarva.



“We were astounded to find fresh wood in trees that started to grow in the late 1200s and had died almost 500 years ago, which is much older than we originally expected. Somehow they have kept from decomposing for several centuries in this humid climate”, Thun says. “This is quite extraordinary - I would go as far as to call it sensational.”

Thun suspects the trees stayed so fresh for two reasons. The first is that many of the trees had either remained upright or fallen on rocks, avoiding exposure to the wet ground. Secondly, pines are full of resin, which protects them against wood-eating bacteria. At death, pines release large amounts of resin, which could have helped delay decomposition.

“Many of the trunks we dated turned out to have seeded in the early 1200s, and had lived for more than 100 years at the time of the Black Death around 1350”, Thun says. “That means that the dead wood has ‘survived’ in nature for more 800 years without breaking down.”

The Norwegians living along the coast of present-day Sogndal in the 12th and 13th centuries apparently favored the robust pines, Thun's study shows.

Timber from a stave church—a distinctive medieval church built by the Vikings—matches wood from the forest that contains the dead trees.

The Norse peoples likely trekked to inland forests to hunt and harvest trees for their buildings, he added.