Ancient Trees Give Context to Recent Droughts
Researchers sampled oak and conifer trees across North Africa, including this Atlas cedar in Morocco. Credit: Ramzi Touchan.
Some of the worst droughts to hit North Africa in the last 900 years have occurred recently—in the late 20th century—according to an analysis of tree rings that has provided the most lengthy and detailed climate record yet for this sub-tropical region on the Mediterranean.
In a paper published this week in the journal Climate Dynamics, a team of scientists found that Morocco, Algeria and Tunisia weathered frequent and severe droughts during the 13th and 16th centuries, as well as the late 20th century. This new and expanded record is expected to help governments plan for future water shortages in a region where instrumental weather records go back less than 50 years and that has recently seen a resurgence of devastating droughts. Moroccan droughts in the early 1980s, for instance, caused food shortages and civil unrest, dried up lakes and sent the country’s foreign debt soaring.
"Water issues in this part of the world are vital," said lead author Ramzi Touchan
, a tree ring scientist at the University of Arizona. "This is the first regional climate reconstruction that can be used by water resource managers.”
In their study
, scientists reconstructed North Africa’s droughts by analyzing the growth rings of ancient oak and conifer trees, some more than 1,100 years old. Trees in a semi-arid environment like northwest Africa grow smaller rings during dry weather and wider rings when it’s wet, making them an excellent proxy for rainfall. Over seven years, the team sampled more than 700 trees at 39 sites to assemble the most extensive network of climate data yet for this region. Different tree species from the same area were sampled to improve accuracy. In many places these trees are disappearing due to logging, population growth and massive die-offs
thought to be related to rising aridity.
The tree ring chronology showed that northwest Africa experienced widespread regional droughts before 1500, and again, in the late 20th century. The drying trend over the last 30 years also appears to be consistent with climate model predictions under global warming; as Earth’s average temperature rises, the northern part of Africa may continue to dry out.
"Progressive drying of the Earth's subtropical zones as a consequence of greenhouse gas emissions is a robust projection of most climate models,” said study coauthor Kevin Anchukaitis
, a tree ring scientist at Columbia University’s Lamont-Doherty Earth Observatory. “Reconstructing the climate history of these regions is critical to both better understanding the potential magnitude of future drought and for evaluating how accurate these climate models may be."
In this study, Anchukaitis worked on the statistical analysis and drought reconstruction. Though the tropics are his specialty, an environment in some ways more forgiving than the parched subtropics, he stressed the importance of expanding the network of tree ring chronologies into arid regions where “drought is, has been, and will be a critical determinant of the well-being of both human populations and ecosystems.”
The study provides additional evidence that Moroccan drought is triggered by different oceanic and atmospheric conditions than droughts in Algeria or Tunisia. Drought in Morocco is strongly linked to air-pressure anomalies in the North Atlantic Ocean called the North Atlantic Oscillation, while drought in Algeria and Tunisia appears more closely linked to warm sea temperatures in the tropical Atlantic.
Researchers hope to expand the network's reach across North Africa to Libya and parts of Algeria, and go back far enough in time to make comparisons with the archaeological record. "If we can bridge this gap, it will be a discovery for the world," said Touchan.
Other researchers: David Meko, University of Arizona; Mohamed Sabir of the National School of Forest Engineering, Morocco; Said Attalah of the University of Ourgla, Algeria; and Ali Aloui of the Institute of Sylvo-Pastoral of Tabarka, Tunisia. The study was funded by the National Science Foundation.