Current regional dry spell appears to be the most severe in more than 900 years
The Mediterranean region may already be feeling the impacts of human-caused climate change, according to a new tree ring study that compared an ongoing drought in the region with historic climate conditions.
Based on the findings, NASA scientists concluded the current drought — spread across Cyprus, Israel, Jordan Lebanon, Palestine, Syria and Turkey — is the worst in at least 900 years. Nearly all climate models project that global warming will dry up the Mediterranean region.
In addition to identifying the driest years, the new tree ring analysis helped the researchers find geographic patterns that help identify the causes of drought. The data help show the range of natural variation in Mediterranean drought occurrence, which will allow scientists to differentiate droughts made worse by human-induced global warming.
The research is part of NASA’s ongoing work to improve the computer models that simulate climate now and in the future. The results were accepted for publication in the Journal of Geophysical Research-Atmospheres, a publication of the American Geophysical Union.
“This paper shows that the behavior during this recent drought period is different than what we see in the rest of the record,” said Yochanan Kushnir, a climate scientist at Lamont Doherty Earth Observatory, who was not involved in the research. That means the Levant region may already be feeling the affects of human-induced warming of the planet, Kushnir said.
“If we look at recent events and we start to see anomalies that are outside this range of natural variability, then we can say with some confidence that it looks like this particular event or this series of events had some kind of human caused climate change contribution,” said Ben Cook, lead author and climate scientist at NASA’s Goddard Institute for Space Studies and the Lamont Doherty Earth Observatory at Columbia University in New York City.
“The magnitude and significance of human climate change requires us to really understand the full range of natural climate variability,” Cook said. The range of how extreme wet or dry periods were is quite broad, but the recent drought in the Levant region, from 1998 to 2012, stands out as about 50 percent drier than the driest period in the past 500 years, and 10 to 20 percent drier than the worst drought of the past 900 years, Cook said.
The study was based on a tree-ring record called the Old World Drought Atlas, including tree rings from living and dead from the entire region, from northern Africa, Greece, Lebanon, Jordan, Syria and Turkey.
Combined with existing tree-ring records from Spain, southern France, and Italy, these data were used to reconstruct patterns of drought geographically and through time over the past millennium. Having such a large area covered allowed the science team not only to see variations in time, but also geographic patterns across the region.
Most historic droughts have been widespread across the region, affecting the Mediterranean Basin from east to west, according to co-author Kevin Anchukaitis, a climate scientist at the University of Arizona in Tucson.
“Both for modern society and certainly ancient civilizations, it means that if one region was suffering the consequences of the drought, those conditions are likely to exist throughout the Mediterranean basin,” Anchukaitis said. “It’s not necessarily possible to rely on finding better climate conditions in one region than another, so you have the potential for large-scale disruption of food systems as well as potential conflict over water resources,” he said.
In addition, the science team found that when the northern part of the Mediterranean–Greece, Italy, and the coasts of France and Spain–tended to be dry when eastern North Africa was wet, and vice versa. These east-west and north-south relationships helped the team understand what ocean and atmospheric conditions lead to dry or wet periods in the first place.
The two major circulation patterns that influence when droughts occur in the Mediterranean are the North Atlantic Oscillation and the East Atlantic Pattern. These airflow patterns describe how winds and weather tend to behave depending on ocean conditions. They have periodic phases that tend to steer rainstorms away from the Mediterranean and bring in dryer, warmer air. The resulting lack of rain and higher temperatures, which increase evaporation from soils, lead to droughts.