A new NASA study has shown that the drought that began in 1998 in the eastern Mediterranean Levant region may be the worst drought in 900 years. The new findings have implications when it comes to studying climate change.

In this latest study, published in the journal Geophysical Research Letters, the researchers reconstructed the Mediterranean's drought history by studying tree rings as part of an effort to understand the regions climate and what shifts water to or from the area. Thin rings indicate dry years while thick rings show years when water is plentiful.

In addition to identifying the driest years, the researchers discovered patterns in the geographic distribution of droughts that provides a "fingerprint" for identifying the underlying causes. Together, the data shows a range of natural variation in Mediterranean drought occurrence, which will allow scientists to differentiate droughts made worse by human-induced global warming.

"The magnitude and significance of human climate change requires us to really understand the full range of natural climate variability," said Ben Cook, lead author of the new study and climate scientist at NASA's Goddard Institute for Space Studies. "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."

Between the years 1100 and 2012, the researchers found droughts in the tree-ring record that corresponded to those described in historical documents written at the time. While the range for how extreme wet or dry periods were was broad, 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. It was also 10 to 20 percent drier than the worst drought of the past 900 years.

The two major circulation patterns that influence drought 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.

"The Mediterranean is one of the areas that is unanimously projected [in climate models] as going dry in the future [due to man-made climate change]," said Yochanan Kushnir, a climate scientist who was not involved in the research. "This paper shows that the behavior during this recent drought period is different than what we see in the rest of the record."

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