A recent 14-year dry spell in the Middle East was the worst drought in the past 900 years, according to a new NASA study released this week.

NASA's researchers examined records of rings of trees in several Mediterranean countries to determine patterns of dry and wet years across a span of 900 years. They concluded that the years from 1998 to 2012 were drier than any other period, and that the drought was likely caused by humans.

The study's lead author Ben Cook said the range of extreme weather events in the eastern Mediterranean has varied widely in the past nine centuries, but the past two decades stand out.

"This recent drought falls outside the range of natural variability," he said. Drought has continued in parts of the Middle East, he added.

Cook is a climate scientist at NASA's Goddard Institute for Space Studies and the Lamont Doherty Earth Observatory at Columbia University in New York City.

The researchers used records of tree rings in Northern Africa, Greece, Lebanon, Jordan, Syria and Turkey, and combined the data with records from Spain, southern France and Italy to examine patterns of drought across time in the region.

They studied rings of trees, both living and dead, that were sampled all over the region. Rings in the trunks of trees represent years. Thin rings indicate dry years; thick rings show years when water was abundant.

An artificial lake with depleted levels of water in Qaraoun, Lebanon, on Dec. 2, 2010. (Mohamed Azakir/Reuters)

Cook said the research supported other studies indicating human causes of extreme climate events.

Last year, researchers at Columbia University and the University of California Santa Barbara found that drought triggered a collapse in agriculture in Syria and the migration of 1.5 million farmers to the cities, straining resources.

The water shortage was one of several contributing factors that had worsened the situation in Syria in the lead-up to the outbreak of that country's devastating civil war in 2011.

Michael Mann, director of the Earth System Science Center at Penn State University, said the NASA study is one of several worrying reports about unprecedented climate conditions.

Mann was not involved in NASA's study.

In an email to The Associated Press, Mann noted that tree rings "have their limitations and uncertainties," but said "the authors have done a reasonable job in assessing the uncertainties."