The reason is an atmospheric high-pressure system due to climate change, according to a study released Monday that warns of serious impacts on wine and olive production.
However, in a new modeling study published in the journal Nature Geoscience, U.S. researchers found that this high-pressure system “has changed dramatically over the past century and that these changes in North Atlantic climate are unprecedented over the past millennium.”
Using climate model simulations over the past 1200 years, the study found that this high-pressure system began to grow and cover a larger area about 200 years ago, when human greenhouse gas pollution increased.
It expanded even more dramatically in the 20th century as global warming continued.
The researchers then analyzed hundreds of years of rainfall preserved in Portuguese stalagmites and found that as the Azores High expanded, winters in the western Mediterranean became drier.
The study predicts that rainfall could drop by another 10 to 20 percent by the end of this century, which would make Iberian agriculture “one of the most vulnerable in Europe,” according to the authors.
They warn that the Azores High will continue to expand in the 21st century as greenhouse gas concentrations rise, increasing the risk of droughts in the Iberian Peninsula and threatening important crops.
“Our results have important implications for projected changes in the hydroclimate of the western Mediterranean region in the 21st century,” the authors said.
According to the study, the Azores High acts as a precipitation “gatekeeper” in Europe, with dry air flowing down during the summer months and causing hot, dry conditions in much of Portugal, Spain and the western Mediterranean.
During the coldest and wettest winter period, the high-pressure system swells and brings in westerly winds that carry rain inland.
This winter precipitation is “vital” to both the ecological and economic health of the region, but it has become particularly important in the latter half of the 20th century.
While previous research has been unable to decipher the effects of the natural variability of the Azores high, the authors of the study show that the expansion of the Azores high during the industrial age is linked to increasing greenhouse gas concentrations in the atmosphere.
One study cited in the latest research estimates that the area suitable for growing grapes in the Iberian Peninsula could shrink by at least a quarter and possibly almost disappear by 2050 due to severe water shortages.
Meanwhile, researchers have predicted a 30 percent decline in production in the olive-growing regions of southern Spain by 2100.
Growers are already looking at ways to adapt to the changing climate, such as moving vineyards to higher elevations and experimenting with more heat-tolerant varieties.
Last year, scientists found that a severe spring frost that devastated vines in France was most likely due to climate change, as plants sprouted earlier and were therefore more susceptible to damage.