Spain, Portugal dryness ‚unprecedented 'in 1,200 years – or maybe not, say critics

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Some scientists contacted by Carbon Brief have their doubts about the reasons given for the reported expansion of the Azores high. An assistant professor at the University of North Carolina, Charlotte, warns that “the title statement is not justified by the study”. Another assistant professor, at the University of Dartmouth, told Carbon Brief ‘that changes in the size and intensity of the Azores high could also have been driven by changes in aerosol levels, rather than changes in greenhouse gases emissions. (For example, the passing of the US Clean Air Act in the 1970s saw pollution levels drop significantly, causing localised warming.) That the authors did not investigate this factor is “a curious omission” he says.’ On top of that, Prof Richard Seagar – a research professor at Columbia University – told Carbon Brief that the expansion in the Azores high could also “easily be explained by the long-term variability and changes of the North Atlantic Oscillation”.
In short, this attempt to put the blame on humans for the more recent climatic conditions related to this phenomenon is already starting to look shaky.
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Parts of Portugal and Spain are the driest they have been in a thousand years due to an atmospheric high-pressure system driven by climate change, according to research published Monday, warning of severe implications for wine and olive production.

The Azores High, an area of high pressure that rotates clockwise over parts of the North Atlantic, has a major effect on weather and long-term climate trends in western Europe says Phys.org.

But in a new modeling study published in the journal Nature Geoscience, researchers in the United States found this high-pressure system “has changed dramatically in the past century and that these changes in North Atlantic climate are unprecedented within the past millennium”.

Using climate model simulations over the last 1,200 years, the study found that this high-pressure system started to grow to cover a greater area around 200 years ago, as human greenhouse gas pollution began to increase.

It expanded even more dramatically in the 20th century in step with global warming.

The authors then looked at evidence of rainfall levels preserved over hundreds of years in Portuguese stalagmites, and found that as the Azores High has expanded, the winters in the western Mediterranean have become drier.

The study cites projections that the level of precipitation could fall a further 10 to 20 percent by the end of this century, which the authors say would make Iberian agriculture “some of the most vulnerable in Europe”.

They warn that the Azores High will continue to expand during the 21st century as greenhouse gas levels rise, leading to an increasing risk of drought on the Iberian Peninsula and threatening key crops.

“Our findings have important implications for projected changes in western Mediterranean hydroclimate throughout the twenty-first century,” the authors said.

Wither vines

The Azores High acts as a “gatekeeper” for rainfall into Europe, according to the study, with dry air descending in the summer months to cause hot, arid conditions in much of Portugal, Spain and the western Mediterranean.

In the cool, wetter winter period, the high-pressure system swells, sending westerly winds carrying rain inland.

This winter rain is “vital” for both the ecological and economic health of the region, but it has been decreasing, particularly over the second half of the 20th century.

While previous research had not untangled the effects of natural variability on the Azores High, the authors said their findings show its expansion during the industrial era is linked to the rise of atmospheric greenhouse gas concentrations.

Full article here.