Climate change major factor in more frequent Mediterranean droughts
Wintertime droughts are increasingly common in the Mediterranean region, and human-caused climate change is partly the cause, according to a new analysis by scientists at the Cooperative Institute for Research in Environmental Sciences (CIRES) and colleagues at NOAA. In the lands surrounding the Mediterranean Sea, 10 of the driest 12 winters since 1902 have struck in just the last 20 years.
“The magnitude of drying that has occurred is too great to be explained by natural variability alone,” said Martin Hoerling of NOAA’s Earth System Research Laboratory in Boulder, Colo., lead author of a paper published online in the Journal of Climate Oct. 27, 2011. “This is not encouraging news for a region that already experiences water stress because it implies natural variability alone is unlikely to return the region’s climate to normal.”
Hoerling’s team uncovered a pattern of increasing wintertime dryness that stretched from Gibraltar to the Middle East. The scientists used observations and climate models to investigate several possible culprits including natural variability, a cyclical climate pattern called the North Atlantic Oscillation (NAO), and climate change caused by greenhouse gases released into the atmosphere during fossil fuel use and other human activities.
Climate change from greenhouse gases explained roughly half the increased dryness of 1902-2010, the team found. This means that other processes – none specifically identified in the new investigation – may also have contributed to increasing drought frequency in the region.
The team also found a surprising coincidence between the observed increase in winter droughts and in the projections of climate models that include known increases in greenhouse gases and aerosols. Both observations and model simulations show a sudden shift to drier conditions in the Mediterranean beginning in the 1970s.
The physical reasons for the relationship between climate change and Mediterranean drought involved sea surface temperatures, the researchers reported. In recent decades, greenhouse-induced climate change has caused somewhat greater warming of the tropical oceans than other ocean regions. That pattern has acted to drive drought-conducive weather patterns around the Mediterranean. The timing of the ocean change coincides closely with the timing of increased droughts, the scientists found.
The Mediterranean has long been identified as a “hot spot” for substantial impact from climate change in the latter decades of this century, because of water scarcity in the region and climate modeling that projects increased risk of drought.
“The question has been whether this projected drying has already begun to occur,” Hoerling said. “The answer is yes, in winter.”
Coauthors on the paper “On the Increased Frequency of Mediterranean Drought” are CIRES Fellow Judith Perlwitz and CIRES scientists Jon Eischeid, XiaoWei Quan,Tao Zhang, Philip Pegion.