Human activities may increase winter droughts

Friday 28th Oct 2011 by theWeather Club

According to a new analysis by the US based National Oceanic and Atmospheric Administration (NOAA), scientists and colleagues at the Cooperative Institute for Research in Environmental Sciences (CIRES), winter droughts are becoming increasingly common in the Mediterranean region, and human-caused climate change is partly responsible. In the last 20 years, 10 of the driest 12 winters have taken place in the lands surrounding the Mediterranean Sea.

"The magnitude and frequency of the drying that has occurred is too great to be explained by natural variability alone," said Martin Hoerling, Ph.D. of NOAA's Earth System Research Laboratory, lead author of a paper published online in the Journal of Climate this month. "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." This is potentially a very serious issue as the region gets most of its water during the winter, and Hoerling's team uncovered a pattern of increasing winter dryness stretching from Gibraltar to the Middle East.

Scientists used observations and climate models to investigate several potential causes, including natural variability, a cyclical climate pattern called the North Atlantic Oscillation (NAO), and climate change caused by greenhouse gases released into the atmosphere by human activity. The team found that Climate change from greenhouse gases explained roughly half the increased dryness of 1902-2010. This means that other processes - none specified - processes have contributed to increasing frequency of winter droughts.

The study also showed a close correlation between the observed increase in winter droughts, and the projections of climate models that include known increases in greenhouse gases. Both observations and model simulations show a sudden shift to drier conditions in the Mediterranean beginning in the 1970s. In this analysis, sea surface temperature patterns emerged as the primary reason for the relationship between climate change and Mediterranean drought. In recent decades, greenhouse-induced climate change has caused greater warming of the tropical oceans compared to other ocean regions. That pattern acts to drive drought-conducive weather patterns around the Mediterranean. The scientists found that timing of ocean temperature changes coincided closely with the timing of increased drought occurrence.

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, a rapidly increasing population, and climate modelling that projects increased risk of drought. "The question has been whether this projected drying has already begun to occur in winter, the most important season for water resources," Hoerling said. "The answer is yes."