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Blocking patterns: How global warming might have worsened US drought

According to NOAA, September 2011 through August 2012 was the warmest 12-month period on record and summer 2012 was the third warmest summer on record for the continental US. In addition, about 63% of the country was experiencing moderate to exceptional drought at the end of August. Experts believe that small changes triggered by global warming could be intensifying these extremes. One possible contributor is the decline in Arctic sea ice, which reached a record low this past summer. More open ocean and an increase in heat escape to the atmosphere in polar regions tend to slow the jet stream and cause it to meander further north and south. The movement of the meanders also slows, causing weather patterns to persist for longer periods of time. In addition, earlier snowmelt in the spring and a later onset of snow in the fall causes the soil to dry out and contributes to warmer temperatures in spring and summer. Another study has attributed extremes in other countries (Russia, Brazil, Pakistan and Colombia) to record warm sea-surface temperatures in the northern Indian Ocean, Gulf of Mexico, Caribbean Sea and the tropical Pacific. The atmosphere, already warmed by climate change and capable of holding more moisture caused more intense storms to form in these regions and their associated release of heat to the atmosphere caused significant changes in circulation patterns worldwide. Experts believe that the direct impact of global warming on most of these weather patterns is small, although this small contribution has been enough to trigger feedbacks that intensify the warming indirectly.

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