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article imageCalifornia's drought caused by climate change? Maybe not

By Karen Graham     Sep 30, 2014 in Environment
After examining the data from 16 extreme weather events occurring last year, from severe floods in Colorado to California's on-going drought, scientists have come to a startling conclusion. The drought cannot be linked to climate change.
In a special supplement to the September issue of the Bulletin of the American Meteorological Society, 20 different research groups examined 16 severe weather events around the world that occurred in 2013. In the report released on Monday, researchers said that climate change is fueling much of extreme heat found in many parts of the world. But the link between climate change and other extreme weather events proved to be less certain.
The report pointed out that any human influence on heavy rains, droughts and heavy storms is sometimes doubtful, and are more indicative of natural variations. A number of the groups focused their studies on the Australian heat waves and the three-year California drought. This gave the groups the advantage of comparing and contrasting the strengths and weaknesses of their methodologies.
World map showing areas where researchers studied weather events.
World map showing areas where researchers studied weather events.
NOAA
In assessing drought conditions, several factors have to be taken into consideration. They include snowpack, temperatures, soil moisture and the lack of precipitation, as well as the severity and length of time the conditions last. Stephanie Herring, the report's lead editor from NOAA's National Climatic Data Center said when you added land and water use to the equation, the severity and influence of those factors become more difficult to assess.
The three-year California drought
There was much discussion of the infamous high-pressure ridge, dubbed the "Ridiculously Resilient Ridge" that sat off the California coast in 2013-2014. The ridge has blocked rainstorms from reaching the Pacific Coast in the last few years, and the scientists suggested this may be partially the result of climate change.
Mono Lake California  August 2014.
Mono Lake California, August 2014.
Mary Phillips
However, and this is big, they could not say conclusively that global warming is responsible for California's persistent drought. Thomas Karl, is the director of the National Oceanic and Atmospheric Administration’s (NOAA) National Climatic Date Center and one of the authors of the new report. He said “It’s a lot easier to associate heat extremes with human influences than it is some of the other extremes."
The scientific teams concluded that persistent drought experienced by the state was caused by the high-pressure ridge that sat off the California coast. They found that the persistence of this ridge was similar in structure, but spatially more extensive than similar atmospheric conditions that have led to droughts in California's past.
The report also points to the timing of the event as playing a role in the severity of the drought. According to the report: "The event began suddenly in January 2013, abruptly truncating what had initially appeared to be a wet rainy season following very heavy precipitation during November–December 2012." But because the event persisted through January, the rainy-season was delayed by at least four months.
The human factor came into play when decision making by level-heads was needed to determine water allocation to urban, agricultural, and environmental interests. Not knowing before hand the length of the drought conditions made those decisions all the more difficult. Knowing the extent and complexity of the drought, as well as the fact that global warming has increased the chance of extreme North Pacific high-pressure ridges, makes it all the more necessary that we understand the link between the two in accessing future droughts.
Heat wave grips the outback in January  2013.
Heat wave grips the outback in January, 2013.
Screen grab
Natural variability played a role around the globe
It was the conclusion of the researchers that human-caused ( anthropogenic ) climate change has played a role in heat waves. Anthropogenic climate change sometimes makes these heat waves 10-times more likely to occur. But natural variability played a role in many other weather events.
From flooding in India to the deep snow in the Spanish Pyrenees Mountains and the California drought, these varied weather conditions all have one thing in common, according to the researchers. They are examples of natural variability playing a much larger role in extreme precipitation events. In summarizing their finding about global warming, the researchers said, "Although global warming has been described as "pausing" since 2000, global temperatures remain at anomalously high levels, and warm annual and seasonal temperature extremes continue to far outpace the occurrence of cold annual extremes."
More about Drought, California, Climate change, high pressure systems, Rainfall
 
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