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California's three year drought is the worst seen for over 1200 years, researchers have found.

Scientists looking at the cumulative effects of temperature, low precipitation and other factors said that it all adds up to the worst conditions in more than a millennium.

The also claim that worse could be to come, with droughts almost certain to occur again in the future. 

The dry bed of the Stevens Creek Reservoir in California: A combination of record high temperatures and sparse rainfall during California'sthree-year drought have produced the worst conditions in 1,200 years.

A combination of record high temperatures and sparse rainfall during California's three-year drought have produced the worst conditions in 1,200 years, according to a study accepted for publication by the American Geophysical Union (AGU). 

'The current California drought is exceptionally severe in the context of at least the last millennium and is driven by reduced though not unprecedented precipitation and record high temperatures,' the report's authors said in the study released late Thursday.

The study by the University of Minnesota and the Woods Hole Oceanographic Institution said that warm, dry conditions have shrunk the supply of surface water from reservoirs, streams and the Sierra Nevada snowpack in the state, even as demand from people and farms has gone up, resulting in unprecedented scarcity. 

Daniel Griffin, an assistant professor in the Department of Geography, Environment and Society at the University of Minnesota, and Kevin Anchukaitis, an assistant scientist at Woods Hole Oceanographic Institution, asked the question, 'How unusual is the ongoing California drought?' and collected new tree-ring samples from blue oak trees in southern and central California. 

'California's old blue oaks are as close to nature's rain gauges as we get,' says Griffin. 

'They thrive in some of the driest environments where trees can grow in California.' 

These trees are particularly sensitive to moisture changes and their tree rings display moisture fluctuations vividly.

'We were genuinely surprised at the result,' says Griffin, a NOAA Climate & Global Change Fellow and former WHOI postdoctoral scholar.

'This is California - drought happens. 

'Time and again, the most common result in tree-ring studies is that drought episodes in the past were more extreme than those of more recent eras. 

'This time, however, the result was different.

'One thing is clear, drought is going to continue to happen. 

'This is the kind of thing we get to see in the future.'

The low water level of California's Lake Kaweah, February 5, 2014. Now in its third straight year of unprecedented drought, California is experiencing its driest year on record, dating back 119 years and possible the worst in the past 500 years.

While there is good evidence of past sustained, multi-decadal droughts or so-called 'megadroughts'' in California, the authors say those past episodes were probably punctuated by occasional wet years, even if the cumulative effect over decades was one of overall drying. 

The current short-term drought appears to be worse than any previous span of consecutive years of drought without reprieve.

It comes as California is experiencing a wet start to December that could result in 12-inches (30 cm) of rain and yards (meters) of snow over the next two weeks, according to the forecasting service Accuweather.

In October, the AGU published a study by NASA's Goddard Institute for Space Studies in New York City saying that the 1934 U.S. drought, which caused the upheaval known as the Dust Bowl, was the worst in 1,000 years.

With an exceptionally wet winter, parts of California might emerge from the drought this year. 'But there is no doubt,' cautions Anchukaitis, 'that we are entering a new era where human-wrought changes to the climate system will become important for determining the severity of droughts and their consequences for coupled human and natural systems.'