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OSLO (Reuters) - Climate records back to Viking times show the 20th century was unexceptional for rainfall and droughts despite assumptions that global warming would trigger more wet and dry extremes, a study showed on Wednesday.
Stretching back 1,200 years, written accounts of climate and data from tree rings, ice cores and marine sediments in the northern hemisphere indicated that variations in the extremes in the 20th century were less than in some past centuries.
"Several other centuries show stronger and more widespread extremes," lead author Fredrik Ljungqvist of Stockholm University told Reuters of findings published in the journal Nature. "We can't say it's more extreme now."
Pinning down links between global warming and rainfall is vital to planning billion-dollar investments in everything from irrigation for food production to flood defenses along rivers.
Ljungqvist said many existing scientific models of climate change over-estimated assumptions that rising temperatures would make dry areas drier and wet areas wetter, with more extreme heatwaves, droughts, downpours and droughts.
The 10th century, when the Vikings were carrying out raids across Europe and the Song dynasty took power in China, was the wettest in the records ahead of the 20th, according to the researchers in Sweden, Germany, Greece and Switzerland.
And the warm 12th century and the cool 15th centuries, for instance, were the driest, according to the report, based on 196 climate records. Variations in the sun's output were among factors driving natural shifts in the climate in past centuries.
Ljungqvist said the findings did not mean current climate change, blamed on rising man-made greenhouse gas emissions, was less of a threat than thought.
"Absolutely not," he said, adding that the pace of warming had increased in recent years and that the 20th century was the warmest in the records.
Last December, 195 nations agreed to shift from fossil fuels and aim for zero net greenhouse gas emissions by 2100 to rein in rising temperatures.
Other climate experts who were not involved in the study said it highlighted complexities in predicting global warming.
"This paper adds to the growing evidence that the simple paradigm of 'wet-gets-wetter, dry-gets-drier' under a warming climate does not apply over land areas," said Ted Shepherd, a professor at the University of Reading.
James Renwick, of Victoria University of Wellington, said it was always hard to match century-long data with recent decades of warming.
"We know that human-induced climate change is already affecting the hydrological cycle", he wrote, with evidence such as recent drought in Syria that he said was the worst in 900 years.
Reporting By Alister Doyle; Editing by Richard Balmforth