Paris (AFP) - Predictions of unprecedented rainfall extremes in the 20th century driven by global warming turned out wrong, a study said Wednesday, casting doubt on methods used to project future trends.

A massive trawl of Northern Hemisphere rainfall data for the last 1,200 years revealed there had been more dramatic wet-dry weather extremes in earlier, cooler centuries before humans set off fossil fuel-driven global warming.

This is problematic, said a study in the journal Nature, as the same data models used to anticipate that global warming would cause record rainfall extremes in the 1900s, are the basis for projections of things to come.

"It might be more difficult than often assumed to project into the future," the study's lead author Fredrik Ljungqvist of Stockholm University told AFP of the findings.

"The truth can be much, much more complicated."

The UN's climate science panel, the consensus authority, contends that dry areas will become ever drier and wet ones wetter as the global temperature rises in response to greenhouse gas emissions.

But the new work said sky-high temperatures in the 20th century did not directly translate into record extremes between wet and dry weather, as many had expected.

This meant that "much of the change is not only driven by temperature, but some internal, more random variability," explained Ljungqvist.

"It's therefore very, very hard also to predict (precipitation extremes) with models."

Over the study period, drought was most severe during the 12th century, which was a warm one, and the 15th which was cold, said the scientist.

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For the study, a team of experts in history, climate, geology and mathematics, compiled drought and rainfall data for Europe, North Asia and North America, and reconstructed 12 centuries worth of "water history".

They considered geologically preserved evidence of stream flow, lake levels, marine and lake sediments, tree rings and historical records.

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The team's reconstruction for the 20th century differed vastly from climate models which had suggested wet areas should have been wetter, and dry ones drier, than ever before.

"In the past, on a longer timescale, there have been even larger variabilities," said Ljungqvist.

This divergence "certainly adds fuel to the fiery debate" on the link between warming and rainfall extremes, Matthew Kirby of California State University's Department of Geological Sciences wrote in a comment published by Nature.

"Do their results invalidate current predictive models? Certainly not. But they do highlight a big challenge for climate modellers, and present major research opportunities both for modellers and climate scientists..."

James Renwick of the Victoria University of Wellington said the predicted wet and dry extremes are "very likely" to materialise in the 21st, century.

Extreme drought and downpours are among many risks that scientists warn about in a warmer world. Others include land-gobbling sea level rise, crop and water shortages, disease spread and wars over dwindling resources.

In December, the nations of the world signed a pact to limit average global warming to no more than two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels, when the fossil fuel burning began.

Research suggests we may already have reached 1 C.