LARAMIE (AP) - A study of tree rings dating back 750 years show previous droughts have in the West have persisted for decades.

The findings of researchers from the University of Wyoming, U.S. Geological Survey and Middlebury College in Vermont are being published in the scientific journal "Geophysical Research Letters."

"This is only the region's fourth major drought since Euro-Americans settled the West," said Stephen Jackson, a professor in the University of Wyoming Department of Botany.

"Droughts occurring in the late 1500s and 1740s, chronicled in the tree rings, were more severe than the lengthy droughts in the 1890s, the 1930s, and the 1950s.

"Those earlier droughts were much more severe than anything we have experienced over the past century, with reduced moisture during periods as long as two or three decades."

Jackson and Steve Gray, a doctoral student, studied rings of limber pines, Douglas firs and ponderosa pine growing in rock outcroppings of the Big Horn Basin and Absaroka Range.

They also examined the rings of pinon pines in the Uinta Mountains and Flaming Gorge area of northeastern Utah and southwest Wyoming.

"These are very tough trees that can survive prolonged droughts," Jackson said. "These ancient trees form a living environmental monitoring network that provides a historical perspective showing that droughts have been much worse in the past, and could happen again."

Drought causes tree rings to grow much thinner. Thinner tree rings in a widespread area shows drought, whereas thinner rings in a small area suggests that another factor, such as insects, might have been the cause.

Tree rings indicate that "megadroughts" have occurred when the tropical Pacific turned cold at the same time that the North Atlantic warmed, according to Jackson.

Records show that this happened simultaneously with a drought in the 1950s and the same thing is happening now.

"If these slow shifts in ocean regime can be identified in their early stages, then perhaps they can be used to assess the probability of disastrous, multiyear droughts across the North American continent and elsewhere," Jackson said.

"The tree rings suggest that when these conditions have happened in the past, they have persisted a long time, more than just a few years," says Jackson.

"We can't predict the duration of the current drought from the tree-ring data, but our tree-ring studies suggest that we cannot afford to be complacent about the current drought. It could last a long time."

The tree-ring studies reveal that the historic drought periods did not occur at regular intervals and did not occur consistently throughout the Rocky Mountain region, suggesting complex links between seasonal precipitation and ocean temperatures.

"We hope to use the information on the relationship between ocean temperature regimes and North American climate to guide us in more effective long-term water management and to anticipate climatic effects on ecosystems," he said.