By Timothy Gardner

NEW YORK, Aug 16 (Reuters) - The extent of Arctic sea ice will likely have melted to a record low this September partially due to man-made greenhouse gas emissions, researchers at the University of Colorado said on Thursday.

There is a 92 percent chance that Arctic sea ice extent in September will melt to its lowest level at least since the 1970s, when satellite measuring efforts began, the researchers said. They had predicted a 33 percent chance of a record low in April, but changed the forecast after a rapid disintegration of sea ice during July.

Such high levels of ice melting could have wide implications in coming years such as changes in temperature and rain patterns across much of the United States.

"Similar to the way the El Nino pattern affects weather in the United States, more ice melt could change rain patterns and temperature patterns in the middle of the United States, which could have economic impacts on farmers," Sheldon Drobot, who leads Arctic ice forecasting at CU-Boulder's aerospace engineering department, said in an interview.

It could also open the Northwest Passage along the northern coast of North America and connect the Atlantic and Pacific Oceans to shipping by as early as 2020 or 2025, he said. That could be a cheaper option for many shippers than the Panama Canal.

Whatever the effects, a rise in heat-trapping emissions such as carbon dioxide is partially responsible, the university said.

"There is an element of human activity in the cause of this melt," said Drobot. "Natural variations can't explain everything."

High levels of greenhouse emissions from smokestacks and tailpipes have combined with natural fluctuations, such as an increase in cloud-free days over the Arctic this summer, to spur the melt, he said.

Arctic sea ice researchers pay particular attention to the months of September and March because they generally mark the annual minimum and maximum sea ice extent, respectively. Sea ice extent, the area of an ocean covered by at least 15 percent ice, has been declining at least since the late 1970s.

The CU-Boulder department used satellite data from the U.S. Department of Defense and temperature records from the National Oceanic and Atmospheric Administration for the ice forecasts, which it has been producing for five years.

The record low September minimum for sea ice, set in 2005, is 2.15 million square miles (5.6 million sq km). For 2007, the highest probability minimum extent is 1.96 million square miles (5 million sq km), and there is a 25 percent chance the low will fall to 1.88 million square miles (4.9 million sq km), Drobot said.

The melt itself can act as a feedback loop and cause ever more melting, because water has a darker surface than snow. "Water acts like a sponge sucking up a lot more solar energy," Drobot said.

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