Shipping on the U.S. Great Lakes is suffering as a result of low water levels, a climate scientist and industry analysts say.

Water levels have remained below average for nearly 15 years. While the levels increased slightly in July and August following a wet, rainy spring, lakes Huron and Michigan remained at record lows about 18 inches below their average in July.

"In order to restore water levels, you need a series of wet years like we had this past spring, as the amounts of water involved are extraordinarily large," Paul Roebber, founder of Innovative Weather at the University of Wisconsin, Milwaukee was quoted as saying Wednesday in The Guardian newspaper.

The $34 billion Great Lakes shipping sector transports more than 160 million tons of cargo each year. Vessels traveling on the Great Lakes can typically transport cargo nearly $20 cheaper per ton compared to railways or trucks, using less fuel. Iron ore, coal and limestone, for example, are dependent on lake travel.

When water levels are low, however, shippers can't load their vessels with as much cargo.

The falling water level "causes a lot of inefficiencies," Mark Barker, president of The Interlake Steamship Co, one of the biggest shipping companies on the lakes, told The Guardian.

Interlake's biggest ship, the Paul R. Tregurtha, this spring had to cut its load by 6,000 tons per trip due to the low water levels, sharply reducing revenue. The vessel normally carries up to 68,000 tons of cargo.

"The aggregate impact over time will be to raise the cost of commodities, which in turn will raise the price of manufacturing goods, which in turn raises the price to the consumer," Richard D. Stewart, director of the Transportation and Logistics Research Center at the University of Wisconsin-Superior, told The New York Times.

In late July, six Great Lakes senators wrote to President Obama urging him "to fully incorporate the risks and impacts to the Great Lakes" as he charts a plan for confronting climate change.

"This year, Great Lakes water levels reached new historic lows severely hampering commercial shipping, jeopardizing recreational boating and fishing, devastating the tourism industry, threatening electric power generation, compromising water supply infrastructure, and exacerbating problems caused by invasive species," they wrote.

The lawmakers noted that the heavy rains and subsequent flooding this year exacerbated shipping problems, because excessive sedimentation further restricted shipping channels.

"If low water levels persist, harbors will continue to close and vessels will continue to light load, reducing our global competitiveness," the senators said.