Two unusually wet years have finally ended the lengthiest period on record of low Great Lakes water levels — a blessing for long-suffering cargo shippers and recreational boaters — although scientists said Tuesday it's uncertain whether the recovery is temporary or heralds a trend.

Four of the giant lakes — Superior, Huron, Michigan and Erie — were above their average monthly levels in November, while Lake Ontario was slightly below. In September, all five were simultaneously above average for the first time since the drop-off began in the late 1990's. A newly released forecast predicts little change over the next six months.

It's a dramatic and remarkably swift rebound from January 2013, when Michigan and Huron — which are linked and have the same level — reached their lowest point since the government began keeping records nearly a century earlier. The others were mired in a prolonged slump, although Ontario was better off because hydropower dams help regulate its ups and downs.

"On Superior, Michigan and Huron, we haven't seen two-year water level increases of this magnitude" in recorded history, said Keith Kompoltowicz, a hydrologist with the U.S. Army Corps of Engineers district office in Detroit.

The wild swing illustrates the uncertainty that may confront the eight states and two Canadian provinces where the lakes are an economic pillar and source of drinking water for more than 30 million people. Climate models suggest global warming may fuel increasingly severe storms and droughts, along with temperature extremes that can influence evaporation rates.

Scientists cautious in predictions

"That's certainly a risk going forward," said Jim Noel of the NOAA Ohio River weather forecasting centre. "When we have a warming climate system, the bottom line is that it yields greater year-to-year variability."

But scientists avoided specific predictions beyond their six-month forecast, saying too many factors were uncertain.

What is clear is that the recent comeback was propelled by heavy rain and snow over the lakes themselves and within their watershed, which increased runoff from land, Kompoltowicz said.

Evaporation also influences water levels. Scientists say it played a leading role in the 15-year low period, as shrinking ice cover boosted winter evaporation rates. But more recent studies suggest heavy evaporation takes place before ice sheets form, which could partially offset the benefit of ice in slowing evaporation.

Even the ice during last winter's deep freeze — which at one point covered more than 90 per cent of the lakes' combined surface area — was a minor cause of the water level surge compared to the rain and snow, said Drew Gronewold, a hydrologist with the NOAA Great Lakes Environmental Research Laboratory in Ann Arbor.

Shipping to benefit

Regardless of the cause, higher water is giving a break to the shipping industry, which for years has paid a stiff price as shallow harbours and channels forced vessels to lighten their loads of iron ore, coal and other bulk cargo.

"It's true that Mother Nature helped us out this year after that brutal winter, but water levels fluctuate," said Glen Nekvasil, spokesman for the Lake Carriers Association. "There's no guarantee they'll be this way next year."