Great Lakes ice cover shows climate change's existence

Spiel · April 3, 2009

Great Lakes ice cover shows climate change's existence -- and its complexity

March 23, 2009

Looking for some solid evidence that global warming is forcing slow but certain changes on the Great Lakes region?

Take a peek inside the freezer: Winter ice cover in the middle of Lake Erie and the other Great Lakes has been diminishing by 1.3 percent a year for three decades.

That's more than a 30 percent decline since the 1970s, lake scientists say.

But those same scientists affirm that an expansive and long-lasting ice cover this winter -- Lake Superior remained partially frozen early this week -- is evidence that the lakes are also heavily influenced by natural cycles that temper the overall warming trend.

Confused? Don't be: The Great Lakes region, home to the largest freshwater lake system on the planet, might also illustrate the complexities and uncertainties about how global climate change affects regional climate.

But they also eventually agree that whatever other forces are at work in any given year -- El Nino, La Nina or perhaps this year a dual low-pressure system over Iceland -- the Great Lakes system is also clearly being affected by a warming global climate.

"We are seeing the impact of global warming here in the Great Lakes -- but the natural variability is at least as large a factor," said Jia Wang, an ice research climatologist at the GLERL offices.

Wang said global climate change and regional climate patterns are competing over the Great Lakes.

"No one thing dominates," he said. "That makes it difficult to separate them and find clear signals -- and why some people maybe don't think it is happening.

"But it is happening."

Less ice and later ice

Accurately measuring ice cover across a lake system that spans 94,000 square miles in two countries is no small task, however.

Wang, an ocean expert who is fairly new to lake studies, works with and a pair of veteran researchers -- current dean George Leshkevich and the now-retired Ray Assel. Together, they combine data from ships, shore, satellite and sometimes airplanes to create composite ice charts that give them solid ice numbers for nearly half a century.

Their combined research tells them this: Despite wild swings in ice cover from year to year, the overall coverage on the lakes is diminishing -- especially in the deepest, middle portions of each lake.

In fact, those deep waters hold the key to understanding overall ice loss -- explaining with simple physics why we may not notice ice depreciation along our usually frozen shorelines, particularly in Lake Erie.

"The deeper the water, the greater the heat storage from summer, and it freezes later than the shallow areas," Assel said. "Now, increase the air temperature and the lake takes in more heat and stores it longer, to the point that many of the midlake areas are freezing over less."

Assel's records bear this out, showing that ice formation at near-shore areas has decreased only 0.65 percent, per year since 1972. But ice on the deepest parts of the lakes has declined more than twice as much, at 1.31 percent per year, on average.

Worse, the diminishing ice cover is part of a "negative feedback loop," a cycle where each problem feeds on the other.

"With more solar input because of a lack of ice, the deep waters of the lake get warmer," Leshkevich said. "Which leads to less ice cover, which leads to evaporation, which leads to shallower waters, which warm faster ... and so on."

Finally, warmer air -- nighttime temperatures in Ohio and across most continental areas have been increasing slightly for decades -- and water temperatures are not only reducing the breadth and thickness of midlake ice, but also how long it lasts.

"Ice is forming on average later in season and dissipating earlier in season," said Leshkevich, even taking into account annual fluctuation like this year's heavy ice. "But the trend line shows the downward truth."

Problems stem from ice loss

And that's not good for the lakes, scientists and environmentalists say.

Ice cover generally protects the lakes from significant winter evaporation. Open water, on the other hand, is easily sucked up by colder air above -- which we experience all too often as heavy lake-effect snow.

That increased evaporation is also lowering lake levels, however. Some studies, for example, project that Lake Erie could lose up to 15 percent of its surface area over the next 30 years.

Here's where it gets tricky again: A smaller Lake Erie would freeze more easily as its increasingly shallow waters would give up heat easily each winter. That could lead to more, not less, ice cover, at least for our lake -- unless climate change affects winter temperatures dramatically over time.

Wang said that scenario -- one with an uncertain future -- is a perfect example of how different physical forces are competing on the lake. He also said computer modeling doesn't yet seem to have a good grip on which one will win out in the end -- or if they'll simply remain in tension indefinitely.

But there are other potential problems from ice loss here and maybe more so in the other four, deeper Great Lakes.

They include: destruction of the eggs of fall-spawning fish by winter waves from an open lake; erosion of coastal areas unprotected by shore ice; and less winter recreation on the lakes such as snowmobiling or ice fishing.

There might be one short-term advantage to decreased ice: Shipping may someday be practical through the winter months more often. The locks at Sault Ste. Marie now close each year in mid January and re-open in late March.

That's small consolation, however, for shipping companies facing declining profits because they would have to haul less cargo to pass through low-water areas.

Ultimately, no one is cheering the lessening of Great Lakes ice cover -- certainly not shippers or ice fishermen, but not even the typically dispassionate scientists.

"We're losing ice and that's not good," Leshkevich said. "People can argue whether it's a signal of a warming climate or just natural variability or a combination of both -- but unfortunately, it is happening."

Click here to see the full-size graphic (pdf).Mich.