and then we can make the Edmund Fitzgerald an amusement park

[Guest guest]

Special Report Earth

Global warming is shrinking the Great Lakes

30 May 2007

NewScientist.com news service

Jessica Marshall Duluth

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Tom Mackay reckons his backyard in Duluth shows what is happening in

Lake Superior as well as any place. In November 2005, the metre-tall

wooden " Bay Ness Monster " statue he installed in the water just off

his home dock was submerged up to its gaping mouth. Today, his would-

be water serpent is high and dry.

 

For residents of this lakeside Minnesota city, located more than 3000

kilometres by boat from the open Atlantic, the transformation is

disturbing. Lake Superior, the largest body of fresh water in the

world by surface area, is experiencing its lowest water levels since

the record set in 1926. The lake is down by 34 centimetres from a year

ago, and more than half a metre below its long-term mean. At least

part of the drop can be attributed to a multi-year drought that has

been particularly severe since 2006. More troubling, however, is

evidence that global warming is driving a long-term shrinkage of this

massive natural reservoir.

 

A rapidly warming lake is the key to understanding the change, says

Jay Austin, a limnologist at the University of Minnesota Duluth's

Large Lakes Observatory. Earlier this year he reported that Superior's

surface waters had warmed by about 2.5 °C since 1979 - far more than

average air temperatures in the region during the same period

(Geophysical Research Letters, vol 34, p L06604). Austin's findings

link the warming to a reduction in winter ice cover on the lake. The

less ice is present to reflect sunlight, the more solar energy the

lake can absorb. On average, the onset of summer warming of the lake

is happening half a day earlier each year. The reduced ice cover also

contributes to shrinkage by allowing more evaporation. " Most of the

evaporation goes on in winter, " Austin says, as cold, dry air swoops

over the warmer lake. Without the ice cap to block evaporation, water

losses increase.

 

" Lake Superior's surface temperature has warmed by 2.5 °C since 1979 -

far more than average local air temperatures " Cynthia Sellinger of the

National Oceanic and Atmospheric Administration's Great Lakes

Environmental Research Laboratory in Ann Arbor, Michigan, agrees.

While the lake's level has dropped precipitously since last year,

Sellinger has tracked a longer-term decline of an average of 10

millimetres per year since 1978. Evaporation has increased by an

average of 4.6 millimetres per year over the same period, she says,

while precipitation has decreased by 4.1 millimetres per year. These

drops are consistent with climate change models, Austin says, which

predict a decline in Great Lakes levels of 0.5 to 2.5 metres with a

doubling of atmospheric carbon dioxide.

 

The falling water level is already affecting Lake Superior's shipping

industry. Freighters carry less cargo now for fear of running aground.

Natural beds of wild rice growing in the lake's shoreside wetlands and

harvested by Native Americans are also threatened. The long-term

effects of prolonged warming on Lake Superior's aquatic ecosystems are

not yet known.

 

Austin has found similar increases in temperature and length of

warming season in lakes Michigan and Huron, which are both at lower

than average levels. In contrast, lakes Erie and Ontario have captured

plenty of rain in recent years, Sellinger says, including the remnants

of hurricanes Katrina and Dennis, and are at above average levels.

 

Last month the US-Canadian International Joint Commission, which

manages waters on the boundary, commissioned a study of water

management of the upper Great Lakes to determine whether managing lake

outflow differently could improve levels.

 

Meanwhile, Mackay's friends have had to find a deeper place than his

dock to moor their boats for the season. " If we get some rain,

hopefully we're back in business, " Mackay says. Austin is less

optimistic. This is the season when the lake should be rising quickly,

and it's not keeping pace, he says. " 2007 is shaping up to be a very

low year.

 

 

>From issue 2606 of New Scientist magazine, 30 May 2007, page 8-9

 

 

 

“The Earth is not dying - she is being killed. And those who are killing her

have names and addresses.†— Utah Phillips