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May 2007 News
Lake Superior warming fast: Researchers surprised by strong trends
By Katie Alvord*
HOUGHTON -- At first, Jay Austin thought he’d made a mistake. When the University of Minnesota, Duluth,
(UMD) oceanographer crunched the numbers from three Lake Superior weather buoys, he found a surprisingly rapid rise in the lake’s average summer surface temperature -- about 4 degrees Fahrenheit since 1980, a much greater rate of change than seen in previous decades. "I spent a long time trying to figure out what I was doing wrong," says Austin.
"I could not explain why the trend at these buoys over the last 25 years was so strong."
But ultimately, his calculations proved sound. Austin and his colleague Steve Colman, also of UMD, went on to complete a study that both explains the striking temperature rise and documents significant recent shifts in Lake Superior’s climate.
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This photo, taken near Hancock, Mich., shows ice melting along the Lake Superior shoreline in late
March 2007. Researchers have found that Lake Superior's declining ice
cover has contributed to a rapid warming of the lake's surface waters. (Photo © 2007
Katie Alvord. Reprinted with permission.)
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Other researchers have already looked at specific pieces of this puzzle, says Austin.
"What was
different about what we did is we snapped a bunch of these pieces together," he notes.
Austin and Colman reviewed data from several sources to get a handle on changes in Lake Superior
from 1980 to 2005. They analyzed air and water temperatures as well as wind data from the three weather
buoys. They studied ice cover data from the Great Lakes Environmental Research Laboratory in Ann Arbor,
which has documented a decline in ice cover for all the Great Lakes. They looked at air temperatures from
every weather station within 500 kilometers of Lake Superior.
When analyzed, the data showed that not only has the lake become warmer,
windier and less icy
since 1980, but also that its surface waters have warmed twice as fast as the region’s air.
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Graphs show recent trends in regional air temperature (red, above), Lake Superior surface water temperature
(blue, above), and ice cover (black, below). Straight lines show trends, jagged lines show yearly variations. Note
rate of change for water temp is twice that for air temp. (Graphs courtesy Jay Austin, University of Minnesota, Duluth.
Reprinted with permission.)
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As Austin explains, he and Colman found two factors that together could account for this rapid
warming trend. One is the well-documented rise in the region’s average air temperatures over the last 25
years.
"Global average temperatures have been increasing over the last
century," Austin points out.
As it happens, he adds, the Upper Midwest is warming faster than the global average.
But rising air temperatures account for only part of Lake Superior’s recent warming.
The second factor showed itself when Austin and Colman looked at data for ice cover and noticed a
strong correspondence, year by year, between low ice cover and high summer lake temperatures. They also
noted a trend toward an earlier "spring overturn" in the lake’s waters.
The spring overturn is the date at which the lake warms enough to become stratified, with
warm water layers atop cooler ones. After spring overturn, Lake Superior’s surface waters generally warm
quickly to an August maximum.
Ice affects this process because it reflects sunlight, Austin explains, while darker open water
absorbs solar radiation as heat.
"Take the ice away, and the lake is capable of absorbing more radiation
or more heat earlier in the season than it would normally," Austin says.
Austin and Colman believe that warming air and declining ice cover have contributed about equally
to Lake Superior’s quick temperature rise.
"This rapid warming trend had not been documented before," says Austin. He believes it may be one
of the largest temperature changes observed in a natural system in the last 25 years.
Austin and Colman found another trend in the Lake Superior weather data.
"Wind speeds have increased dramatically," Austin says. "We think it’s plausible that the change in surface water
temperature might be leading to these increased wind speeds." He speculates this could be
attributed to a reduction in temperature and density differences between air layers at the surface and those higher in the
atmosphere.
The paper detailing Austin and Colman’s results has been published in Geophysical Research
Letters.
Austin says predicting the future is tricky, and there will always be variability from year to
year. But if current climate trends continue, then by about 2040, he estimates,
"in an average year on Lake Superior, you won’t have ice in the
winter."
From local to global and back
Austin and Colman are among a growing number of regional officials and researchers examining how
climate change may already affect air, water and ice within the Lake Superior basin.
This regional work is taking place as the International Panel on Climate Change (IPCC) completes
its Fourth Assessment Report on the state of the world’s climate. The IPCC is releasing its 2007 assessment in segments, and the next will be released Friday, May
4.
The first segment, released in February, concluded that human activities such as burning of fossil
fuels, which adds heat-trapping carbon dioxide (CO2) to the atmosphere, are
"very likely" the primary cause of the observed rise in global temperatures.
The report states, "Warming of the climate system is unequivocal, as is now evident from
observations of increases in global average air and ocean temperatures, widespread melting of snow and
ice, and rising global mean sea level."
Warming of Lake Superior’s climate is evident, too, says Jay Austin.
"There’s a very, very strong climate change signal in Lake
Superior," he notes. "It’s not something you have to go far away to observe.
Regardless of what’s causing it, it is happening; and the effects are not remote. They are right
here."
Among other effects, scientists have noticed a range of changes related to declining ice on the lake. For the ongoing wolf-moose study at Isle Royale National Park, for instance, less ice is a problem.
In conducting the study, says Isle Royale Superintendent Phyllis Green, ice pack is a critical logistical factor. Researchers rely on ice to access and move around the island in ski planes, and
they rely on tracks in snow to find and count wolves and moose.
"These warmer winters have put a number of logistical problems into conducting winter studies, as well as having a good feel for what’s going on out there," notes Green.
This last winter, reports wildlife ecologist Rolf Peterson, researchers barely had a long enough
snow and ice season to perform their field work.**
Globally, the 2006-07 northern winter/southern summer -- defined by meteorologists as running from
December 1 to February 28 -- was the warmest ever recorded, according to Jay
Lawrimore, chief of the
National Climatic Data Center’s Climate Monitoring Branch.
"One particular season is not evidence by itself of climate change," says
Lawrimore. But, he
adds, "This boreal winter record is additional evidence that changes are continuing to occur and there
will likely be more global temperature records set in coming years and decades."
A few local temperature records were set this winter by the same conditions that reduced ice at
Isle Royale. Kevin Crupi, meteorologist at the Marquette National Weather Service office, says December
2006 tied the record for warmest December ever at the Houghton County Airport. December’s mean temperature of 27.3 degrees was 7.1 degrees above normal, Crupi reports. From Dec. 11 to 13, the mercury stayed above freezing for three consecutive days. And 16 high maximum or minimum temperature records were broken or tied in the 38 days from
Dec. 1 to Jan. 7.
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Note lack of snow in this Dec. 29, 2006, photo of Copper Harbor and Lake Fanny Hooe. December 2006 tied the record
for the warmest December ever in the Copper Country. (Photo © 2006 Kraig Klungness.
Reprinted with permission.)
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Even so, this winter turned out not to be the warmest ever for Houghton/Hancock, due to chilly conditions in late January and February. However, the local winter season still ended with an average temperature 2.7 degrees higher than normal.
At the same time, Houghton’s 2006-07 winter saw lower-than-normal precipitation. Just 104.8 inches of snow fell at the Houghton County Airport, well below the 30-year normal for that site of 162.8 inches, says
Crupi.
"Dry weather has pretty much prevailed over the Keweenaw since last May," Crupi notes.
Does a warmer lake mean lower levels?
Recent drought conditions affecting the Lake Superior basin are strikingly consistent with
forecasts from global climate change models, says Bob Krumenaker, Superintendent at Apostle Islands
National Lakeshore and a former park biologist at Isle Royale. Krumenaker has studied the issue of
climate change to aid in planning for Apostle Islands and other national park units.
Yet another trend forecast by climate change models, he says, is a drop in lake levels. Lake
Superior has approached record low levels over the past year. As of April 26, the lake’s level was about 16
inches below its long-term average, based on measurements by the U.S. Army Corps of Engineers.
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Little Sand Bay Marina at Apostle Islands National Lakeshore in November 2004
(above) and again in February 2007 (below). Dock angles show drop in Lake Superior's levels, at
near record lows.
(Photos © 2004 and 2007 and courtesy Bob Krumenaker, Apostle Islands National Lakeshore.
Reprinted with permission.)
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Lower lake levels can have widespread effects, costing shippers money due to reduced load sizes, creating hazards for small boaters, exposing water intake pipes, and more. At Apostle Islands, Krumenaker reports, the lower levels will require dock work and dredging. At Isle Royale, says Phyllis Green, lower lake levels have meant the cancellation of one boat service from Grand Portage, Minnesota, to Windigo for the 2007 season.
As agency officials deal with these problems, Jay Austin and Steve Colman are crunching more numbers, exploring how Lake Superior’s rising temperature and declining ice might contribute to the lower levels.
That work is still speculative, according to Austin. But he expects to find a link.
"If water temperatures are going up faster than air temperatures, you can show that that is going to lead to
excess evaporation,” he says. Similarly, decreased ice cover will increase evaporation from the lake.
"Preliminary work I’ve done suggests these might have a large enough effect to be causing the drops in
lake levels we’ve seen over the last 25 years," Austin notes.
Noel Urban, environmental engineering faculty member at Michigan Tech
University (MTU), also notes evaporation’s potential role in lowering lake levels. He points out that global climate change models predict
evaporation changes in this region in response to higher temperatures. Lower lake levels, he suspects,
probably reflect a climate-related increase in evaporation and plant transpiration as well as decreased
runoff to the lake.
Urban was part of the team that worked on MTU’s five-year Keweenaw Interdisciplinary
Transport Experiment in Superior project (KITES), coordinated by Sarah Green of
MTU’s Department of Chemistry. KITES studied a number of processes within Lake Superior, including the cycling of CO2 into and
out of the lake. As a result of their studies, Urban says, KITES came to the unexpected conclusion that
Lake Superior is a net source of CO2, releasing it into the atmosphere.
This results from bacteria within the lake releasing CO2 as they break down organic matter. It’s
important because so far, Urban notes, the lake’s effect on atmospheric CO2 hasn’t been accounted for by
people who measure atmospheric CO2 in order to assess CO2 uptake by forests.
"If we’re trying to understand either what the lake is doing or what the watershed is doing in
response to climate change [and] in relation to CO2 emissions, we need to be measuring the lake as
well," Urban explains.
There is not yet enough data to determine whether CO2 releases from the lake have changed in
response to rising temperatures, Urban says.
Like Urban, Jay Austin indicates that Lake Superior has been poorly studied and so remains poorly
understood.
"We understand so little about how it works right now that it’s very difficult to understand what
the impacts of these changes are going to be," says Austin. "For as large an effect as it is, I’m
surprised by how few people are studying it."
Austin also thinks studies of changes in Lake Superior might have wider applications.
"By better understanding how water, ice and air in Lake Superior
interact," he says, he hopes scientists can "gain some insight into what’s going on in much more complicated, much more important systems like the Arctic Ocean."
Scientists believe that the widely-reported melting of sea ice in the Arctic could have significant effects on global climate.
An advantage of Lake Superior, Austin points out, is that it’s easier and less costly to study than a place like the Arctic.
Noel Urban agrees that many gaps remain in our knowledge of Lake Superior. He is currently
working on a National Science Foundation study that will develop a carbon-cycling model for the
lake and compare its results to field measurements. "Part of what’s exciting about it is the chance to study the interactions of the lake with its
watershed," says Urban.
As yet, not enough is known to predict what further regional shifts in climate might mean for flora and fauna in and around the lake,
Urban notes. But if weather and temperature patterns continue changing, he says,
"our data suggest that there may be some very marked biological responses."
COMING SOON: Are climate changes affecting plants and animals around Lake Superior? What some scientists
have already observed.
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