Low water levels expose the sandy lake bottom on Lake Michigan. Photo by Jeff J. Cashman.
For people living around the Great Lakes, water levels this past month have appeared much lower than many will remember. The upper Great Lakes reached near-record low water levels in October. This was most evident on Lakes Michigan and Huron, where lake levels dropped to less than two inches (4 cm) above record lows and 28 inches (71 cm) below the long-term average. All five lakes, plus Lake St. Clair, remain below their long-term averages.
Rock and sand recently exposed by low water levels made stretches of the northern Lake Michigan shoreline look like a moonscape. Recreational boaters had trouble navigating the shallow water this fall, and shipping companies lightened loads to compensate for low water. Lakes Michigan and Huron hovered just above a record low set nearly 50 years ago, and Lake Superior was within five inches (11 cm) of a record low set in 1925.
A 2002 National Geographic magazine story, Down the Drain: The Incredible Shrinking Great Lakes, documents declining lake levels and the potential economic and ecological consequences for the region. Ten years later, the story continues to unfold, as water levels remain lower than normal.
Experts blame the recent low water on the unusually warm and dry weather over the past year. Rain events in October, including Hurricane Sandy, delayed the inevitable, but forecasters predict Lakes Superior, Michigan, and Huron will likely reach historic low levels in the late fall or winter, a time of year that the lakes are normally already dropping due to high rates of evaporation.
Low water levels are not the only climate-related trend being observed on the Great Lakes. Ice cover is also declining. The Great Lakes have lost 71% of their ice cover since 1973, according to a study by the Great Lakes Environmental Research Laboratory (GLERL). This past winter, the Great Lakes, including Lake Superior, were virtually ice free with just 5% ice coverage, the second lowest on record. Similar to the global assessment conducted in 2000, loss of ice cover is being reported on lakes throughout North America, Europe, and Asia.
Summer lake temperatures are also on the rise. As mentioned in one of my previous posts about warming lakes, the Great Lakes are among many lakes in the northern hemisphere experiencing a rapid warming trend. Lake Superior, the largest freshwater lake in the world by surface area and third largest by volume (after Baikal in Siberia and Tanganyka in Africa), is also one of the most rapidly warming lakes in the world.
Because lower lake levels are considered one of the potential consequences of climate change, I was curious to find out whether there was any connection to what is being observed on the Great Lakes.
I recently had the opportunity to talk with John Lenters, a lake and climate scientist, while we attended a meeting of the Global Lake Ecological Observatory Network (GLEON) in Mulranny, Ireland. When comparing notes about our personal connections to Lake Superior, I learned that this accomplished scientist, with a laid-back, Midwestern manner, first fell in love with the Big Lake as a 14-year-old boy while on a backpacking trip in Isle Royale National Park. “Although the trip was grueling, I was awed by Lake Superior and realized I wanted to study lakes,” Lenters told me.
Now an associate professor at the University of Nebraska–Lincoln (UNL), Lenters studies lake-climate interactions in the Great Lakes region, the Alaskan Arctic, and western Nebraska. Given the global implications of his research, he joined GLEON in 2008 and helped to form the new Global Lake Temperature Collaboration (GLTC), hosting their first meeting at UNL this past June. With his boyhood dream as inspiration, he and his collaborators are leading the way to learning more about how climate change is affecting lakes around the world, including the Great Lakes.
On Lake Superior, Lenters and his collaborators are studying the interactions among evaporation, ice cover, and water temperature. Their research builds on work by others in the region (and elsewhere) and provides new insight on factors affecting water levels.
Surface Water Temperatures Increasing on the Great Lakes
Similar to Lenters’ findings in a 2004 paper, which found Lake Superior to be warming more rapidly than summer air temperatures, Jay Austin (a GLTC collaborator) led a study of lake temperature trends at the University of Minnesota-Duluth’s Large Lakes Observatory (LLO). Published in Geophysical Research Letters, the LLO study found that summer surface water temperatures on Lake Superior have increased approximately 4.5°F (2.5°C) during the period 1979–2006.
The LLO study found that the decline in winter ice cover leads to an earlier start of the summer stratified season, a natural process in lakes when water near the surface warms, while deeper waters remain a more constant, cooler temperature. The earlier the lake becomes stratified in summer, the longer the warming period. “This results from a progressively earlier start of the summer stratified season, in response to a significant decline in average winter ice cover,” the study states. “Given a longer summer stratified season, surface waters can be heated to higher temperatures than that expected from increases in air temperature alone.”
Researchers also found a clear relationship between ice cover and summer water temperatures, which tend to be cooler following a winter with extensive ice cover. In contrast, winters with less ice cover tend to be followed by a summer with warm surface water temperatures. This is exactly what happened this year on Lakes Superior, Michigan, and Huron. The lakes were relatively ice free in 2011–12 and reached record-high water temperatures in the summer.
New Insight About the Interaction Between Ice Cover and Evaporation
Measuring evaporation rates on lakes as large as Superior is a very difficult and intensive process, so until recently, researchers in the Great Lakes region relied on models instead. The models correctly account for the various factors that impact evaporation rates, including when the lake surface is covered by ice. But with ice cover shrinking on all of the Great Lakes, scientists began to wonder what impact this would have on observed evaporation rates. Understanding this new dynamic required the installation of new monitoring stations on all five of the Great Lakes. Results are being shared through a network of researchers monitoring evaporation rates.
In the past, experts assumed that as ice cover decreased, evaporation would increase, since more of the lake’s surface is exposed to the air during winter months. But a new paradigm is emerging.
“Some of our recent work challenges the standard paradigm that more ice cover means less evaporation,” Lenters told me. Evaporation rates are increasing as the climate changes, but the relationship to water temperature and ice cover is not as simple as previously thought.
Katherine Van Cleave, Lenters’ former student at UNL, recently completed a study of these interactions for her master’s thesis. Her study includes an analysis of the first direct observations of nearshore evaporation rates on the Great Lakes, using a high-tech monitoring station on Granite Island, near Marquette, Michigan. She also looked at some of the primary climatic factors driving this variability. Although her study period, from October 2010 to April 2012, does not include this past summer, the impact of the warm 2012 water temperatures on evaporation rates is entirely consistent with her findings.
Research by Lenters at Granite Island and a study with other collaborators at Stanard Rock (published in the Journal of Great Lakes Research) has examined seasonal and annual evaporation rates on Lake Superior. Together with research by Van Cleave, they found that evaporation rates in late winter and early spring (when ice cover is typically at a maximum) are generally minimal, even in years with low or no ice cover. The highest rates of evaporation, on the other hand, occur during the fall and early winter and, during particularly cold years, can actually lead to greater ice cover later in the winter and spring. “Evaporation is a cooling process,” explained Lenters, “and the more rapidly it occurs, the more likely the lake is to reach freezing temperatures and form extensive ice cover.”
Evaporation from a lake is similar to how we humans perspire to cool our bodies on a hot summer day. It is a process that transfers heat from the lake back into the atmosphere. When a lake evaporates, heat is released to the atmosphere. The more the lake “sweats,” the more it cools.
So instead of simply thinking of ice cover as a “cap” on evaporation, we need to realize that the reverse is also true – that strong evaporation can lead to high ice cover. In other words, says Van Cleave, “this ‘standard paradigm’ of decreasing ice cover, increasing water temperatures, and increasing evaporation may not stand as a full explanation of the role of evaporation in these processes. More evaporation in the fall will cool the lake quicker, leading to an earlier onset of ice cover.”
But the lack of ice cover affects evaporation in another important way – by impacting water temperatures and evaporation rates much later in the year. “Ice cover was found to be a strong determinant of summer water temperature, and this in turn, can lead to changes in late-summer evaporation rates,” Van Cleave concluded.
Regime Shifts in the Great Lakes Ecosystem
Van Cleave made an interesting discovery after looking at long-term data for Lake Superior: Certain changes have not been linear through time. Scientists use statistical analysis to see if patterns emerge in their data and to determine whether certain parameters are increasing or decreasing with time. “Lake Superior experienced a pronounced change during the winter of 1997–98 when ice cover reached, at the time, record low levels,” her report states. “This was followed by record-warm summer water temperatures and near-record evaporation rates (surpassed only by 1987).”
“A step-change occurred in 1997–98 that resulted in a drop of ice duration of nearly 40 days, a 5.4°F (3°C) increase in summer water temperature, and a near doubling of July-August evaporation rates,” Van Cleave concluded. Ecologists refer to an abrupt change such as this as a regime shift, and although some evidence indicates that the lake recovered somewhat, Van Cleave found that these more recent trends ”are not statistically significant, suggesting that the 1998 regime shift has largely been sustained.”
Given the extreme conditions of this past year, Lenters wonders whether Lakes Superior, Michigan, and Huron are in the midst of another such regime shift.
Lake Levels Remain Below the Long-term Average
The U.S. Army Corps of Engineers (Corps) began keeping coordinated water level records in 1918. They base “record events” on calculations of monthly average lake level. Lake Michigan-Huron, considered one lake for hydrological studies because of the connection at the Straits of Mackinac, was 576.6 feet (175.74 meters) above sea level in October. The all-time record low for all months occurred in March 1964, when the lake dropped to 576.0 feet (175.58 meters).
The Great Lakes Water Level Dashboard, a handy, interactive online tool provided by NOAA, helped me to better visualize historic water level trends going back to 1861. I was reminded of a period of high water on the upper lakes during the 1970s and 1980s, when everyone was concerned about erosion along the lakeshore and houses were falling into Lake Michigan. What also jumped out are the below-average water levels after the 1997–98 event that Van Cleave described.
Great Lakes water levels normally fluctuate throughout the year and from one year to the next depending on climate conditions. The lakes naturally cycle between periods of high water and low water, but abrupt changes in annual water levels are not unusual. These fluctuations are due to climate variability and are considered vital to a healthy ecosystem.
One variable the Corps constantly monitors is the supply of water to each Great Lake, which is made up of rainfall on the lake surface, runoff to the lake, and evaporation from the lake. In a teleconference with the media, Keith Kompoltowicz, chief of the Watershed Hydrology Branch in the Corps’s Detroit District Office, explained that this supply is the primary driver of water level fluctuations and that this past year, evaporation was much greater than precipitation and runoff combined. “Any time there is a scenario like that, lake levels will likely decline,” he said.
Lake levels are expected to continue dropping as part of the normal seasonal decline through the fall and winter. “Evaporation usually wins out at this time of year,” said Kompoltowicz.
Lakes Superior, Michigan, and Huron have been fluctuating below average levels since the extreme event 15 years ago. Corps officials acknowledged that the upper lakes have not recovered from this extreme event and are not likely to anytime soon. “We would need several months and seasons in a row of very wet weather to get us back to long-term average,” said Kompoltowicz.
This extended period of low water raises questions about whether climate change is contributing to declining lake levels, but the Corps maintains the position that it’s difficult to know, because the lakes continue to fluctuate within their normal range.
Low Lake Levels Renew Debate About Potential Causes
Controversy usually arises about potential causes whenever lake levels are low. Numerous theories abound. People ask whether a diversion in Chicago to the Mississippi River watershed might be to blame. Others point to erosion or dredging in the St. Clair River. John Allis, Chief of the Corps’s Great Lakes Hydrology and Hydrography Office, dismissed these claims, citing studies that show the Chicago diversion is more than offset by a diversion into Lake Superior from Canada.
Allis referred to a study of historic dredging and sand removal operations on the St. Clair River. “Studies show that the net impact of historic dredging and erosion is about 10 to 15 inches lower water levels in Lakes Michigan and Huron,” he said. “The last dredging project was completed in the 1960s, and since 1967, the only dredging that has been done on the St. Clair River is maintenance dredging to keep the rivers at authorized depths.”
The Corps says that when water levels are low, they get asked about whether structures could be built to restrict flow in the St. Clair River. “Recent studies show that any of those projects could range from $50–200 million to construct,” said Allis. “Although water is low right now, there are many groups that would not support the construction of structures because of concerns about what would happen in high water.” He explained that projects to mitigate historic water losses were de-authorized in the late 1970s when the lakes approached record high levels.
The International Joint Commission (IJC) studied the impacts of dredging and erosion in the St. Clair River on water levels in the upper Great Lakes. Among other things, the International Upper Great Lakes Study (IUGLS) evaluated remedial measures for historic dredging projects and erosion. “The Study Board found that there had been some erosion in the St. Clair River between 1962 and 2000, but the riverbed had stabilized since then, making it unclear whether action would be appropriate,” said IJC Public Information Officer John Nevin. Further details can be found in the summary report.
The five-year, $14.6 million study by the Study Board also examined options for regulating water levels and flows in the upper Great Lakes system, consistent with the Boundary Waters Treaty of 1909. In March, the Study Board released its final report and recommended a new regulation plan for Lake Superior outflows, one that is “more robust than the existing plan and provides benefits, especially to the environment,” said Nevin. The new plan will not change regulation in a way that helps the situation on Lakes Michigan and Huron. “If we were to try to do that, it would damage Lake Superior,” he said. “It really can’t be done.”
The focus on past diversions and dredging operations is not surprising, given the complex nature of more subtle but very real changes underway. I asked Lenters for his opinion on other theories that explain why the lakes are so low this year. “No one bottled it up and took it away or diverted it to the Mississippi,” he said. “Together with the low precipitation we’ve seen this year, the lake water simply evaporated more quickly.”
Management Agencies Study Effects of Climate Change
With nearly 20% of the world’s surface freshwater at play and millions invested in restoration efforts, the stakes are incredibly high for understanding how natural climate variability and human-induced climate change affect the Great Lakes.
The IUGLS evaluated the impacts of climate change on lake levels in the Great Lakes region with state-of-the-art climate research. Projections suggest that “lake levels are likely to continue to fluctuate, but still remain within a relatively narrow historical range – while lower levels are likely, the possibility of higher levels cannot be dismissed.” Nevin explained it another way. “Low lake levels are not a new normal,” he said. “We expect to see lake levels fluctuate as we have in the past.”
The IUGLS acknowledges that despite uncertainties in the models used, “it is clear that evaporation is increasing and likely will increase for the foreseeable future.” The study further states, “Analysis indicates that in the Lake Michigan-Huron basin this increased evaporation is being largely offset by increases in local precipitation.” The outlook for Lake Superior is more cautious:
“In the Lake Superior basin, however, increasing evaporation over the past 60 years has not been compensated for by increased precipitation. As a result, the water supply has been declining in general in the basin. This trend is consistent with the current understanding of climate change. Unless changes in the precipitation regime occur, which is possible, [net basin supply] in Lake Superior will continue to decline, on average, despite the possibility of higher supplies at times.”
These findings convinced the Study Board to recommend that “further climate analysis be undertaken to explore these dynamics” in order to provide more certainty in water supply estimates. Will changes in precipitation offset increased evaporation rates? Can lake levels recover from extreme events or are we seeing a new normal? These are some of the questions yet to be answered.
The IUGLS acknowledges that the Great Lakes basin is a complex system whose dynamics are only partially understood. In addition to further research, the Study Board recommends a more adaptive approach to future management – one that places climate change considerations in the mix. As the experience on Lake Tahoe shows, an important first step was acknowledging that climate change is a major driver in the ecosystem. Lake managers there are now focused on restoration projects that build the lake’s resilience to changes that are already underway.
Lenters explained that this past year was like a “perfect storm” of conditions leading to high rates of evaporation and low water levels on Lake Superior. “Record low ice cover in 2011–12, an extreme heat wave in March, and a warm, dry summer led to record-high summer lake temperatures,” he said. “As a result, we saw higher-than-normal evaporation rates earlier in the season.”
Lake Superior evaporation – which is typically very low during spring and early summer – doesn’t normally begin increasing until early August. But this year it began as early as late June. Together with the summer’s below-normal rainfall, lake levels began their annual decline in the summer rather than in the fall. This would explain the near-record low lake levels in October. “Lake Superior’s rapid warming is like a canary in the coal mine,” Lenters told me. “We’re seeing changes in ice cover, water temperature, and evaporation that indicate major shifts are underway on the world’s largest lake.”
Lisa Borre is a lake conservationist, freelance writer and sailor based in Annapolis, MD. With her husband, she co-founded LakeNet, a world lakes network that was active from 1998 to 2008, and co-wrote a sailing guide called “The Black Sea.” She is a native of the Great Lakes region and served as coordinator of the Lake Champlain Basin Program from 1990 to 1997.