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Great Lakes water levels have swung from record lows to record highs. Here’s why.

Lake Michigan waves batter the shoreline and cause erosion along North Shore Estates Road in Ferrysburg, Mich., on Oct. 23. (Cory Morse/The Grand Rapids Press/AP)

The Great Lakes water levels broke records this past July and August, with some basins experiencing the highest levels ever recorded since 1918. Unusually high water has plagued their shores this year, causing beach erosion and disappearing waterfronts.

Even as we head into the winter months, water levels “remain well above average and near record highs levels,” according to the Great Lakes Water Level Outlook, released this week by the U.S. Army Corps of Engineers.

Water levels have not always been this high, however, with record low levels observed on lakes Michigan and Huron in 2013, part of an erratic pattern that could become normalized with our changing climate.

It’s important to examine the drivers behind the fluctuating lake levels and how the warming climate fits in.

Lakes Superior, Erie and Ontario saw their highest levels on record for the months of June and July. Lake St. Clair, which connects Lakes Huron and Erie, also set its highest level on record for those two months.

In August, Lake Superior tied its record high, while lakes St. Clair and Erie set records. Although water levels on Lake Michigan were also abnormally high, it just missed setting records over the summer.

Lake St. Clair’s July level of 577.56 feet set a record high for any month, compared with the average lake level of 574.8 feet. This may seem like a small difference, but this increase spread over an entire basin increases incidents of coastal flooding.

The height of each lake is determined by Net Basin Supply (NBS), which is evaporation subtracted from the sum of precipitation and runoff into the lakes. When the inflow (precipitation and runoff) is greater than evaporation, you get rising lakes. If the evaporation exceeds the inflow, lakes drop.

The record-breaking water levels this summer can be attributed to tremendous amounts of precipitation and excess runoff during the spring and summer.

The Great Lakes region saw above-average rain for the three consecutive months from April to June, contributing to the overflowing basins.

The heavy spring and summer rains are part of a longer-term trend. The United States saw a 4 percent increase in precipitation between 1901 and 2015, with the Great Lakes region alone seeing a 10 percent rise.

“We’re seeing some of the highest water levels in recorded history on the Great Lakes, and that’s the result of very wet weather experienced over the last several years,” said Keith Kompoltowicz, chief of watershed hydrology for the Detroit district of the U.S. Army Corps of Engineers.

This trend toward increased precipitation is expected to continue as the atmosphere warms because of climate change. A warmer atmosphere can hold more moisture, which can condense and turn into precipitation. A marked increase in heavy precipitation events has been observed across the United States during the past several decades, a trend linked to climate change and observed in the Great Lakes region.

But a warmer atmosphere also means water can more readily evaporate, which brings in the third part in our lake level equation. As temperature increases, the energy in a water particle increases, and the molecules can “escape” into the surrounding air. As we head into the colder months of late fall and early winter, evaporation increases because of the greater temperature difference between the chilly air above the water, and the comparatively mild lake surface waters. This competes with increased precipitation amounts to determine lake levels.

In other words, during dry spells, because of the warming climate, water levels can drop more quickly, leading to unusually low levels — which is what was seen in 2013.

So if the lake levels can be high, and also low, what’s the big deal?

Over short time scales (a few years), intensified precipitation one season could spur high water levels and associated flooding. Likewise, hotter summers and increased evaporation could result in lower water levels the next season.

Over long time scales, the water levels usually reach equilibrium, as all basins are interconnected via channels such as the St. Marys River. However, with climate change, inconsistent lake level extremes across the Great Lakes are expected. Retreating beaches, water-covered bridges and even flooded basements and dysfunctional septic fields are all consequences of high water levels, while low water harms commercial shipping on the lakes, reducing the amount cargo ships can carry.

A 2019 climate change assessment for the Great Lakes conducted by the Environmental Law and Policy Center found that the most recent research shows both the potential for drops and rises in lake levels into the future.

The erratic seesaw of highs and lows year to year may be the new norm, and residents of the Great Lakes will need to prepare for a future with unstable water levels.

Kim Frauhammer is a meteorologist and climate scientist living in Denver, Colo. She received her Bachelor’s and Master’s degrees from the University of Michigan.