Earlier this month WBEZ reported that Lake Michigan water levels are at a record low. Today the lake levels are still dropping, putting the livelihoods of shippers, boaters and whole coastal towns at risk.

That news is not getting old, either: As of Jan. 28, the lake was two inches below the previous record set in 1965 (down from just one inch in early January). It was more than five feet below the record high of 1987. A person of an average height can stand on dry land today in spots where 26 years ago she would have been up to her neck in water.

A few commenters on this story asked about the reasons for today’s low levels in Lake Michigan. The short answer is that there is no short-term answer. Lake levels are subject to long-term fluctuations caused by weather and precipitation patterns.

The U.S. Army Corps of Engineers tests the lake levels in all five Great Lakes daily, and they have data on lake levels going back to 1860. That data shows relatively consistent fluctuations of several feet of depth, usually over the space of a decade or more. In one instance, the water in Lake Michigan went up three whole feet in only three years (1926-1929). Between 1965 and 1987, the levels went up five feet. Now they’re back down, but our environmental concerns are drastically different than they were fifty years ago. As Greg Buckley, the City Manager of Two Rivers, Wis. put it, “In ‘64 nobody talked about climate change.”

The big picture

The Great Lakes shapes and sizes have been in flux since the lakes were formed over 10,000 years ago by receding glaciers at the end of the last Ice Age. As recently as 9,000 years ago, what is now Lake Michigan covered a much larger surface area, and drained out to the Mississippi River system through outlets to the south. What is now Chicago was completely submerged.

It took another five millenia for the waters to recede to more or less their current level, by which point the St. Lawrence River far to the east had become the lakes’ main outlet. Simultaneously, the land surrounding the newly-formed glacial lakes began to rebound; without the weight of the massive glaciers pushing it down, the Great Lakes basin landforms rise on their own at a rate of about three inches every hundred years. And according to the Great Lakes Information Network, sometime in the last 10,000 years the lakes were a full five feet higher than any levels recorded by the U.S. government.

Glacial change is powerful - but it’s slow. Neither a few inches per century of naturally rising lands nor five feet of lake level loss in 10,000 years compares to Lake Michigan’s recent decline of five feet over less than 50 years.

Bottled water is chump change

Some have suggested that bottled water and municipal water use are draining the lakes.

“Water withdrawals for drinking water, for bottled water, and for municipal use… are unlikely to be a significant factor in lowering lake levels,” said Dr. David Allan, a professor in the School of Natural Resources and the Environment at the University of Michigan (and co-creator of a new Great Lakes mapping project). “If you look at it from a water budget perspective, inputs and outputs, the inputs in the form of precipitation and runoff, and the outputs in terms of evaporation and flow…those values just dwarf the water use of human activity. They’re just a small fraction of the total water budget.”

Many cities and towns that draw water from Lake Michigan end up returning that water, used and semi-sanitized, to the same water basin. And although a controversial legal loophole allows Great Lakes water to be bottled and sold, the lake water that’s disappearing to China enclosed in Nestle company plastic is a fraction of Chicago’s daily use alone. It’s not enough to drain the lake by a long shot.

But bottling and some municipal water uses are a net loss to the lake. Illinois is unique in that on the small Illinois slice of the coast, water is pumped out of Lake Michigan to give Chicago and surrounding suburbs showers, fire hydrants and delicious drinking water – but it doesn’t return. After it’s used once, that water drains out into the Chicago River system.

The Chicago River has a special relationship to Chicagoans’ consumption habits. It used to flow into Lake Michigan and return Chicago’s runoff and sewage. But since 1900, when Chicagoans decided they didn’t want to drink their own sewage, it’s been flowing the other way, out into the Mississippi River system. Even though a federal court decision keeps a cap on Illinois’ water diversion, nearly 2 billion gallons a day leave Lake Michigan for a single use in the Chicago area and never trickle back down.

Despite that gargantuan-sounding number, Allan says the impact on overall water levels is tiny. After all, we’re talking about 4 percent of the entire world’s surface fresh water in Lake Michigan alone. That’s 1,180 cubic miles of water. A cubic mile of water holds more than a trillion gallons.

So when it comes to drinking water, the concern for northeastern Illinois is not so much that the lake will run out, but that Illinoisans could hit that federal cap as soon as the year 2030. That means Illinois is going to have to limit use, keep the groundwater in the area clean enough to drink, or renegotiate the deal.

Of course, scientists will continue to disagree on how urgently coastal communities need to reduce their diversions.

What we think we know

There are a few factors most researchers can agree are affecting lake levels.

1. Precipitation. The lake’s major sources of water replenishment are rivers and streams, runoff, and rain directly over the lake. The water basin is the whole area that drains into that lake, and the area of the Great Lakes water basin is about 295,200 square miles. Last year’s massive drought meant reduced precipitation in many parts of the basin. And when water evaporates from Lake Michigan, the movement of weather systems generally dictates that it comes back down further to the East, raining on Ohio or New York. And of course, no water system is contained: at the far eastern end of the Great Lakes basin, water flows out of Lake Ontario, into the St. Lawrence River, and towards the Atlantic. Moisture that leaves here headed east is unlikely to make a quick return.

2. Surface temperature. The sun has an upper hand on any human attempts to control or extract water from the lake. Evaporation across the lake’s broad surface is the most reliable cause of water loss. And after the hottest year ever in 2012, we’re now experiencing the second consecutive warm winter in the region. Those combined factors mean the lake’s surface temperature stays a little warmer, and when the sun shines down, the water disappears even more rapidly than usual. Nearly a foot of the water lost to Lake Michigan disappeared in the hot spell between 2011-2012.

3. Ice cover. It’s probably obvious that ice cover on the lakes is the inverse of warm surface temperatures. And through the winter months it can serve as a protective layer against evaporation. Great Lakes ice cover has declined 71 percent since 1973 due to rising temperatures.

4. Dredging in the St. Clair River. The U.S. Army Corps of Engineers says Chicago’s perpetual diversion of water out of the system via the Chicago River is more than offset by other diversions into the Great Lakes basin from the north. But they concur with researchers who say that further east, the deepening of the St. Clair River accounts for over a foot of permanent loss in Lakes Michigan and Huron. The St. Clair, which connects Lake Huron with Lake St. Clair near Detroit, has been dredged to keep shipping channels open since the mid-1800s. Lakes Erie and Ontario, which receive the flow diverted through the St. Clair, are not facing the same low water crisis.

Climate change, right?

Is climate change causing the water to disappear? As Allan puts it, “the answer is a decisive maybe.”

Looking back at the loss of five feet of water over the last thirty years, he said, “you’d like to be able to say what fraction of that drop is due to climate change. And I don’t know how one would do that.”

But climate is the major factor in changing lake levels, so even if cause is immeasurable, a correlation between climate change and low water is hardly a stretch. Global temperatures are rising, the Great Lakes region is warming, the lakes are heating up, which means more evaporation and less ice cover.

“There’s a whole argument that says falling lake levels are consistent with climate change,” Allan said. “What I don’t think we have the ability to do at the present time is say, ‘our models tell us that lake levels should drop by x amount.’”

The return of the glaciers

What’s left of the ancient glaciers is now melting away, but this time the runoff isn’t flowing into inland freshwater lakes. So as lake levels go down, the salty seas are rising. As in the lakes, a process that might have happened naturally over many thousands of years has been accelerating exponentially in recent decades.

A saltwater invasion threatens coastal crops and raises the likelihood of full-scale destruction of coastal communities by storms and flooding. Plus, rising sea waters threaten fresh-water aquifers along the ocean coasts, which makes the preservation of clean potable water like the Great Lakes all the more pressing.

A master index of Great Lakes stressors

Dr. Allan and a team of researchers at the Great Lakes Environmental Assessment and Mapping Project (GLEAM) recently launched a website that breaks down environmental stressors including temperature change, ice cover, and loss of precipitation into a series of individual maps and a total “stress index”. Check out this video for a guide to how to use the site.

“I would caution people that the whole mapping tool is designed to be something of a 10,000 foot look at the Great Lakes,” he said. “The data get fuzzier the more you zoom in. But you can certainly get a broad picture.”

The stress index across most of Lake Michigan’s coast on the GLEAM map is very high. For example, those wide beaches Chicagoans have enjoyed in recent summers also mean shallower water just off the coasts, which can cause a host of problems including increased presence of dangerous bacteria. So much for a summer of safe Lake Michigan swimming. Check out the site for more information, but before you go, watch our instructional guide.