Chicago is sinking 10 to 20 centimeters (4-8 inches) each century, scientists at the National Oceanic and Atmospheric Administration (NOAA) have calculated. That is roughly 1 to 2 millimeters every year – about the same rate as Venice. 

"It's a slow process, but it's a persistent one," said Daniel Roman, chief geodesist with the National Oceanic and Atmospheric Administration, reports Tony Briscoe at The Chicago Tribune.

It is due to glacial melt that took place some 10,000 years ago, which caused some areas depressed under the weight of the ice sheet to bounce back up. But it's not just America's second city that has been affected by this geological phenomenon. It is areas across the northern United States and Canada. 

To understand how the process works, imagine a tube of paint that has been squeezed in the middle. Pressure applied to the tube (which, in this case, represents the Earth's crust) pushes the paint (the Earth's mantle) to the sides. This then causes the areas surrounding the point of pressure to bloat. But later, when that pressure is removed, the paint (or mantle) starts to drift back to the center. 

Here, the metaphorical thumb is an ice sheet and the movement we are now seeing is what happens when that pressure is lifted. Because Chicago sits on what was the fringe of the most recent glacial intrusion, it rebounded soon after the disappearance of the ice sheet. Now, it is starting to fall as some of the mantle returns to Canada, into areas that once felt the strain of the thickest parts of the ice sheet. 

So, what does this sinking (and rising) mean in practice? It could cause problems for the city's sewage system, the pipes of which slope down to the sewage main. However, perhaps the biggest dilemma it poses is rising water levels in North America's Great Lakes. 

Take, for example, Lake Michigan. The far northern end is rising but the rest of the lake is sinking. This is creating a tilting effect that equates to higher water levels in the southern part of the lake. If the overall volume of water remains the same, that shift could see Chicago's lakefront rise 10 centimeters (4 inches) over the coming century. Similar patterns could see lake levels in Milwaukee increase by 10 centimeters (5 inches), and Canada's Hudson Bay increase by 90 centimeters (3 feet) per century. 

But that's not all. All this movement within lakes will affect the movement of water between lakes. For example, the eastern end of Lake Superior is rising, whereas the western end is sinking. This, the researchers say, could slow the lake's outflow.

"If you're tilting one direction, the water flows might change direction or water might accumulate in a way different than you expected in the past," Roman explained.  

"That's important for on-land and near-shore environments. You might get more water, but not where you want it."

Although it's hard to predict exactly how this will manifest.

[H/T: The Chicago Tribune]