Pisa and Climate Change

Our visit to Pisa was a lesson in engineering, architecture, and climate change. We arrived as early as we could, ahead of the mid-day press of tourists, and here was our first good view of Pisa’s most famous landmark:

Cathedral and bell tower, with a glimpse of the baptistry (a separate building) on the left. Inspect this and the next photo carefully and you will see that the tower has a slight curve to it.

The baptistry, cathedral, and bell tower were all built of marble. As you might know, construction of the bell tower presented some interesting challenges. According to our tour guide, the owners hired an architect (maybe low bid?) to design and build the tower, but it started to lean almost immediately after construction started. By the time the foundation and first three or four stories were done, the tilt was so alarming that the owners stopped work, fired the architect, and refused to pay him. They eventually found a second architect to resume construction, and he built the next few stories with a corrected alignment (slanted slightly away from the direction of tilt) in an attempt to fix the problem. Leaning continued to increase, though, so the owners stopped work, fired the second architect, and refused to pay him, too. You would think word got around, but the owners somehow found a third architect to complete the tower. He added stories and topped out the tower with even more of a misalignment in an attempt to compensate for the tilt. It didn’t work, but this is why the tower has an ever-so-slight banana curve to its profile. But what caused it to lean?

Tourists mugging for the camera in front of the leaning tower of Pisa.

In an earlier post we mentioned that some of our tour guides blamed climate change for various ills of society such as the rise and fall of empires, changes in trade routes, inflation, war, and so on. At least some of them understood that climate change has always been with us, that it can cause problems, and that humans have been rather resourceful at finding ways to adapt. However, we were taken by surprise when our guide said the Leaning Tower leans because of climate change. This is not directly true, and may not even be indirectly true. Let’s discuss why the tower leans.

If you set a pot or dish on top of a water-soaked sponge, what happens? The weight of the pot causes the sponge to get thinner, or consolidate, and water seeps out of the pores of the sponge as it gets thinner. What if the sponge is sealed in plastic wrap so the water cannot escape? The water would help carry the load, and the sponge might get a little thinner, but not much. What if you stick a syringe into the plastic-wrapped sponge and use it to pull out some of the water? The sponge will get thinner, or consolidate, under the load of the pot. And what if you took the load off the sponge? It would spring back to its original thickness, more or less.

Think of the soil under the bell tower as two sponges next to each other: one stiff and the other stiffer, and both soils containing air or water in their pores. You start building the tower, which puts a load on top of the soil, and the soil starts to consolidate as the weight of the tower forces water (or air) out of the pores. The tower starts to sink, ever so slowly. However, the stiffer soil does not consolidate as much as the softer soil, so one side of the tower sinks more than the other. Simply put, this is what happened at the Leaning Tower of Pisa. As they built the tower higher the load on the soil increased. Larger loads led to more consolidation, but it was worse on the side of the tower founded on softer soil, so the tower tilted towards that side. By they time they noticed the tilt, it was too late to fix given the technology available at the time.

But what about climate change? IF climate change led people to drill wells and pump water from near the tower (like our syringe withdrawing water from the sponge), then the water withdrawal would have led to more consolidation, causing the tower to sink and lean more. But did climate change cause people to pump water out of the ground near the tower? Not that we know of, but maybe the tour guide knew better. Or not.

Top of the bell tower, seen over adjacent buildings. Notice that the top story does not align with the ones below.

This was probably more than you ever wanted to know about why the Leaning Tower leans. It is funny, though, to see how many old structures have a bit of a tilt to them, particularly if they are a lot taller than they are wide. Maybe the folks on the top part of the tower, in the photo above, need to get away from the low side and move to high side to help keep it stable!

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