Cities Are Sinking

Ideal Material

If you were to design the perfect industrial material, what would it be like? You’d want it to be strong and lightweight. Easy to form into different shapes. You’d like it to be resistant to corrosion. And to heat. Ideally, you could easily combine it with other materials for more uses. And re-uses. It would be best if all these qualities came in something cheap and abundant. And in fact, they do. This ideal material … is aluminum. It’s the third-most abundant element in Earth’s crust, behind oxygen and silicon. It’s a small, highly charged atom, so it bonds well to others to form valuable alloys. Adding even a small fraction of copper makes aluminum much harder, for use in things like jet engines. A little magnesium makes it much more malleable, for things like packaging. A little boron raises aluminum’s electrical conductivity almost to that of copper, to make a cheaper, lighter wire ideal for transmission lines. It’s made from bauxite ore, in a process that requires a great deal of electricity. But it takes only 5% of that energy to recycle it. As a result, of the nearly 1 billion tons of aluminum ever produced, three-quarters of it is still in use—recycled in a closed loop so that its many benefits can be enjoyed again and again. That truly is an ideal material.

Cicada Cacophony

It’s going to be a very noisy summer! This year, more cicadas will fill the air with—song?—than any summer since 1803. Cicadas evolved 150 million years ago, and 3,000 species now live on nearly every continent. These are called “annual” cicadas. They live a few years underground, then crawl up a tree and shed their crusty “underground” skin for a final molt into their winged adult form.  They live for a few weeks, and the males screech all day long—which the females find attractive. They mate, the females inject their eggs beneath the bark of a tree, and they both die. The nymphs hatch and fall to the ground, where they burrow into the soil to grow and repeat the process. If you live in the South or Southwest, you see—and hear—the green or brown annual cicada. But in the middle of the US—and only here—a special cicada has evolved, with black bodies and red eyes. Two types exist: one lives underground for 13 years, and the other for 17. No one knows exactly how they time it, but billions emerge at once. Then, every 221 years, as in 2024, the 13- and 17-year cycles align … and they all come out together. The last time this happened, Thomas Jefferson was negotiating the Louisiana Purchase. If he knew what a racket he was in for, he might have rethought the deal.

Leonardo’s Rule of Trees

Leonardo da Vinci was one of our most brilliant thinkers. He’s known as an inventor, scientist, mathematician, architect, sculptor and, of course, painter of the most famous portrait in the world, the Mona Lisa. But unless you’re also a painter, you probably haven’t heard of Leonardo’s Rule of Trees. Intent to paint them realistically, he carefully analyzed trees and recognized a pattern, which he encoded in this formula: The thickness of the limbs that grow from a trunk, when combined, equals the thickness of the trunk. The thickness of all the branches that grow from each limb, equals the thickness of that limb. Trees are fractal, meaning the pattern established in their larger branches repeats in smaller ones—so Leonardo’s rule applies all the way to the twigs. Using his formula to design his trees, his results were so convincing that other painters adopted his rule, too—and have used it since. Recently, scientists wanted to figure out why this pattern exists. One researcher posited that trees evolved this way to withstand force, from wind or the weight of fruits. And in wind tunnel tests, this held up. Other scientists, who hoped Leonardo’s rule might also hold for the vascular pattern of trees, found it did not. But he never intended it to—only to depict their outer structure. Which, 500 years later, it still does remarkably well.

If Earth Had No Moon

What if Earth didn’t have a moon? Well, nearly everything about our home planet would be different—so much, we’d hardly recognize it. Our Moon is a quarter the size of Earth, far bigger than most planets’ moons. Scientists think it formed when a smaller planet crashed into Earth 4 billion years ago, knocking off a chunk that eventually became our Moon. Its large size is key to its dramatic effects on Earth. The gravity of the Moon pulls on the oceans to produce tides. Tidal range would be only a third its size otherwise. Tides define coastal ecosystems and influence what kind of life can live there. Tides also shape ocean currents—like the ocean conveyor we discussed on another EarthDate. Currents moderate global weather and temperature, helping create the stable climate that has allowed human civilization to thrive. Tidal friction has also slowed Earth’s rotation. Billions of years ago, a day on Earth lasted just 10 hours. Without the Moon, it would be even shorter than that! The Moon also stabilizes the tilt of Earth on its axis, which stabilizes our seasons. Planets without moons have much more dramatic seasonal variations. Adapting to erratic seasons and temperatures, and days as short as 6 hours—the plants and animals that could live on Earth would likely be very different. Without the Moon, humans might not be here at all!

Swimming in the Sahara

In 1933, Hungarian explorer László Almásy set out across the Sahara in search of a legendary lost city. What he discovered was far more magical and, at first, completely unbelievable. In a remote ravine, Almásy found a shallow cave. Inside he saw cave paintings of human figures with outstretched arms and legs. They appeared to be flying. But no, Almásy realized, they were swimming—in the Sahara! Other cave paintings showed antelope, giraffe, elephants and lions alongside more human figures. Almásy realized they depicted a Sahara of grasslands with abundant water. His theories were met with skepticism but sparked further investigation. And geologic studies eventually proved him right! Those cave paintings dated from what’s now called the African Humid Period, which lasted from the end of the last Ice Age until 5,000 years ago.  It was caused by a thousand years of African monsoons, brought on by changes in the wobble of Earth’s axis. These greened the Sahara and formed vast lakes.& More than 150 excavations uncovered the remains of aquatic species like hippos, crocodiles and more than 30 types of fish. Further investigation showed the Sahara has alternated between desert and wet grasslands every 20,000 years or so—for hundreds of thousands of years.