Storm in a Teacup: The Physics of Everyday Life(47)

But now the ground has heated up. And just like the toaster heating element, it must give away light energy because of its temperature. It’s relatively cool, so we can’t see the glow. But in the longer-wavelength infrared, the warm ground is a lightbulb. And this is where the greenhouse effect comes into play. Most of the atmosphere will just let these infrared waves through. But some gases—water, carbon dioxide, methane, and ozone—punch above their weight. Even though they make up only a fraction of the total atmosphere, they absorb the infrared waves very strongly. They’re known as the greenhouse gases. As you look out across the landscape, you can see the visible light leaving the surface, but you can’t see the infrared. If you could, you’d see that it faded away as it got farther from the ground. The atmosphere is absorbing the infrared waves as they travel upward. It won’t be long before those molecules will give up their new energy, and send it out again as more infrared waves. But here’s the important bit. When the new waves are sent out, they’ll be sent in all directions equally. Only some will travel upward and out of the atmosphere. Some will travel back downward, and be reabsorbed by the ground. So some of the traveling energy is trapped in the atmosphere. That extra little bit of heating is what keeps our planet warmer than it should be, allowing liquid water to exist. A new balance has to be established; ultimately, the same amount of energy must both arrive and leave, otherwise we’d continually be getting hotter. So the Earth heats up until it can give away enough infrared waves to balance the books.

This is the “greenhouse effect.”?? Most of it is natural—there’s lots of water and carbon dioxide in our atmosphere, and everything is in balance when the average surface temperature is 57°F. But as fossil fuels burn, humans are adding carbon dioxide to the atmosphere, so that more of the infrared energy traveling upward is trapped. This shifts the balance. So the planet will heat up until a new balance is achieved. The amounts of carbon dioxide involved are very small: CO2 made up 313 parts per million of the atmosphere in 1960, and 400 parts per million in 2013. Compared with all the other molecules up there, it’s a tiny increase. But these molecules select certain waves to absorb. Methane will absorb even more infrared than carbon dioxide. So these gases matter. The greenhouse effect is what made our planet habitable, but it also has the potential to change the temperature significantly. It’s all happening with waves that we can’t see directly. But we can measure the consequences already.

There are all sorts of waves rippling around our world—giant radio waves, minuscule visible light waves, ocean waves, ponderous deep sound waves emitted by whales underwater, and the high-frequency echo-sounding beacons sent out by bats. Each type is zooming through and past the others, but has no effect on them. But we have one more question to answer. What happens when a wave meets another of exactly its own type? The answer is beautiful if you are holding an iridescent pearl, but something to be avoided if you’re trying to hold a mobile phone conversation.

Pinctada maxima can be found parked on the seabed, just a few yards below the surface of a turquoise sea near Tahiti and other South Pacific islands. When it’s feeding, the two halves of its shell part slightly and it sucks in sea water, gallons each day. The mollusk inside the shell quietly filters out any valuable specks of food and then expels the cleaned water to rejoin the ocean. You could swim right over the top of it and never notice—the outside of the shell is coarse and unremarkable, mottled in beige and brown. These vacuum cleaners of the ocean look the part: functional and unglamorous. The inside of an oyster was never meant to be seen. And yet Cleopatra, Marie Antoinette, Marilyn Monroe, and Elizabeth Taylor were all proud owners of what happened when an oyster’s innards made the best of a bad job: pearls. Pinctada maxima is the South Pacific pearl oyster.

Occasionally, an irritant finds its way into the wrong bit of the oyster. Since it has no way of expelling the intruder, the oyster coats it in something harmless, the same stuff that it coats the inside of its shell with. It’s the mollusk version of sweeping something under the carpet, except that it makes the carpet to fit rather than using one that’s already there. The coating is made of tiny flat platelets cemented together with organic glue and stacked up on top of each other. Once it has begun the coating process, the oyster just keeps going. It was recently discovered that the pearl turns as it forms, going around perhaps once every five hours. The tides and seasons come and go, sharks and manta rays and turtles pass by overhead, and the oyster quietly sits there, filtering the ocean as the growing pearl pirouettes slowly in the dark.

Serenity reigns for years, until our oyster has a terribly bad day and is yanked out of the ocean by a human and pried open. As sunlight hits the pearl for the first time, waves of light bounce off its shiny white surface. But they don’t just bounce off the platelets at the top; some make it through to the next few layers and bounce off those instead, or maybe bounce a few times inside the layers before they make their way out. So now we have a situation where there’s a single type of wave—let’s consider just the green light from the sun—and it’s overlapping with other waves of exactly the same type. The waves still don’t affect each other, but they do add up. Sometimes, the green light wave that bounces off the top surface lines up exactly with the green light wave that has bounced off the next surface down. The peaks and the troughs of the wave shape match perfectly. So they keep going out into the world together, a strengthened green wave. But perhaps the red light arriving from the same angle, and bouncing off the layers in exactly the same way, doesn’t line up as perfectly. The peaks from one red wave line up with the troughs of the other red wave. Add them together, and there’s nothing left to travel in that direction.